The pump-probe signals shown in Figure 3.6 illustrate several aspects of the stimulated Raman response of triiodide. Vibrational coherences are observed throughout the visible spectral range as in earlier work on this system.16-23 The modulation depth of the oscillations is greatest in the range, 18000-21000 cm−1 (475-555 nm). Coherences at detection wavenumbers below 20000 cm−1 (500 nm) are dominated by diiodide, whereas signals acquired at detection wavenumbers above 20000 cm−1 possess significant contributions from ground state wavepacket motions of triiodide. The vibrational coherences detected below 20 000 cm−1 are similarly assigned to the ground electronic state of diiodide, because the excited state potential energy surface is
Figure 3.6. (a) Transient absorption signals (in mOD) obtained for triiodide with a 400-nm
pump pulse and continuum probe pulse. (b) The coherent component of the signal is isolated by subtracting sums of 2 exponentials from the total signal presented in panel (a). (c) Fourier transformation of the signal between delay times of 0.1 and 2.5 ps shows that the vibrational frequency decreases as the detection wavenumber decreases. Dispersion in the vibrational frequency reflects sensitivity to high-energy quantum states in the anharmonic potential of diiodide.19
The photodissociation process is known to cause the period of vibrational motion to evolve as the delay increases.22 In Figure 3.6(b), such “chirped” wavepacket dynamics are
evidenced by time evolution in the orientations of the nodal contour lines of the signal. It has been established that the strength of the chemical bond weakens as the symmetry of triiodide breaks immediately following light absorption.16-23 The reactive wavepacket departs from the Franck–Condon geometry as one of the bonds ruptures, thereby giving rise to the time
dependence of the vibrational frequency observed in Figure 3.6(b). The dependence of the vibrational period on the detection wavenumber reflects sensitivity to highly excited states in the anharmonic potential of the diiodide product. Kühne and Vöhringer determined that experiments with visible probe pulses are sensitive to states with 10-30 vibrational quanta.19
3.4.2. 2DRR Response of The Diiodide Photoproduct
The 2DRR response of the diiodide photoproduct is detected using the two-color approach described in Figure 3.4. The signals shown in Figure 3.7 are Fourier transformed to
reveal peaks in the upper right and lower left quadrants of the 2DRR spectrum. The resonances appear near 100 cm−1 in both dimensions, which indicates that the experiment is sensitive to states of diiodide that possess roughly 20 vibrational quanta.19 Vibrational resonances are not detected in the other two quadrants of the 2DRR spectrum as in our earlier study of ground state wavepacket motions in triiodide.26 The locations of the peaks in the experimental 2DRR
spectrum agree with the prediction made for terms 5-8 in Figure 3.3 (i.e., the terms this pulse sequence is designed to detect).
Figure 3.7. 2DRR signals associated with terms 5-8 are obtained using the two-color approach
described in Figure 3.4. (a) The total signal possesses both coherent and incoherent components. (b) The coherent (Raman) component of the signal is isolated by subtracting sums of two
exponentials from the total signal presented in panel (a). (c) The two-dimensional Fourier transformation of the signal in panel (b) reveals resonances in the upper right and lower left quadrants. This pattern of 2DRR resonances is consistent with calculations based on terms 5-8 (see Figure 3.3), which this experiment is designed to detect.
