Name: an experimental protocol for femtosecond nir/uv - xuv pump-probe experiments with free-electron lasers
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Description: Watch this Scientific Journal Video about An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers at JoVE.com

The authors thank Evgeny Savelyev, C&#233;dric Bomme, Nora Schirmel, Harald Redlin, Stefan D&#252;sterer, Erland M&#252;ller, Hauke H&#246;ppner, Sven Toleikis, Jost M&#252;ller, Marie Kristin Czwalinna, Rolf Treusch, Thomas Kierspel, Terence Mullins, Sebastian Trippel, Joss Wiese, Jochen K&#252;pper, Felix Brau&#946;e, Faruk Krecinic, Arnaud Rouz&#233;e, Piotr Rudawski, Per Johnsson, Kasra Amini, Alexandra Lauer, Michael Burt, Mark Brouard, Lauge Christensen, Jan Th&#248;gersen, Henrik Stapelfeldt, Nora Berrah, Maria M&#252;ller, Anatoli Ulmer, Simone Techert,&#160;Artem Rudenko, Daniela Rupp, and Melanie Schnell, who participated in the FLASH beamtime during which the specific data shown and discussed here were acquired and who contributed to the analysis and interpretation. The work of the scientific and technical teams at FLASH, who have made the experiment possible, is also gratefully acknowledged. D.R. acknowledges support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02-86ER13491. The experiments at FLASH were also supported by the Helmholtz Gemeinschaft through the Helmholtz Young Investigator Program. We acknowledge the Max Planck Society for funding the development and the initial operation of the CAMP end-station within the Max Planck Advanced Study Group at CFEL and for providing this equipment for CAMP@FLASH. The installation of CAMP@FLASH was partially funded by the BMBF grants 05K10KT2, 05K13KT2, 05K16KT3 and 05K10KTB from FSP-302

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The authors declare no competing interests.

Due to the complexity of the experimental setups, pump-probe experiments with free-electron lasers require a high level of expertise and experience and need very careful preparation and detailed discussions with the scientific teams that operate the free-electron laser, the optical laser, and the end-station, both before and during the experiment. While performing the actual experiment, precise determination of spatial and temporal overlap and close monitoring of all diagnostics and timing systems, as described in this protocol, are essential.
Note that most of the methods described here are only applicable for a specific photon energy range of the FEL since they rely on effects that strongly depend on the photon energy. For example, the determination of the &#34;rough&#34; temporal overlap using scattered light directed on a photodiode was found to work well for photon energies up to ~250 eV. At higher photon energies, the signal generated by the FEL pulses becomes so small that it is hard to detect. In that case, an open-ended SMA cable that can be brought very close (less than a millimeter) to or even into the FEL beam was found to produce a more reliable signal to perform the procedure described in step 3.1) of the protocol. Similarly, the best target for determining the &#34;fine&#34; timing, described in step 3.2), is strongly dependent on the photon energy. For FEL pulses in the XUV and soft X-ray region above 65.7 eV and ~57 eV photon energy (corresponding to the 4d ionization thresholds in xenon and CH3I, respectively), Xe and CH3I were found to be suitable targets for the procedure described in step 3.2. The method using CH3I was found to work for photon energies up to 2 keV (above which it has not yet been tested), while the method using Xe has been tested up to 250 eV. For photon energies below 50 eV, the bond softening process in H2 can be used19. At photon energies above 400 eV, a similar process in N2 is also suitable20. Alternative approaches involve the change in reflectivity of a solid sample25,26,30 or the formation of side bands in the photoelectron spectrum31,32.
In order to achieve the best temporal resolution, it is necessary to sort the experimental data on a shot-by-shot basis in the data analysis to compensate for the arrival time jitter between the FEL and the optical laser pulses, as described in step 5. However, the quality of the pump-probe data and, in particular, the achievable temporal resolution, strongly depends on the performance of the FEL during the experiment and on the pulse durations of the optical laser pulses and the FEL pulses that can be provided during that time. For the exemplary data shown here, the pulse duration of the UV pulses was estimated to be 150 fs (FWHM) and the FEL pulse duration was estimated to be 120 fs (FWHM). Although the total arrival time-jitter of approximately 90 fs (rms) before jitter-correction could be reduced to approximately 27 fs (rms) using the procedure described here12, the resulting improvement of the total temporal resolution of the experiment was rather small because of the relatively long pulse durations of the FEL and the optical laser. Both can, however, be reduced substantially, in which case the impact of the jitter correction scheme will be more significant. For example, a new optical laser is currently being installed at FLASH, which will have a pulse duration (in the near-infrared) below 15 fs, while new FEL operation modes are also being tested that can produce FEL pulses with pulse durations of a few femtoseconds or even below. These developments will soon enable pump-probe experiments combining FEL and optical laser pulses with an overall temporal resolution of only a few tens of femtoseconds.
While the increased availability of short and intense XUV and X-ray pulses produced by FELs has spawned a number of NIR/UV - XUV pump-probe experiments such as the one described here, similar pump-probe experiments can also be performed with high harmonic generation (HHG) sources33,34,35. The main limitation of the FEL-based experiments is typically the achievable temporal resolution, which is fundamentally limited by the synchronization between the FEL and the optical laser or by the precision with which the relative timing between the pump and the probe pulses can be measured. This is not the case for a HHG-based pump-probe experiment, where the XUV and NIR pulses are intrinsically synchronized with sub-cycle precision and which can therefore, in general, have a much higher temporal resolution. The major advantage of the FEL-based experiments, on the other hand, is the several orders of magnitude higher photon fluence, which enables experiments, e.g., on dilute targets that are not be feasible with current HHG sources, especially at higher photon energies in the soft X-ray regime. For the foreseeable future, pump-probe experiments with FELs and HHG will therefore remain complementary, with some overlap in the XUV region where both can be used for similar investigations. Some of the steps to perform these experiments are also similar, and some of the methods described here can therefore also be applied for HHG-based pump-probe experiments.

