This work was supported by the U.S. Army Research Office through Contract W911NF-18-1-0051.

NOTE: All commercially acquired instruments and parts such as the laser, vacuum pumps, chamber, time-of-flight tube and microchannel plate detector were installed and operated according to the manufacturer&#8217;s instructions or user&#8217;s manual. Laser safety goggles designed for the operating laser intensities and wavelengths should be worn.
1. Construction of TOF-MS26
<ol>
	<li>Design and build an ultrahigh vacuum (UHV) chamber that has enough space to accommodate a standard stack of ion optics26 and provisions to mount optical windows on 2 &#190;&#8221; flanges on either side of the ion optics (Figure 1).</li>
	<li>Attach the stack of ion optics mounted on a 1-m flight tube to the chamber. 
	Note: To save space on the optical table, it is easiest to mount the ion optics and flight tube vertically.</li>
	<li>Insert a 1/16&#8221; stainless steel tube into the chamber between the extractor and repeller plates, thread the tube out of the chamber, and connect it to &#188;&#8221; stainless steel tubing27. Attach one or more variable leak valves to the &#188;&#8221; stainless steel tubing. 
	Note: Glass tubes containing molecular samples or gas tanks may be attached to this tubing for sample inlet.</li>
	<li>Attach an 18-mm microchannel plate stack in Z-stack configuration29 on the end of the flight tube.</li>
	<li>Attach two optical windows (1 mm thickness, 50 mm diameter, fused silica) mounted on 2 &#190;&#8221; flanges to the chamber. 
	Note: The laser beams propagate through these two windows through the space between the repeller and extractor plates.</li>
	<li>Wire the ion optics and the detector to high-voltage power supplies via current feedthroughs and BNC cables.</li>
	<li>Attach one turbomolecular pump to the chamber near the ion optics and a second pump to the end of the flight tube near the detector (Figure 1). Connect both pumps to an appropriate backing pump. 
	CAUTION: When attaching a turbomolecular pump to the end of a vertically-mounted flight tube, take care to ensure that the TOF system does not lean to one side due to the weight of the pump. This issue can be mitigated by attaching the vacuum chamber to the optical table.</li>
	<li>Turn on the pumps and wait 24 h. The pressure in the chamber should be below 10-8 torr with no sample. If pressure is high, check for leaks and tighten the nuts or bake the chamber until desired pressure is reached.</li>
</ol>
2. Construction of Optical Pump and Probe Paths
Note: A diagram of the pump and the probe optical paths is given in Figure 1.
<ol>
	<li>Provision of femtosecond laser pulses 
	note: Femtosecond laser pulses (800 nm) were provided by a commercial Ti:Sapphire regenerative amplifier source operated according to the manufacturer&#8217;s manual.
	<ol>
		<li>Turn the laser on and wait for about 30 min for it to stabilize.</li>
		<li>Position a 90:10 (%R:%T) beam splitter after the laser output to generate two replica, which will be used to construct the pump and probe beam lines. Check the laser power of both replicas to ensure adequate power delivery.</li>
		<li>Direct the reflected beam into the optical parametric amplifier (OPA) and optimize the output power using the procedures in the manual.</li>
	</ol>
	</li>
	<li>Preparation of the pump optical path
	<ol>
		<li>Set the OPA software to select the desired wavelength.</li>
		<li>Direct the output beam from the OPA through the &#955;/2 wave plate (WP) and polarizer (P).</li>
		<li>Block the p-polarized beam and direct the s-polarized beam to the concave (f = -10 cm) and convex (f = 50 cm) mirrors to expand its diameter by a factor of 5.</li>
		<li>Direct the expanded beam to the dichroic mirror (DC).</li>
	</ol>
	</li>
	<li>Preparation of the probe optical path
	<ol>
		<li>Direct the beam that passes through the 90:10 beam splitter to the convex mirror (f = 20 cm) and concave mirror (f = -10 cm) to reduce its diameter by a factor of 2.</li>
		<li>Direct the down-collimated beam to a hollow retro-reflector mounted on a motorized linear delay stage. Adjust the mounting knobs of the two flat mirrors prior to the retro-reflector to ensure that the beam position after the retro-reflector does not change when the stage is moved along its full travel range. 
		Note: This ensures that the pump-probe spatial overlap will be maintained over the full scan range.</li>
		<li>Insert a tunable neutral density (ND) filter after the delay stage to attenuate the power of the probe pulse, insert an iris after the ND filter to adjust the beam diameter, and direct the beam to the dichroic mirror (DC).</li>
	</ol>
	</li>
	<li>Measurement of pump and probe pulse durations 
	Note: The durations of the pump and probe pulses are measured with a home-built second harmonic generation-frequency resolved optical gating (SHG-FROG) setup. Details on the construction of a SHG-FROG setup, measurement process, and data retrieval algorithms are described elsewhere30,31,32. In our experiments, pulse durations out of the OPA are typically around 20 fs and that of the 800 nm pulse around 30 fs19,20,27. However, OPAs can introduce higher-order pulse distortions, so it may be necessary to implement pulse-compression using, for instance, chirped mirrors10,11,12,13 or an acousto-optic modulator16.
	<ol>
		<li>Block either the pump or probe beam. Direct the remaining beam into the FROG with flat mirrors placed after the dichroic mirror that combines the pump and probe pulses.</li>
		<li>Ensure that the two beam replicates in the FROG overlap in the &#946;-barium-borate (BBO) crystal. Adjust the beam alignment and delay stage until a third beam is visible between the two original beams.</li>
		<li>With an iris and a f = 10 cm lens, isolate and focus the beam into a fiber-optic mount connected to the spectrometer and computer.</li>
		<li>Collect the FROG scan and retrieve the pulse shape with the appropriate software and retrieval algorithm.</li>
		<li>Repeat steps 2.4.1-2.4.3 for the other beam. Remove the mirrors directing beams to the FROG.</li>
	</ol>
	</li>
	<li>Rough spatial overlap of the pump and probe beams 
	Note: If both pump and probe beams are visible, step 2.5.1 may be skipped.
	<ol>
		<li>Insert a 15 mm-diameter BBO crystal after the dichroic to double the wavelengths of both beams, thereby making them visible. 
		Note: It is easiest to use an OPA wavelength of ~1200-1300 nm for this step to make an orange 600-650 nm beam that is easily distinguishable from the blue 400 nm probe beam. Take care to ensure that the most intense region of the pump beam passes through the center of the crystal. The crystal angle should be optimized such that both the orange and blue pulses are easily visible, although this angle may not correspond to the maximum intensity of a given color.</li>
		<li>Adjust the pump and probe beam alignments using the mirror mounts prior to the dichroic such that the beams propagate collinearly through the TOF-MS chamber and out from the other side. 
		Note: The probe beam has a smaller diameter and should be centered in the middle of the pump beam.</li>
	</ol>
	</li>
	<li>Rough temporal overlap of pump and probe beams 
	Note: The method described here is limited to the oscilloscope resolution and can only determine the zero-delay position to within several millimeters of travel on the delay stage.
	<ol>
