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

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’s instructions or user’s manual. Laser safety goggles designed for the operating laser intensities and wavelengths should be worn.

1. Construction of TOF-MS²⁶

1. 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 ¾” flanges on either side of the ion optics (Figure 1).
2. 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.
3. Insert a 1/16” stainless steel tube into the chamber between the extractor and repeller plates, thread the tube out of the chamber, and connect it to ¼” stainless steel tubing²⁷. Attach one or more variable leak valves to the ¼” stainless steel tubing.
   Note: Glass tubes containing molecular samples or gas tanks may be attached to this tubing for sample inlet.
4. Attach an 18-mm microchannel plate stack in Z-stack configuration²⁹ on the end of the flight tube.
5. Attach two optical windows (1 mm thickness, 50 mm diameter, fused silica) mounted on 2 ¾” flanges to the chamber.
   Note: The laser beams propagate through these two windows through the space between the repeller and extractor plates.
6. Wire the ion optics and the detector to high-voltage power supplies via current feedthroughs and BNC cables.
7. 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.
8. 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.

2. Construction of Optical Pump and Probe Paths

Note: A diagram of the pump and the probe optical paths is given in Figure 1.

1. 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’s manual.
   1. Turn the laser on and wait for about 30 min for it to stabilize.
   2. 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.
   3. Direct the reflected beam into the optical parametric amplifier (OPA) and optimize the output power using the procedures in the manual.
2. Preparation of the pump optical path
   1. Set the OPA software to select the desired wavelength.
   2. Direct the output beam from the OPA through the λ/2 wave plate (WP) and polarizer (P).
   3. 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.
   4. Direct the expanded beam to the dichroic mirror (DC).
3. Preparation of the probe optical path
   1. 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.
   2. 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.
   3. 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).
4. 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 elsewhere^30,31,32. In our experiments, pulse durations out of the OPA are typically around 20 fs and that of the 800 nm pulse around 30 fs^19,20,27. However, OPAs can introduce higher-order pulse distortions, so it may be necessary to implement pulse-compression using, for instance, chirped mirrors^10,11,12,13 or an acousto-optic modulator¹⁶.
   1. 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.
   2. Ensure that the two beam replicates in the FROG overlap in the β-barium-borate (BBO) crystal. Adjust the beam alignment and delay stage until a third beam is visible between the two original beams.
   3. 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.
   4. Collect the FROG scan and retrieve the pulse shape with the appropriate software and retrieval algorithm.
   5. Repeat steps 2.4.1-2.4.3 for the other beam. Remove the mirrors directing beams to the FROG.
5. 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.
   1. 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.
   2. 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.
6. 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.
   1. 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.
   2. 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.
   3. Remove the photodiode detector.

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.

1. Calibration of absolute peak intensity of the pump pulse²⁸
   1. 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.
   2. Adjust the wave plate (WP) rotation angle (Figure 1) to maximize the power of the pump beam measured before the lens.
   3. 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.
   4. 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.
   5. 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.
   6. Check for ion signals of Xe⁺ (and higher charge states) in the oscilloscope directly or via a computer connected to the oscilloscope.
   7. 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.
   8. Record the Xe mass spectrum using the data acquisition software.
   9. Decrease the laser power by rotating the WP angle to obtain a power ~20 mW lower than the previously measured power.
   10. 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.
   11. 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.
2. 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.
   1. Block the pump beam and direct the probe beam along a straight path after the dichroic mirror using two flat mirrors.
   2. 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.
   3. Minimize the probe beam energy using the variable ND filter and add additional ND filters to attenuate the pulse energy below ~100 nJ.
   4. 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.
   5. Adjust the position of the translation stage to obtain the smallest, most intense laser spot. This location corresponds to the focus of the beam.
   6. 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.
   7. Remove the mirrors directing the probe beam to the camera and return the focusing lens to its position in front of the TOF-MS.
3. 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.
   1. Connect the desired sample to the TOF-MS chamber and adjust the pressure to the range of 1-5 x 10^-7 torr.
   2. Unblock the pump and probe beams and ensure that they are aligned into the TOF-MS chamber.
   3. 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.
   4. 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).
   5. 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.
   6. 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.
4. Cross-correlation^6,34
   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.
   1. 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).
   2. 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 µm over a range of 120 µm 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.

4. Pump-probe Measurements

1. Preliminary checks before taking measurements
   1. Check the experimental setup to confirm that both beams are propagating collinearly through the window of the chamber (Figure 1).
   2. 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.
   3. 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.
   4. 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.
   5. 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.
2. Data acquisition
   1. Adjust the pump and probe pulse energies to obtain desired ion signals.
   2. 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 fs^19,20.
   3. 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.
3. Data processing
   1. 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.
   2. 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.