The availability of short and intense extreme ultraviolet (XUV) and X-ray pulses from free-electron lasers (FELs)1,2 has opened up new opportunities for femtosecond pump-probe experiments exploiting the site- and element-specificity of the inner-shell photo-absorption process3,4,5,6. Such experiments can be used, e.g., to investigate molecular dynamics and charge transfer processes in liquids7 and gas-phase molecules8,9,10,11,12, and for real-time observations of catalytic reactions and ultrafast surface chemistry13,14 with a temporal resolution of 100 femtoseconds or below. If the pump-probe experiment is performed by combining a synchronized optical femtosecond laser with the FEL, which was the case in all of the examples mentioned above, the intrinsic arrival-time jitter between the optical laser and the FEL pulses has to be measured on a shot-by-shot basis and corrected for in the data analysis in order to achieve the best temporal resolution possible.
Within a large collaboration, several pump-probe experiments combining optical lasers with a free-electron laser have recently been performed9,10,11,12, both at the FLASH XUV FEL15,16 and the LCLS X-ray FEL17 facilities, and an experimental protocol for performing and analyzing these experiments has been developed, which is presented in the following. The method is demonstrated for an exemplary experiment performed at the FLASH free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging11,12. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques and can also be adapted to other FEL facilities. While some of the individual steps presented here or variations thereof have already been discussed in the literature18,19,20, this protocol provides a comprehensive description of the key steps, including some that take advantage of the most recent technical improvements in the synchronization and in the timing diagnostics, which have considerably improved the stability and the temporal resolution for pump-probe experiments12,21.
The following protocol assumes a pump-probe end-station, such as the CAMP instrument at FLASH22, equipped with an ion time-of-flight, an ion momentum imaging, or a velocity map imaging (VMI) ion spectrometer; an effusive or supersonic gas jet; and a synchronized near-infrared (NIR) or ultraviolet (UV) femtosecond laser, whose pulses can be overlapped collinearly or near-collinearly with the free-electron laser beam, as sketched schematically in Figure 1. Furthermore, an appropriate suite of diagnostics tools such as a removable beam viewing screen (e.g. a paddle coated with Ce:YAG powder or a thin Ce:YAG crystal) in the interaction region, a fast photodiode sensitive to both FEL and laser pulses, and a bunch arrival-time monitor (BAM)23,24 or &#34;timing tool&#34;25,26,27 are required, all of which are usually integrated in the pump-probe end-station or are provided by the FEL facility, if requested before the experiment. Finally, the shot-by-shot jitter correction assumes that the experimental data is recorded and accessible on a shot-by-shot basis and linked to the shot-by-shot measurements of the bunch arrival-time time jitter by using a unique &#34;bunch ID&#34; or by another equivalent scheme.
At FLASH, the specific systems that are crucial for pump-probe experiments are:
<ul>
	<li>The active, all-optical feedback and stabilization system of the pump-probe laser to the master laser oscillator, which includes a balanced optical cross-correlator that stabilizes the pump-probe laser&#39;s oscillator output to the master laser oscillator, and a cross-correlator (&#34;drift correlator&#34;) to correct for slow drifts of the laser amplifier with respect to the oscillator21.</li>
	<li>The bunch arrival-time monitors (BAMs) that measure the shot-to-shot variations in the electron bunch arrival time at various positions in the accelerator with respect to the master laser oscillator23,24. They can be used for an active-feedback loop to stabilize the timing of the electron bunches with respect to the master laser oscillator, thus reducing slow drifts in the arrival time. Moreover, the BAM located closed to the experiment (BAM 4DBC3) can be used for a shot-to-shot jitter correction in the data analysis, which is in detail in step 5.1 of the experimental protocol.</li>
	<li>The pump-probe laser streak camera, which measures the relative timing between the pump-probe laser output and the dipole radiation generated by the electron bunch at the end of the accelerator before it is guided into the beam dump28.</li>
	<li>The focus camera, which images the &#34;virtual&#34; laser focus by using the part of the laser beam that is leaking through the last turning mirror behind the focusing lens in order to parasitically monitor slow spatial drifts of the optical laser.</li>
</ul>
Similar systems are available at other FEL facilities and are crucial for performing a reliable pump-probe experiment.

<table>
	<tbody>
		<tr>
			<td>Xenon</td>
			<td>Linde</td>
			<td></td>
			<td>minican</td>
		</tr>
		<tr>
			<td>CH3I (methyl iodide)</td>
			<td>Sigma Aldrich</td>
			<td>67692</td>
			<td>or other suitable sample</td>
		</tr>
		<tr>
			<td>FEL pump-probe endstation</td>
			<td>CAMP@FLASH or LAMP@LCLS</td>
			<td></td>
			<td>or a similar endstation at another FEL facility</td>
		</tr>
		<tr>
			<td>fast XUV photodiode</td>
			<td>Opto Diode Corp.</td>
			<td>AXUVHS11</td>
			<td></td>
		</tr>
		<tr>
			<td>bias T</td>
			<td>Tektronix</td>
			<td>PSPL5575A</td>
			<td></td>
		</tr>
		<tr>
			<td>fast ( &#8805;10 GHz) oscilloscope</td>
			<td>Tektronix</td>
			<td>TDS6124C</td>
			<td></td>
		</tr>
	</tbody>
</table>

an experimental protocol for femtosecond nir/uv - xuv pump-probe experiments with free-electron lasers

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques.

If the FEL and the optical laser pulses are spatially overlapped in the interaction region of the ion spectrometer, the temporal overlap, i.e., the delay value T0, at which FEL and laser pulses arrive exactly at the same time, can be found by varying the delay between FEL and NIR pulses and by analyzing the ratio of the Xe2+ to Xe3+ ion yield as a function of delay, as explained above in section 3.2.1. When the NIR pulse arrives after the FEL pulse (which needs to have a photon energy of 67.5 eV or higher), the Xe3+ ion yield is increased due to post-ionization of excited, metastable Xe2+ ion that are created during the Auger decay process following the Xe(4d) inner-shell ionization18, as shown in Figure 2. Plotting the ratio of the Xe2+ to Xe3+ ion yield as a function of delay thus yields a step function, which can be fitted to extract the exact value of T0.
A similar step function can be obtained by varying the delay between FEL and laser pulses and by analyzing the ion time-of-flight traces or ion momentum images of highly charged iodine ions, such as I3+ or I4+, created in the ionization of CH3I, as explained above in step 3.2.2). In this case, a low-energy contribution will appear as an additional peak at center of the highly charged iodine peaks in the time-of-flight spectrum or as a bright spot at the center of the corresponding momentum images, as shown in Figure 3. The low-energy ions are created when the CH3I molecules are first dissociated by the laser pulse and the ion fragment is then post-ionized by the FEL pulse9,10. This method can be used if either NIR or UV pulses are used for the pump-probe experiment, as long as the FEL photon energy is higher than 57 eV, which is the iodine 4d inner-shell ionization threshold in CH3I.
In order to correct for the jitter in the relative arrival time of the FEL pulses with respect to the laser pulses, the shot-by-shot data recorded by the bunch arrival-time monitor (BAM), shown in Figure 4, can be used to sort the recorded pump-probe data in the post-analysis, as explained above in section 5. This typically improves the temporal resolution and overall quality of the pump-probe data considerably, as shown in Figure 4 and, in more detail, in Savelyev et al. 201712.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57055/57055fig1.jpg" /> 
Figure 1: Experimental Setup. Sketch of the experimental setup for a UV-pump XUV-probe experiment on gas-phase molecules. The UV (266 nm) laser beam is produced as the third harmonic of an 800-nm Titanium:Sapphire (Ti:Sa) beam using Beta Barium Borate (BBO) crystals and compressed using a prism compressor. It is collinearly overlapped with the XUV FEL beam using a drilled mirror and focused inside a supersonic gas beam at the center of a double-sided velocity map imaging spectrometer22,29. Ion and electron momentum distributions are recorded at opposite ends of the spectrometer using a MCP/phosphor screen assembly followed by a CCD camera. <a href="https://cloudflare.jove.com/files/ftp_upload/57055/57055fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57055/57055fig2.jpg" /> 
Figure 2: Delay-dependence of the Xe ion yield. Xe ion time-of-flight spectrum (decoupled MCP signal recorded by a fast digitizer) at 83 eV photon energy and with the NIR laser pulses arriving 1 &#956;s before (top, black trace) and after (bottom, red trace) the FEL pulses. The change in the Xe2+ to Xe3+ ratio is clearly visible. <a href="https://cloudflare.jove.com/files/ftp_upload/57055/57055fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57055/57055fig3.jpg" /> 
Figure 3: Delay-dependence of the iodine ion yield and momentum. (A) Zoom-in on the I4+ peak in the ion time-of-flight spectrum of CH3I recorded at 727 eV photon energy and with the UV laser pulses arriving before (red line) and after (black line) the FEL pulses. The blue and green line, respectively, show the time-of-flight spectrum for FEL and UV laser pulse alone. This figure has been modified from Boll et al. 201610. (B) Ion momentum image of I3+ ions from CH3I recorded at 107 eV photon energy and with the UV laser pulses arriving before the FEL pulses. (C) Same as (B), but with the UV pulses arriving after the FEL pulses. The color scale in (B) and (C) shows the ion yield in arbitrary units. <a href="https://cloudflare.jove.com/files/ftp_upload/57055/57055fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57055/57055fig4.jpg" /> 
Figure 4: Relative arrival time jitter of the FEL pulses with respect to the optical laser pulses. (A) Shot-by-shot bunch arrival-time monitor (BAM) data for all FEL shots recorded during an exemplary delay scan. The reference value BAM0 was set to the mean BAM value for this scan. (B) Ion yield of low kinetic-energy I3+ ions produced in a UV-XUV pump-probe experiment on difluoroiodobenzene before correction of the shot-to-shot arrival jitter. The red line shows a least-squares fit of a cumulative distribution function (Gauss error function) to the experimental data. The fit parameter &#963; is a measure of the total temporal resolution of the pump-probe experiment. (C) Same as in (B) but with the single-shot images resorted into new delay bins using the BAM data. The error bars represent one standard deviation. Figure adapted from Savelyev et al. 201712. <a href="https://cloudflare.jove.com/files/ftp_upload/57055/57055fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers

This protocol describes the key steps for performing and analyzing pump-probe experiments combining a femtosecond optical laser with a free-electron laser in order to study ultrafast photochemical reactions in gas-phase molecules.

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a ...

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