		<li>Place a fast photodiode detector a few centimeters in front of the window entrance to the TOF-MS chamber in the path of the pump and probe beams. Attach the detector cable to a digital oscilloscope and independently locate the signals of the pump and probe pulses.</li>
		<li>Adjust the position of the motorized delay stage on the probe line such that the pump and probe signals in the oscilloscope are temporally overlapped. If one signal is consistently in front (behind) the other in the oscilloscope, move the mounts holding the motorized delay stage to shorten or lengthen the path length as needed.</li>
		<li>Remove the photodiode detector.</li>
	</ol>
	</li>
</ol>
3. Preliminary Measurements
Note: All data in our experiments were acquired using codes written in-house with commercial instrument control software (Table of Materials). All instrument driver software was obtained from the respective manufacturer.
<ol>
	<li>Calibration of absolute peak intensity of the pump pulse28
	<ol>
		<li>Block the probe beam and insert a f = 20 cm lens mounted on a manual linear translation stage directly before the entrance window to the mass spectrometer.</li>
		<li>Adjust the wave plate (WP) rotation angle (Figure 1) to maximize the power of the pump beam measured before the lens.</li>
		<li>Attach a tank of Xe gas to the TOF-MS chamber inlet and adjust the leak valve controlling gas flow into the chamber such that the pressure gauge reads between 5-10 x 10-8 torr. Ensure that the TOF-MS power supply voltages are off when adjusting sample pressure to avoid damage to the MCP detector due to pressure spikes.</li>
		<li>Connect the output cables from the MCP detector and from the laser signal delay generator to a digital oscilloscope. Set the oscilloscope to trigger off the laser signal.</li>
		<li>Turn on the TOF-MS power supply and check the voltages. Typical values of the voltages for V1, V2, V3, and V4 (Figure 2) are +4,190 V, +3,910 V, 0 V, and -3,000 V, respectively.</li>
		<li>Check for ion signals of Xe+ (and higher charge states) in the oscilloscope directly or via a computer connected to the oscilloscope.</li>
		<li>Adjust the position of the manual translation stage holding the lens to maximize the total ion signal. This step ensures that the pump beam focus overlaps with the 0.5 mm slit shown in Figure 2.</li>
		<li>Record the Xe mass spectrum using the data acquisition software.</li>
		<li>Decrease the laser power by rotating the WP angle to obtain a power ~20 mW lower than the previously measured power.</li>
		<li>Repeat steps 3.1.8-3.1.9 until the laser power is too low to generate measurable Xe+ signal. A total of 10-15 mass spectra at different laser powers should be recorded.</li>
		<li>Using appropriate data analysis software, follow the steps in reference 28 to identify the laser pulse energy corresponding to the absolute saturation intensity for Xe+ (1.12 x 1014 W cm-2)28. This procedure provides an absolute intensity scale for any pump pulse energy used in further experiments.</li>
	</ol>
	</li>
	<li>Calibration of absolute peak intensity of the probe pulse33 
	Note: Due to the weak probe pulse intensity, the Xe calibration method described in step 3.1 cannot be used. Instead, the probe intensity in the experiments may be estimated by measuring the spot size at the focal point with a digital camera32, along with the pulse duration and energy.
	<ol>
		<li>Block the pump beam and direct the probe beam along a straight path after the dichroic mirror using two flat mirrors.</li>
		<li>Remove the focusing lens from its position adjacent to the chamber and place it in the probe beam path, ensuring that the probe beam passes through its center.</li>
		<li>Minimize the probe beam energy using the variable ND filter and add additional ND filters to attenuate the pulse energy below ~100 nJ.</li>
		<li>Place a compact CMOS camera on a manual linear translation stage and connect it to a computer with suitable data acquisition software. Mount the translation stage in the probe beam path with the camera centered near the focal spot of the beam. Locate the beam spot using the software program. Add ND filters and adjust the camera acquisition settings to prevent saturation of the CMOS detector.</li>
		<li>Adjust the position of the translation stage to obtain the smallest, most intense laser spot. This location corresponds to the focus of the beam.</li>
		<li>Acquire a camera image at the focus and fit the spot to a two-dimensional Gaussian function using appropriate data analysis software to determine the beam diameter.</li>
		<li>Remove the mirrors directing the probe beam to the camera and return the focusing lens to its position in front of the TOF-MS.</li>
	</ol>
	</li>
	<li>Determination of pump-probe spatial and temporal overlap in the TOF-MS 
	Note: The completion of the protocol in step 3.1 is assumed. While Xe gas could be used as the sample to determine spatial and temporal overlap, it is recommended to use the target molecule for study because changes in the mass spectrum can be observed over a range of positive time-delays instead of only at zero time-delay, as with Xe.
	<ol>
		<li>Connect the desired sample to the TOF-MS chamber and adjust the pressure to the range of 1-5 x 10-7 torr.</li>
		<li>Unblock the pump and probe beams and ensure that they are aligned into the TOF-MS chamber.</li>
		<li>Maximize the probe power by adjusting the ND filter. Set the pump power with the waveplate to a sufficiently high level to obtain satisfactory ion signal. 
		Note: The probe power should be sufficiently high to induce fragmentation but not so high as to create ions in the absence of the pump pulse.</li>
		<li>Adjust the spatial position of the probe beam with the knobs on the dichroic mirror mount (DC, Figure 1) until either a spike in intensity of all ions is observed (if the stage position is exactly at zero delay) or a significant depletion of the parent molecular ion and/or increase in fragment ion yields are observed (if the stage position corresponds to a positive time-delay).</li>
		<li>If no changes in the ion signals are observed, the stage position is likely at negative time-delay, i.e., probe precedes pump. Adjust the motorized delay stage to a longer path for the probe beam and repeat step 3.3.4 until a change in the mass spectrum is observed.</li>
		<li>Adjust the motorized delay stage position to produce a spike in total ion signal. This position corresponds to zero time-delay. Representative mass spectra of the molecule DMMP taken at zero time-delay with good and poor spatial overlap, along with the mass spectrum taken with only the pump beam, are shown in Figure 3.</li>
	</ol>
	</li>
	<li>Cross-correlation6,34 
	Note: The cross-correlation measurement must be performed on an inert gas such as Xe32. It serves to verify both the pulse durations measured with FROG and the delay stage position corresponding to zero time-delay.
	<ol>
		<li>With Xe gas in the chamber (step 3.1) and the beam overlap optimized (step 3.3), move the motorized stage position to locate zero time-delay (i.e., when Xe+ signal is maximized).</li>
		<li>Scan the motorized translation stage over the delay range of -200 fs to +200 fs in steps of 5 fs. This scan corresponds to steps of 1.5 &#181;m over a range of 120 &#181;m centered at the zero time-delay position. Record mass spectrum at each scan position and integrate Xe+ yields to acquire the time-dependent ion signal34.</li>
