Name: measurement of ultrafast vibrational coherences in polyatomic radical cations with strong-field adiabatic ionization
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Description: Watch this Scientific Journal Video about Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization at JoVE.com

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 accommoda...

<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>

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 ...

This method can help answer key questions about the ultra fast dynamics of polyatomic radical cations such as whether coherent vibrational motions are excited and how coherent exotation drives bond association. The main advantage of this technique is that polyatomic radical cations may be prepared in the ground electronic state, which significantly improves the ability to resolve coherent vibrational motions. Generally, individuals new to this method would struggle with setting up two independent beam path with identical path name in ensuring that the two beams propagate exactly parallelly or collinearly when necessary.First, turn the femtosecond laser on and wait for about 30 minutes for it to stabilize. Position a 90/10 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 before the beam splitter and the power of the transmitted beam to ensure that approximately 10%of the power is transmitted.Direct the reflected beam into the optical parametric amplifier. Then, optimize the output power using the procedures in the manual. Next, set the optical parametric amplifier software to select the desired wavelength.Direct the output beam from the optical parametric amplifier through the half wave plate and polarizer. Block the P polarized beam and direct the S polarized beam to the concave and convex mirrors to expand its diameter by a factor of five. Then, direct the expanded beam to the dichroic mirror.To prepare the probe optical path, direct the beam that passes through the 90/10 beam splitter to the convex and concave mirrors to reduce its diameter by a factor of two. Direct the down collimated beam to a hollow retroreflector 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 retroreflector does not change when the stage is moved along its full travel range.Next, insert a tunable neutral density filter after the delay stage to attenuate the power of the probe pulse, insert an iris after the neutral density filter to adjust the beam diameter, and direct the beam to the dichroic mirror. Now, insert a 15 millimeter diameter Beta-Barium Borate crystal after the dichroic mirror to double the wavelengths of both beams, thereby making them visible. Adjust the pump end probe beam alignments using the mirror mounts prior to the dichroic mirror such that the beams propagate collinearly through the TOFMS chamber and out from the other side.Following this, place a fast photo dio detector a few centimeters in front of the window entrance to the TOFMS chamber in the path of the pump end probe beams. Attach the detector cable to a digital oscilloscope and independently locate the signals of the pump end probe pulses. Adjust the position of the motorized delay stage on the probe line such that the pump end probe signals in the oscilloscope are temporally overlapped.If one signal is consistently in front of the other in the oscilloscope, move the mounts holding the motorized delay stage to shorten or lengthen the path length as needed. Then, remove the photodiode detector. Connect the desired sample to the TOFMS chamber and adjust the pressure.Turn on the TOFMS power supply and check the voltages. Verify the operability of the data acquisition software with respect to the communication with both the motorized delay stage and the oscilloscope. Unblock the pump end probe beams and ensure that they are aligned into the TOFMS chamber.Maximize the probe power by adjusting the neutral density filter. Set the pump power with the wave plate to a sufficiently high level to obtain satisfactory ion signal. Next, adjust the spacial position of the probe beam with the knobs on the dichroic mirror mount until either a spike in intensity of all ions is observed, or a significant depletion of the parent molecular ion and, or increase in fragment ion yields are observed.Adjust the motorized delay stage position to produce a spike in total ion signal, which corresponds to zero time delay. Adjust the pump end probe pulse energies to obtain desired ion signals. In the data acquisition software, specify the scan length and step size.Finally, run the data acquisition software to acquire the mass spectrum at each pump-probe delay. DMMP mass spectra taken at zero time delay and with only the pump pulse are shown here. There is little perceptible difference between the poorly overlapped and pump only spectra, which illustrates how to determine optimal spacial overlap of the pump end probe beams using the ion signals directly.Mass spectral data obtained from one pump-probe scan with the flight time on the abscissa and pump-probe delay on the ordinate are displayed here. The raw data illustrates how changes in ion signals with the pump-probe delay in these experiments can be visualized without additional data workup. Time resolved DMMP ion signals from one pump-probe scan with optimized and poor spacial overlap of pump end probe beams are shown here.These results illustrate the importance of optimizing the pump-probe spacial overlap to acquire high quality transient ion signals in the processed data. DMMP ion and the fragment PO2C2H4 ion transient ion signals taken using 800 nanometer and 1500 nanometer pump wave lengths are displayed here. The fast foray transform of the DMMP ion signals taken with 800 nanometer and 1500 nanometer pumps is shown here.The peak at 750 wave numbers visible for the 1500 nanometer pump illustrates the frequency resolution under the scan settings used. While attempting this procedure, it's important to remember to double check the spacial develop of the pump and the probe beams before recording any experimental data. Following this procedure, ab initio or density functional theory calculations can be performed in order to determine which vibrational modes are coherently excited, as well as identify the electronic energy level or levels accessed by the probe-pulse exotation.Don't forget that working with high powered lasers can be extremely hazardous and precautions such as wearing appropriate safety goggles should always be taken while performing this procedure.

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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, ...

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.

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

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 ...

Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active metal-oxide surfaces in contact with light-absorbing dyes is an ongoing thorny and elusive challenge in physical chemistry. With this in mind, a molecular-dynamics (MD) simulation were performed by using optimized empirical potentials for a well-representative and prototypical dye-sensitized solar cell (DSC) solvated by a widely-studied room temperature ionic liquid (RTIL), in the guise of a [bmim]+[NTf2]- RTIL solvating an N719-sensitizing dye adsorbed onto 101 anatase-titania. In doing so, important insights were gleaned into how using a RTIL as the electrolytic hole acceptor modulates the dynamical and vibrational properties of a N719 dye, estimating the spectra for the DSC photo-active interface via Fourier transformation of mass-weighted velocity autocorrelation functions from MD. The acquired vibrational spectra were compared with the experiment spectra and those sampled from ab initio molecular dynamics (AIMD); in particular, various empirical-potential spectra generated from MD provide insight into how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction. In any event, careful fitting of empirical force-field models has been shown to be an effective tool in handling DSC vibrational properties, when validated by AIMD and an experiment.

A dye-sensitized solar cell was solvated by RTILs; using optimized empirical potentials, a molecular dynamics simulation was applied to compute vibrational properties. The obtained vibrational spectra were compared with experiment and ab initio molecular dynamics; various empirical potential spectra show how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction.