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>

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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>&lt;strong&gt;Mass spectrometer components&lt;/strong&gt;</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>&lt;strong&gt;Vacuum system components&lt;/strong&gt;</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>&lt;strong&gt;Femtosecond laser system&lt;/strong&gt;</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>&lt;strong&gt;Motion control&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Motorized linear translation stage 1&quot; 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&quot; travel</td><td>Thorlabs</td><td>PT1</td><td></td></tr><tr><td>&lt;strong&gt;Detectors&lt;/strong&gt;</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&amp;nbsp;</td><td>LeCroy</td><td>WaveRunner 610Zi</td><td></td></tr><tr><td>&lt;strong&gt;Optics&lt;/strong&gt;</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&quot; diameter</td><td>Thorlabs</td><td>DMLP950L</td><td></td></tr><tr><td>&lt;strong&gt;Software&lt;/strong&gt;</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

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet (VUV) Synchrotron Radiation

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics1-4. Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water1, 5-9.
We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline10 located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds1. Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations11, 12. By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species 1, 3.

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation

A molecular beam coupled to tunable vacuum ultraviolet photoionization mass spectrometer at a synchrotron provides a convenient tool to explore the electronic structure of isolated gas phase molecules and clusters. Proton transfer mechanisms in DNA base dimers were elucidated with this technique.

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy ...

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Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Proton Transfer and Protein Conformation Dynamics in Photosensitive Proteins by Time-resolved Step-scan Fourier-transform Infrared Spectroscopy

Monitoring the dynamics of protonation and protein backbone conformation changes during the function of a protein is an essential step towards understanding its mechanism. Protonation and conformational changes affect the vibration pattern of amino acid side chains and of the peptide bond, respectively, both of which can be probed by infrared (IR) difference spectroscopy. For proteins whose function can be repetitively and reproducibly triggered by light, it is possible to obtain infrared difference spectra with (sub)microsecond resolution over a broad spectral range using the step-scan Fourier transform infrared technique. With ~102-103 repetitions of the photoreaction, the minimum number to complete a scan at reasonable spectral resolution and bandwidth, the noise level in the absorption difference spectra can be as low as ~10-4, sufficient to follow the kinetics of protonation changes from a single amino acid. Lower noise levels can be accomplished by more data averaging and/or mathematical processing. The amount of protein required for optimal results is between 5-100 &#181;g, depending on the sampling technique used. Regarding additional requirements, the protein needs to be first concentrated in a low ionic strength buffer and then dried to form a film. The protein film is hydrated prior to the experiment, either with little droplets of water or under controlled atmospheric humidity. The attained hydration level (g of water / g of protein) is gauged from an IR absorption spectrum. To showcase the technique, we studied the photocycle of the light-driven proton-pump bacteriorhodopsin in its native purple membrane environment, and of the light-gated ion channel channelrhodopsin-2 solubilized in detergent.

Key steps of protein function, in particular backbone conformational changes and proton transfer reactions, often take place in the microsecond to millisecond time scale. These dynamical processes can be studied by time-resolved step-scan Fourier-transform infrared spectroscopy, in particular for proteins whose function is triggered by light.

Monitoring the dynamics of protonation and protein backbone conformation changes during the function of a protein is an essential step towards ...

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 ...

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.

For small chiral species, Coulomb Explosion Imaging provides a new approach to determine the handedness of individual molecules.

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish ...

Measurements of Long-range Electronic Correlations During Femtosecond Diffraction Experiments Performed on Nanocrystals of Buckminsterfullerene

The precise details of the interaction of intense X-ray pulses with matter are a topic of intense interest to researchers attempting to interpret the results of femtosecond X-ray free electron laser (XFEL) experiments. An increasing number of experimental observations have shown that although nuclear motion can be negligible, given a short enough incident pulse duration, electronic motion cannot be ignored. The current and widely accepted models assume that although electrons undergo dynamics driven by interaction with the pulse, their motion could largely be considered &#39;random&#39;. This would then allow the supposedly incoherent contribution from the electronic motion to be treated as a continuous background signal and thus ignored. The original aim of our experiment was to precisely measure the change in intensity of individual Bragg peaks, due to X-ray induced electronic damage in a model system, crystalline C60. Contrary to this expectation, we observed that at the highest X-ray intensities, the electron dynamics in C60 were in fact highly correlated, and over sufficiently long distances that the positions of the Bragg reflections are significantly altered. This paper describes in detail the methods and protocols used for these experiments, which were conducted both at the Linac Coherent Light Source (LCLS) and the Australian Synchrotron (AS) as well as the crystallographic approaches used to analyse the data.

We describe an experiment designed to probe the electronic damage induced in nanocrystals of Buckminsterfullerene (C60) by intense, femtosecond pulses of X-rays. The experiment found that, surprisingly, rather than being stochastic, the X-ray induced electron dynamics in C60 are highly correlated, extending over hundreds of unit cells within the crystals1.

The precise details of the interaction of intense X-ray pulses with matter are a topic of intense interest to researchers attempting to interpret the ...

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.

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 ...

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

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 ...

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.

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

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