	</ol>
	</li>
</ol>
4. Pump-probe Measurements
<ol>
	<li>Preliminary checks before taking measurements
	<ol>
		<li>Check the experimental setup to confirm that both beams are propagating collinearly through the window of the chamber (Figure 1).</li>
		<li>Attach the desired sample to the TOF-MS chamber and gradually release the sample into the chamber using variable leak valve(s) to achieve a target pressure of 1-5 x 10-7 torr. Ensure that the TOF-MS power supply voltages are turned off when adjusting the sample pressure to avoid damage to the MCP detector due to pressure spikes.</li>
		<li>If the vapor pressure of the molecule is too low to produce the desired pressure, gently heat the sample holder until desired pressure is achieved.</li>
		<li>Turn on and check TOF-MS voltages (step 3.1.5). Verify the operability of the data acquisition software with respect to the communication with both the motorized delay stage and oscilloscope.</li>
		<li>Adjust the lens in front of the chamber (step 3.1.7) and pump-probe spatial alignment (step 3.3.4) to optimize ion signal and spatial overlap.</li>
	</ol>
	</li>
	<li>Data acquisition
	<ol>
		<li>Adjust the pump and probe pulse energies to obtain desired ion signals.</li>
		<li>In the data acquisition software, specify the scan length and step size. 
		Note: Typical scan lengths in our experiments range from 1000-5000 fs and step sizes range from 5-20 fs19,20.</li>
		<li>Run the data acquisition software to acquire the mass spectrum at each pump-probe delay. 
		Note: Typically, the mass spectrum recorded at each time-delay in a scan is averaged over 1000 laser shots. To obtain sufficiently high signal-to-noise ratios, 10-20 scans are taken at the desired settings (i.e., scan length, step size, pump and probe powers) and averaged. To minimize the effects of laser power drift, scans may be taken in alternating directions of delay-stage travel. All data are saved as tab-delimited text files. Representative raw mass spectral data from a single scan taken on DMMP for a scan length of 1250 fs with step size of 5 fs are shown in Figure 4.</li>
	</ol>
	</li>
	<li>Data processing
	<ol>
		<li>Identify the flight time range for each mass peak of interest (illustrated by the bracketed regions in Figure 4) and integrate over these ranges in each mass spectrum. The outputs represent the time-resolved signals of each ion of interest. For example, time-resolved ion signals of DMMP parent molecular ion with good and poor pump-probe spatial overlap obtained from one pump-probe scan are shown in Figure 5.</li>
		<li>Repeat step 4.3.1 to obtain the desired number of scans (e.g., 10-20)19,20 at the same scan settings. Average each time-resolved ion signal over all scans taken. Representative averaged ion signals are shown in Figure 6.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Letokhov, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photophysics+and+Photochemistry.">Photophysics and Photochemistry.</a> Physics Today. 30 (5), 23-32 (1977).</li><li>Bloembergen, N., Yablonovitch, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infrared+laser+induced+unimolecular+reactions.">Infrared laser induced unimolecular reactions.</a> Physics Today. 31 (5), 23-30 (1978).</li><li>Zewail, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser+selective+chemistry-is+it+possible?.">Laser selective chemistry-is it possible?.</a> Physics Today. 33 (11), 25-33 (1980).</li><li>Brif, C., Chakrabarti, R. L., Rabitz, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+of+quantum+phenomena:+past,+present+and+future.">Control of quantum phenomena: past, present and future.</a> New Journal of Physics. 12, 075008 (2010).</li><li>Bloembergen, N., Zewail, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Energy+redistribution+and+the+question+of+mode+selective+laser+chemistry+revisited.">Energy redistribution and the question of mode selective laser chemistry revisited.</a> The Journal of Physical Chemistry. 88 (23), 5459-5465 (1984).</li><li>Rosker, M. J., Dantus, M., Zewail, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femtosecond+real-time+probing+of+reactions.+I.+The+technique.">Femtosecond real-time probing of reactions. I. The technique.</a> The Journal of Chemical Physics. 89 (10), 6113-6127 (1988).</li><li>Zewail, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femtochemistry:+Recent+progress+in+studies+of+dynamics+and+control+of+reaction+and+their+transition+states.">Femtochemistry: Recent progress in studies of dynamics and control of reaction and their transition states.</a> The Journal of Physical Chemistry. 100 (31), 12701-12724 (1996).</li><li>Warren, W. S., Rabitz, H., Dahleh, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coherent+Control+of+Quantum+Dynamics:+The+Dream+Is+Alive.">Coherent Control of Quantum Dynamics: The Dream Is Alive.</a> Science. 259 (5101), 1581-1589 (2003).</li><li>Kosma, K., Trushin, S. A., Fu&#223;, W., Schmid, E. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cyclohexadiene+ring+opening+observed+with+13+fs+resolution:+coherent+oscillations+confirm+the+reaction+path.">Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path.</a> Physical Chemistry Chemical Physics. 11 (1), 172-181 (2009).</li><li>De, S., Magrakvelidze, M., Bocharova, I. A., Ray, D., Cao, W., Znakovskaya, I., Li, H., Wang, Z., Laurent, G., Thumm, U., Kling, M. F., Litvinyuk, I. V., Ben-Itzhak, I., Cocke, C. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Following+dynamic+nuclear+wave+packets+in+N2,+O2+and+CO+with+few-cycle+infrared+pulses.">Following dynamic nuclear wave packets in N2, O2 and CO with few-cycle infrared pulses.</a> Physical Review A. 84 (4), 043410 (2011).</li><li>Bryan, W. A., McKenna, J., English, E. M. L., Wood, J., Calvert, C. R., Torres, R., Murphy, D. S., Turcu, I. C. E., Collier, J. L., McCann, J. F., Williams, I. D., Newell, W. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isolated+vibrational+wavepackets+in+D2+:+Defining+superposition+condition+and+wavepacket+distinguishability.">Isolated vibrational wavepackets in D2+: Defining superposition condition and wavepacket distinguishability.</a> Physical Review A. 76 (5), 053402 (2007).</li><li>Calvert, C., Bryan, W., Newell, W., Williams, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time+resolved+studies+of+ultrafast+wavepacket+dynamics+in+hydrogen+molecules.">Time resolved studies of ultrafast wavepacket dynamics in hydrogen molecules.</a> Physics Reports. 491 (1), 1-28 (2010).</li><li>Kelkensberg, F., Lefebvre, C., Siu, W., Ghafur, O., Nguyen-Dang, T. T., Atabek, O., Keller, A., Serov, V., Johnsson, P., Swoboda, M., Remetter, T., L&#39;Huillier, A., Zherebtsov, S., Sansone, G., Benedetti, E., Ferrari, F., Nisoli, M., L&#233;pine, F., Kling, M. F., Vrakking, M. J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+dissociative+ionization+and+wave-packet+dynamics+studied+using+two-color+XUV+and+IR+pump-probe+spectroscopy.">Molecular dissociative ionization and wave-packet dynamics studied using two-color XUV and IR pump-probe spectroscopy.</a> Physical Review Letters. 103 (12), 123005 (2009).</li><li>Erattupuzha, S., Larimian, S., Baltuska, A., Xie, X., Kitzler, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-pulse+control+over+double+ionization+pathways+in+CO2.">Two-pulse control over double ionization pathways in CO2.</a> The Journal of Chemical Physics. 144 (2), 024306 (2016).</li><li>Rudenko, A., Makhija, V., Vajdi, A., Ergler, T., Schurholz, M., Kushawaha, R. K., Ullrich, J., Moshammer, R., Kumarappan, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strong+field-induced+wave+packet+dynamics+in+carbon+dioxide+molecule.">Strong field-induced wave packet dynamics in carbon dioxide molecule.</a> Faraday Discussions. 194, 463-478 (2016).</li><li>Pearson, J. B., Nichols, S. R., Weinacht, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+fragmentation+driven+by+ultrafast+dynamic+ionic+resonances.">Molecular fragmentation driven by ultrafast dynamic ionic resonances.</a> The Journal of Chemical Physics. 127 (13), 131101 (2007).</li><li>Bohinski, T., Tibbetts, K. M., Tarazkar, M., Romanov, D. A., Matsika, S., Levis, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strong+field+adiabatic+ionization+prepares+a+launch+state+for+coherent+control.">Strong field adiabatic ionization prepares a launch state for coherent control.</a> The Journal of Physical Chemistry Letters. 5 (24), 4305-4309 (2014).</li><li>Tibbetts, K. M., Tarazkar, M., Bohinski, T., Romanov, D. A., Matsika, S., Levis, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlling+the+dissociation+dynamics+of+acetophenone+radical+cation+through+excitation+of+ground+and+excited+state+wavepacket.">Controlling the dissociation dynamics of acetophenone radical cation through excitation of ground and excited state wavepacket.</a> The Journal of Physics B: Atomic, Molecular, and Optical Physics. 48 (16), 164002 (2015).</li><li>Ampadu Boateng, D., Gutsev, G. L., Jena, P., Tibbetts, K. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrafast+coherent+vibrational+dynamics+dynamics+in+dimethyl+methylphosphonate+radical+cation.">Ultrafast coherent vibrational dynamics dynamics in dimethyl methylphosphonate radical cation.</a> Physical Chemistry Chemical Physics. 20 (7), 4636-4640 (2018).</li><li>Ampadu Boateng, D., Gutsev, G. L., Jena, P., Tibbetts, K. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dissociation+dynamics+of+3-+and+4-nitrotoluene+radical+cations:+Coherently+driven+C-NO2+bond+homolysis.">Dissociation dynamics of 3- and 4-nitrotoluene radical cations: Coherently driven C-NO2 bond homolysis.</a> The Journal of Chemical Physics. 148 (13), 134305 (2018).</li><li>Markevitch, A. N., Romanov, D. A., Smith, S. M., Schlegel, H. B., Ivanov, M. Y., Levis, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sequential+non+adiabatic+excitation+of+large+molecules+and+ions+driven+by+strong+laser+fields.">Sequential non adiabatic excitation of large molecules and ions driven by strong laser fields.</a> Physical Review A. 69 (1), 013401 (2004).</li><li>Lezius, M., Blanchet, M., Rayner, D. M., Villeneuve, D. M., Stolow, A., Ivanov, M. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonadiabatic+multielectron+dynamics+in+strong+field+molecular+ionization.">Nonadiabatic multielectron dynamics in strong field molecular ionization.</a> Physical Review Letters. 86 (1), 51 (2001).</li><li>Lezius, M., Blanchet, V., Ivanov, M. Y., Stolow, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polyatomic+molecules+in+strong+laser+fields:+Nonadiabatic+multielectron+dynamics.">Polyatomic molecules in strong laser fields: Nonadiabatic multielectron dynamics.</a> The Journal of Chemical Physics. 117 (4), 1575 (2002).</li><li>Bohinski, T., Tibbetts, K. M., Tarazkar, M., Romanov, D., Matsika, S., Levis, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+an+electronic+resonance+in+a+ground-state,+gas-phase+acetophenone+cation+via+strong-field+mass+spectrometry.">Measurement of an electronic resonance in a ground-state, gas-phase acetophenone cation via strong-field mass spectrometry.</a> The Journal of Physical Chemistry Letters. 4 (10), 1587-1591 (2013).</li><li>Bohinski, T., Tibbetts, K. M., Tarazkar, M., Romanov, D., Matsika, S., Levis, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+ionic+resonances+in+alkyl+phenyl+ketone+cations+via+infrared+strong+field+mass+spectrometry.">Measurement of ionic resonances in alkyl phenyl ketone cations via infrared strong field mass spectrometry.</a> The Journal of Physical Chemistry A. 117 (47), 12374-12381 (2013).</li><li>Wiley, W. C., McLaren, I. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time-of-Flight+Mass+Spectrometer+with+Improved+Resolution.">Time-of-Flight Mass Spectrometer with Improved Resolution.</a> Review of Scientific Instruments. 26, 1150-1156 (1955).</li><li>Gutsev, G. L., Ampadu Boateng, D., Jena, P., Tibbetts, K. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Theoretical+and+Mass+Spectrometry+Study+of+Dimethyl+Methylphosphonate:+New+Isomers+and+Cation+Decay+Channels+in+a+Femtosecond+Laser+Field.">A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in a Femtosecond Laser Field.</a> The Journal of Physical Chemistry A . 121 (44), 8414-8424 (2017).</li><li>Hankin, S. M., Villeneuve, D. M., Corkum, P. B., Rayner, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intense-field+laser+ionization+rates+in+atoms+and+molecules.">Intense-field laser ionization rates in atoms and molecules.</a> Physical Review A. 64 (1), 013405 (2001).</li><li>Amitay, Z., Zajfman, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+type+of+multiparticle+three-dimensional+imaging+detector+with+subnanosecond+time+resolution.">A new type of multiparticle three-dimensional imaging detector with subnanosecond time resolution.</a> Review of Scientific Instruments. 68 (3), 1387-1392 (1997).</li><li>Trebino, R., Kane, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+phase+retrieval+to+measure+the+intensity+and+phase+of+ultrashort+pulses:+frequency-resolved+optical+gating.">Using phase retrieval to measure the intensity and phase of ultrashort pulses: frequency-resolved optical gating.</a> Journal of the Optical Society of America A. 10 (5), 1101 (1993).</li><li>DeLong, K. W., Trebino, R., Hunter, J., White, W. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Frequency-resolved+optical+gating+with+the+use+of+second+harmonic+generation.">Frequency-resolved optical gating with the use of second harmonic generation.</a> Journal of the Optical Society of America B. 11 (11), 2206 (1994).</li><li>Trebino, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Frequency Resolved Optical Gating. , (2000).</li><li>Ruff, J. A., Siegman, A. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-pulse+laser+beam+quality+measurements+using+a+CCD+camera+system.">Single-pulse laser beam quality measurements using a CCD camera system.</a> Optics Letters. 31 (24), 4907-4909 (1992).</li><li>Trushin, S. A., Kosma, K., Fu&#223;, W., Schmid, W. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sub-10-fs+supercontinuum+radiation+generated+by+filamentation+of+few-cycle+800+nm+pulses+in+argon.">Sub-10-fs supercontinuum radiation generated by filamentation of few-cycle 800 nm pulses in argon.</a> Optics Letters. 32 (16), 2432-2434 (2007).</li><li>Fu&#223;, W., Schmid, W. E., Pushpa, K. K., Trushin, S. A., Yatsuhashi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrafast+relaxation+and+coherent+oscillations+in+aminobenzonitriles+in+the+gas+phase+probed+by+intense-field+ionization.">Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization.</a> Physical Chemistry Chemical Physics . 9 (10), 1151-1169 (2007).</li></ol>

The authors have nothing to disclose.

This protocol enables us to resolve ultrafast vibrational dynamics in polyatomic radical cations through selective preparation of the ions in the ground electronic state. While the standard strong-field ionization procedure using 800 nm can prepare vibrational coherences in ground-electronic state radical cations of first-row diatomics10,11,12,13 and CO214<sup...

Since the invention of the laser in 1960&#39;s, the goal of selectively breaking chemical bonds in molecules has been a longstanding dream of chemists and physicists. The ability to tune both laser frequency and intensity was believed to enable direct cleavage of a target bond through selective energy absorbance at the associated vibrational frequency1,2,3,4. However, early experiments found that intramolecular vibrational redistribution of the absorbed energy throughout the molecule often resulted in non-selective cleavage of the weakest bond4,5. It was not until the development of femtosecond pulsed lasers and the pump-probe technique6 in the late 1980&#39;s that direct manipulation of coherent vibrational states, or &#34;wave packets&#34;, enabled successful control over bond cleavage and other objectives6,7,8. Pump-probe measurements, wherein the &#34;pump&#34; pulse prepares an excited state or ion that is subsequently excited by a time-delayed &#34;probe&#34; pulse, remain one of the most widely used techniques for studying ultrafast processes in molecules9,10,11,12,13,14,15,16,17,18,19,20.
A significant limitation to studying the ultrafast dissociation dynamics of polyatomic radical cations using pump-probe excitation coupled to mass spectrometric detection arises from nonselective fragmentation of the target molecule by the ionizing pump pulse at the Ti:Sapphire wavelength of 800 nm21,22,23. This excess fragmentation results from nonadiabatic multiphoton ionization and can be mitigated by shifting the excitation wavelength into the near-infrared (e.g., 1200-1500 nm)22,23,24,25. At these longer wavelengths, the contribution of adiabatic electron tunneling increases relative to multiphoton excitation in the ionization process22,23. Adiabatic tunneling imparts little excess energy to the molecule and forms predominantly &#34;cold&#34; ground state molecular ions19,22,23. Our previous work has demonstrated that the use of near-infrared excitation significantly improves the preparation of coherent vibrational excitations, or &#34;wave packets&#34;, in polyatomic radical cations as compared to 800 nm excitation19,20. This work will illustrate the difference between strong-field ionization dominated by multiphoton and tunneling contributions with pump-probe measurements taken on the chemical warfare agent simulant dimethyl methylphosphonate (DMMP) using 1500 nm and 800 nm pump wavelengths.
In our pump-probe experiments, a pair of ultrashort laser pulses is time-delayed, recombined, and focused into a time-of-flight mass spectrometer, as shown in our setup in Figure 1. These experiments require a Ti:Sapphire regenerative amplifier producing &#62;2 mJ, 800 nm, 30 fs pulses. The amplifier output is split on a 90:10 (%R:%T) beam splitter, where most of the energy is used to pump an optical parametric amplifier (OPA) for generation of 1200 - 1600 nm, 100 - 300 &#181;J, 20 - 30 fs pulses. The diameter of the IR pump beam is expanded to 22 mm and the diameter of the 800 nm probe beam down-collimated to 5.5 mm and cored using an iris. These collimations result in the pump beam focusing to a significantly smaller beam waist (9 &#181;m) than the probe beam (30 &#181;m), thereby ensuring that all ions formed during the ionizing pump pulse are excited by the time-delayed probe pulse. This configuration is used because the goal of our experiments is to probe the dynamics of the parent molecular ion, which may be formed even at lower intensities near the edges of the focused beam. We note that if the dynamics of more highly-excited ionic species are of interest, then the probe beam diameter should be made smaller than that of the pump.
The pump and probe pulses propagate collinearly and are focused into the extraction region of a Wiley-McLaren time-of-flight mass spectrometer (TOF-MS)26 (Figure 2). Molecular samples placed in a vial are attached to the inlet and opened to the vacuum. This setup requires that the molecule under investigation have a nonzero vapor pressure; for molecules with low vapor pressure, the vial may be heated. The flow of gaseous sample into the chamber is controlled by two variable leak valves. The sample enters the chamber through a 1/16&#34; stainless steel tube approximately 1 cm away from the laser focus (Figure 2) in order to deliver a locally high concentration of target molecule in the extraction region27. The extraction plate has a 0.5 mm slit oriented orthogonal to the laser propagation and ion paths. Because the Rayleigh range of the pump beam is approximately 2 mm, this slit serves as a filter, allowing only ions generated from the central focal volume where the intensity is highest to pass through the extraction plate28. The ions enter a 1 m field-free drift tube to reach the Z-gap micro channel plate (MCP) detector29, where they are detected and recorded with a 1 GHz digital oscilloscope at the 1 kHz repetition rate of typical commercial Ti:Sapphire lasers.

<table>
	<tbody>
		<tr>
			<td>Mass spectrometer components</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>TOF lens stack and flight tube assembly</td>
			<td>Jordan TOF Products, Inc.</td>
			<td>C-677</td>
			<td></td>
		</tr>
		<tr>
			<td>18 mm Z-gap detector assembly</td>
			<td>Jordan TOF Products, Inc.</td>
			<td>C-701Z</td>
			<td></td>
		</tr>
		<tr>
			<td>TOF high voltage power supply</td>
			<td>Jordan TOF Products, Inc.</td>
			<td>D-603</td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum system components</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rotary vane backing pump</td>
			<td>Edwards Vacuum LLC</td>
			<td>RV12</td>
			<td></td>
		</tr>
		<tr>
			<td>Turbomolecular pumps (2)</td>
			<td>Edwards Vacuum LLC</td>
			<td>EXT255H</td>
			<td></td>
		</tr>
		<tr>
			<td>Turbomolecular pump controllers (2)</td>
			<td>Edwards Vacuum LLC</td>
			<td>EXC300</td>
			<td></td>
		</tr>
		<tr>
			<td>Pressure gauge</td>
			<td>Edwards Vacuum LLC</td>
			<td>AIGX-S-DN40CF</td>
			<td></td>
		</tr>
		<tr>
			<td>Chiller for water cooling</td>
			<td>Neslab</td>
			<td>CFT-25</td>
			<td></td>
		</tr>
		<tr>
			<td>Femtosecond laser system</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ti:Sapphire regenerative amplifier</td>
			<td>Coherent, Inc.</td>
			<td>Astrella</td>
			<td>oscillator and amplifier in a single integrated system</td>
		</tr>
		<tr>
			<td>Optical Parametric Amplifer (OPA)</td>
			<td>Light Conversion</td>
			<td>TOPAS Prime</td>
			<td></td>
		</tr>
		<tr>
			<td>Motion control</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Motorized linear translation stage 1&#34; travel</td>
			<td>Thorlabs</td>
			<td>Z825B</td>
			<td></td>
		</tr>
		<tr>
			<td>controller for linear translation stage</td>
			<td>Thorlabs</td>
			<td>KDC 101</td>
			<td></td>
		</tr>
		<tr>
			<td>USB controller hub and power supply</td>
			<td>Thorlabs</td>
			<td>KCH 601</td>
			<td></td>
		</tr>
		<tr>
			<td>Manual linear translation stage 1&#34; travel</td>
			<td>Thorlabs</td>
			<td>PT1</td>
			<td></td>
		</tr>
		<tr>
			<td>Detectors</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Pyroelectric laser energy meter</td>
			<td>Coherent, Inc.</td>
			<td>1168337</td>
			<td></td>
		</tr>
		<tr>
			<td>Thermal laser power meter</td>
			<td>Coherent, Inc.</td>
			<td>5356E16R</td>
			<td></td>
		</tr>
		<tr>
			<td>Si-biased detector 200-1100 nm</td>
			<td>Thorlabs</td>
			<td>DET10A</td>
			<td></td>
		</tr>
		<tr>
			<td>Compact USB CMOS Camera</td>
			<td>Thorlabs</td>
			<td>DCC1545M</td>
			<td></td>
		</tr>
		<tr>
			<td>USB spectrometer</td>
			<td>Ocean Optics</td>
			<td>HR4000</td>
			<td></td>
		</tr>
		<tr>
			<td>1 GHz digital oscilloscope&#160;</td>
			<td>LeCroy</td>
			<td>WaveRunner 610Zi</td>
			<td></td>
		</tr>
		<tr>
			<td>Optics</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Type 1 BBO crystal</td>
			<td>Crylight Photonics</td>
			<td>BBO007</td>
			<td>aperture and thickness may be customized</td>
		</tr>
		<tr>
			<td>Achromatic half wave plate, 1100-2000 nm</td>
			<td>Thorlabs</td>
			<td>AHWP05M-1600</td>
			<td></td>
		</tr>
		<tr>
			<td>Wollaston prism polarizer</td>
			<td>Thorlabs</td>
			<td>WPM10</td>
			<td></td>
		</tr>
		<tr>
			<td>Hollow retro-reflector</td>
			<td>PLX, Inc.</td>
			<td>OW-20-1C</td>
			<td></td>
		</tr>
		<tr>
			<td>Variable neutral density filter</td>
			<td>Thorlabs</td>
			<td>NDC-100C-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Longpass dichroic mirror 2&#34; diameter</td>
			<td>Thorlabs</td>
			<td>DMLP950L</td>
			<td></td>
		</tr>
		<tr>
			<td>Software</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Digital Camera image software</td>
			<td>Thorlabs</td>
			<td>ThorCam</td>
			<td></td>
		</tr>
		<tr>
			<td>Instrument communication interface</td>
			<td>National Instruments</td>
			<td>NI-MAX</td>
			<td></td>
		</tr>
		<tr>
			<td>Graphical development environment for measurement programs</td>
			<td>National Instruments</td>
			<td>LabVIEW</td>
			<td></td>
		</tr>
		<tr>
			<td>Data processing software</td>
			<td>Mathworks</td>
			<td>MATLAB</td>
			<td></td>
		</tr>
	</tbody>
</table>

measurement of ultrafast vibrational coherences in polyatomic radical cations with strong-field adiabatic ionization

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state (&#34;wave packet&#34;) amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

Results obtained for the molecule DMMP21 are presented. Figure 3 shows the DMMP mass spectra taken at zero time-delay with the peak intensities of the 1500 nm pump and 800 nm probe pulses being 8 x 1013 and 8 x 1012 W cm-2, respectively. For reference, the mass spectrum taken with only the pump pulse is also shown. The spectra are averaged over 10,000 laser shots (total acquisition time 12 s). The incre...

Watch this Scientific Journal Video about Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization at JoVE.com

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the ...

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6.8K Views.  Virginia Commonwealth University. We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

Video: Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

A Study of the Complexation of Mercury(II) with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry

In this study we evaluated a method for the characterization of complexes, formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptide, by electrospray ionization orbitrap mass spectrometry. This strategy is based on previous successful characterization of mercury-dicysteinyl complexes involving tripeptides by utilizing mass spectrometry among other techniques. Mercury(II) chloride and a dicysteinyl tetrapeptide were incubated in a degassed buffered medium at varying stoichiometric ratios. The complexes formed were subsequently analyzed on an electrospray mass spectrometer consisting of a hybrid linear ion- and orbi- trap mass analyzer. The electrospray ionization mass spectrometry (ESI-MS) spectra were acquired in the positive mode and the observed peaks were then analyzed for distinct mercury isotopic distribution patterns and associated monoisotopic peak. This work demonstrates that an accurate stoichiometry of mercury and peptide in the complexes formed under specified electrospray ionization conditions can be determined by using high resolution ESI MS based on distinct mercury isotopic distribution patterns.

A Study of the Complexation of MercuryII with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry

The characterization of complexes formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptides by electrospray ionization orbitrap mass spectrometry is presented.

In this study we evaluated a method for the characterization of complexes, formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptide, ...

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The procedure is performed under an inert atmosphere using air- and water-exclusion techniques. The outcome of the polymerization is affected by the ratios of monomer, initiator, and catalyst used as well as the reaction concentration, solvent, and nature of the light source. Temporal control over the polymerization can be exercised by turning the visible light source off and on. Low dispersities of the resultant polymers as well as the ability to chain-extend to form block copolymers suggest control over the polymerization, while chain end-group analysis provides evidence supporting an atom-transfer radical polymerization mechanism.

A method for the atom transfer radical polymerization of functionalized vinyl monomers using perylene as a visible-light photocatalyst is described.

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Fabrication and Testing of Photonic Thermometers

In recent years, a push for developing novel silicon photonic devices for telecommunications has generated a vast knowledge base that is now being leveraged for developing sophisticated photonic sensors. Silicon photonic sensors seek to exploit the strong confinement of light in nano-waveguides to transduce changes in physical state to changes in resonance frequency. In the case of thermometry, the thermo-optic coefficient, i.e., changes in refractive index due to temperature, causes the resonant frequency of the photonic device such as a Bragg grating to drift with temperature. We are developing a suite of photonic devices that leverage recent advances in telecom compatible light sources to fabricate cost-effective photonic temperature sensors, which can be deployed in a wide variety of settings ranging from controlled laboratory conditions, to the noisy environment of a factory floor or a residence. In this manuscript, we detail our protocol for the fabrication and testing of photonic thermometers.

We describe the process of fabrication and testing of photonic thermometers.

In recent years, a push for developing novel silicon photonic devices for telecommunications has generated a vast knowledge base that is now being ...

Fizzy Extraction of Volatile Organic Compounds Combined with Atmospheric Pressure Chemical Ionization Quadrupole Mass Spectrometry

Chemical analysis of volatile and semivolatile compounds dissolved in liquid samples can be challenging. The dissolved components need to be brought to the gas phase, and efficiently transferred to a detection system. Fizzy extraction takes advantage of the effervescence phenomenon. First, a carrier gas (here, carbon dioxide) is dissolved in the sample by applying overpressure and stirring the sample. Second, the sample chamber is decompressed abruptly. Decompression leads to the formation of numerous carrier gas bubbles in the sample liquid. These bubbles assist the release of the dissolved analyte species from the liquid to the gas phase. The released analytes are immediately transferred to the atmospheric pressure chemical ionization interface of a triple quadrupole mass spectrometer. The ionizable analyte species give rise to mass spectrometric signals in the time domain. Because the release of the analyte species occurs over short periods of time (a few seconds), the temporal signals have high amplitudes and high signal-to-noise ratios. The amplitudes and areas of the temporal peaks can then be correlated with concentrations of the analytes in the liquid samples subjected to fizzy extraction, which enables quantitative analysis. The advantages of fizzy extraction include: simplicity, speed, and limited use of chemicals (solvents).

Fizzy extraction is a new laboratory technique for analysis of volatile and semivolatile compounds. A carrier gas is dissolved in the liquid sample by applying overpressure and stirring the sample. The sample chamber is then decompressed. The analyte species are liberated to the gas phase due to effervescence.

Chemical analysis of volatile and semivolatile compounds dissolved in liquid samples can be challenging. The dissolved components need to be brought to ...

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) Mass Spectrometry

There are many techniques that can be employed in the characterization of synthetic homopolymers, but few provide as useful of information for end group analysis as matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). This tutorial demonstrates methods for optimization of the sample preparation, spectral acquisition, and data analysis of synthetic polymers using MALDI-TOF MS. Critical parameters during sample preparation include the selection of the matrix, identification of an appropriate cationization salt, and tuning the relative proportions of the matrix, cation, and analyte. The acquisition parameters, such as mode (linear or reflector), polarization (positive or negative), acceleration voltage, and delay time, are also important. Given some knowledge of the chemistry involved to synthesize the polymer and optimizing both the data acquisition parameters and the sample preparation conditions, spectra should be obtained with sufficient resolution and mass accuracy to enable the unambiguous determination of the end groups of most homopolymers (masses below 10,000) in addition to the repeat unit mass and the overall molecular weight distribution. Though demonstrated on a limited set of polymers, these general techniques are applicable to a much wider range of synthetic polymers for determining mass distributions, though end group determination is only possible for homopolymers with narrow dispersity.

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight MALDI-TOF Mass Spectrometry

A protocol for the matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) characterization of synthetic polymers is described including the optimization of sample preparation, spectral acquisition, and data analysis.

There are many techniques that can be employed in the characterization of synthetic homopolymers, but few provide as useful of information for end group ...

An Efficient Sample Preparation Method to Enhance Carbohydrate Ion Signals in Matrix-assisted Laser Desorption/Ionization Mass Spectrometry

Sample preparation is a critical process in mass spectrometry (MS) analysis of carbohydrates. Although matrix-assisted laser desorption/ionization (MALDI) MS is the method of choice in carbohydrate analysis, poor ion signal and data reproducibility of carbohydrate samples continue to be severe problems. For quantitative analysis of carbohydrates, an effective analytical protocol providing superior data quality is necessary. This video demonstrates sample preparation protocols to improve signal intensity and minimize data variation of carbohydrates in MALDI-MS. After drying and crystallization of sample droplets, the crystal morphology is reformed by methanol before mass spectrometric analysis. The enhancement in carbohydrate signal is examined with MALDI imaging mass spectrometry (IMS). Experimental results show that crystal reformation adjusts crystalline structures and redistributes carbohydrate analytes. In comparison with the dried droplet preparation method in conventional MALDI-MS, reforming carbohydrate crystal morphologies with methanol shows significantly better signal intensity, ion image distribution, and data stability. Since the protocols demonstrated herein do not involve changes in sample composition, they are generally applicable to various carbohydrates and matrixes.

A protocol for enhancing carbohydrate ion signals in MALDI mass spectrometry by reforming crystalline structures during sample preparation processes is demonstrated.

Sample preparation is a critical process in mass spectrometry (MS) analysis of carbohydrates. Although matrix-assisted laser desorption/ionization (MALDI) ...

Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and the more conventional PPTA were introduced. Little is known regarding the long-term stability of these fibers, but as condensation polymers, they are expected to have potential sensitivity to moisture and humidity. Therefore, characterizing the strength of the materials and understanding their vulnerability to environmental conditions is important for evaluating their use lifetime in safety applications. Ballistic resistance and other critical structural properties of these fibers are predicated on their strength. To accurately determine the strength of the individual fibers, it is necessary to disentangle them from the yarn without introducing any damage. Three aramid-based copolymer fibers were selected for the study. The fibers were washed with acetone followed by methanol to remove an organic coating that held the individual fibers in each yarn bundle together. This coating makes it difficult to separate single fibers from the yarn bundle for mechanical testing without damaging the fibers and affecting their strength. After washing, fourier transform infrared (FTIR) spectroscopy was performed on both washed and unwashed samples and the results were compared. This experiment has shown that there are no significant variations in the spectra of poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide) (PBIA-co-PPTA1) and PBIA-co-PPTA3 after washing, and only a small variation in intensity for PBIA. This indicates that the acetone and methanol rinses are not adversely affecting the fibers and causing chemical degradation. Additionally, single fiber tensile testing was performed on the washed fibers to characterize their initial tensile strength and strain to failure, and compare those to other reported values. Iterative procedural development was necessary to find a successful method for performing tensile testing on these fibers.

The primary goal of the study is to develop a protocol to prepare consistent specimens for accurate mechanical testing of high strength copolymer aramid fibers, by removing a coating and disentangling the individual fiber strands without introducing significant chemical or physical degradation.

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to ...

High Resolution Physical Characterization of Single Metallic Nanoparticles

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single nanometer-scale pore. The signal is characteristic of the molecule&#39;s physicochemical properties and its interactions with the pore. We demonstrate that the nanopore formed by the bacterial protein exotoxin Staphylococcus aureus alpha hemolysin (&#945;HL) can detect polyoxometalates (POMs, anionic metal oxygen clusters), at the single molecule limit. Moreover, multiple degradation products of 12-phosphotungstic acid POM (PTA, H3PW12O40) in solution are simultaneously measured. The single molecule sensitivity of the nanopore method allows for POMs to be characterized at significantly lower concentrations than required for nuclear magnetic resonance (NMR) spectroscopy. This technique could serve as a new tool for chemists to study the molecular properties of polyoxometalates or other metallic clusters, to better understand POM synthetic processes, and possibly improve their yield. Hypothetically, the location of a given atom, or the rotation of a fragment in the molecule, and the metal oxidation state could be investigated with this method. In addition, this new technique has the advantage of allowing the real-time monitoring of molecules in solution.&#160;

Here, we present a protocol to detect discrete metal oxygen clusters, polyoxometalates (POMs), at the single molecule limit using a biological nanopore-based electronic platform. The method provides a complementary approach to traditional analytical chemistry tools used in the study of these molecules.

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single ...

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional &#960;-conjugate Systems

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of &#946;-carotene in toluene solution. A C=C stretch band of &#946;-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of &#960;-conjugate systems from simple molecules to complex materials.

Details of signal generation and optimization, measurement, data acquisition, and data handling for a femtosecond time-resolved near-IR stimulated Raman spectrometer are described. A near infrared stimulated Raman study on the excited-state dynamics of &#946;-carotene in toluene is shown as a representative application.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near ...