The data shown in Figure 3.7(b) indicate that the vibrational dephasing rate is slightly faster in 𝜏1 than it is in 𝜏2 (i.e., the line width is slightly larger in ω1 than it is in ω2). We attribute
this difference in line widths to intramolecular relaxation and inertial solvation processes that occur following photodissociation in 𝜏1. The photoproduct is likely far from equilibrium when the vibration fully damps near 2 ps; however, a significant amount of solute-to-solvent
carried by each dimension of the 2DRR signals consistent with interpretations of related optical pump/Raman probe experiments.46-48, 52-54 That is, the relaxation processes detected in the first delay time, 𝜏1, are related to those investigated with traditional pump-probe experiments (e.g., vibrational cooling).16-2316–23 Scanning the second delay time, 𝜏2, essentially yields a snapshot of the vibrational spectrum as the system relaxes in 𝜏1.18
3.4.3. 2DRR Cross Peaks Between Triiodide and Diiodide
The 2DRR spectra theoretically predicted in Figure 3.3 are consistent with the experimental measurements shown in Figure 3.7 (terms 5-8) and our earlier all-UV 2DRR spectra (terms 1-4).26 In both cases, peaks are found in only the upper right and lower left
quadrants because of interferences between numerous terms in the response function. In this section, we test the prediction that signal components corresponding to terms 9-12 will give rise to resonances in all four quadrants of the 2DRR spectrum (see Figure 3.3(c)). This unique pattern of resonances signifies a process in which a vibrational wavepacket transitions between reactant and product states without loss of coherence.
The 2DRR data presented in Figure 3.8 are obtained using the experimental setup
described in Figure 3.5. The pump-repump-probe signals exhibit oscillations in both dimensions, which may be Fourier transformed to produce 2DRR spectra. Signals acquired at several
detection wavenumbers are displayed to illustrate a transition between regimes in which motions of triiodide or diiodide dominate the second dimension, ω2. At a detection wavenumber of 22500
cm-1 (444 nm), where absorption of triiodide is dominant, the pattern of resonances is consistent with terms 1-4 (see Figure 3.3(a)). However, intensities of the vibrational resonances in the upper left and lower right quadrants of the 2DRR spectrum increase as the detection
less than 19500 cm-1 (513 nm), we observe peaks with equal intensities in all four quadrants, which is consistent with the prediction made for terms 9-12 in Figure 3.3(c).
Figure 3.8. 2DRR data are obtained using the two-color approach described in Figure 3.5. Each
column corresponds to a different detection wavenumber: 22500 cm-1 (444 nm) in column 1; 21000 cm-1 (476 nm) in column 2; 19500 cm-1 (513 nm) in column 3; 18000 cm-1 (555 nm) in column 4. (a)-(d) Total pump-repump-probe signal in mOD. (e)-(h) Coherent parts of the pump- repump-probe signals displayed in the first row. (i)-(l) 2DRR spectra are generated by Fourier transforming the signals shown in the second row in delay ranges, 𝜏1 and 𝜏2, between 0.15 and 2.0 ps. The data show that peaks in the upper left and lower right quadrants emerge as the detection wavenumber becomes off-resonant with triiodide. Signals acquired at detection wavenumbers above 21,000 cm-1 (476 nm) are dominated by stimulated Raman processes in the ground electronic state of triiodide (terms 1-4). In contrast, signals acquired at detection
The detection of peaks with equal intensities in all four quadrants is consistent with nonlinearities of the type shown in terms 9-12. The peak positions are also consistent with this assignment. The 112-cm-1 vibrational resonance in is notably independent of the detection wavenumber (as it should be for the reactant). In contrast, the frequency of the vibrational resonance in decreases as the detection wavenumber decreases. For example, we observe resonances in at 110 cm-1 and 100 cm-1 for detection wavenumbers of 22500 cm-1 (444 nm)
and 18000 cm-1 (555 nm), respectively. As discussed in Section 3.4.1, correlation between the vibrational frequency and detection wavenumber is a signature that diiodide contributes to the signal (i.e., the origin of the response transitions from terms 1-4 to terms 9-12 as the detection frequency decreases).51 At present, 2DRR spectra cannot be measured at detection wavenumbers below 18,000 cm-1 (555 nm) in this setup, because of the substantial background that must be removed by chopping the pump and repump laser beams. Nonetheless, the transition between the two aforementioned regimes (i.e., terms 1-4 versus terms 9-12) is made sufficiently clear in the range of detection frequencies where adequate signal strength is obtained.
Cascades of four-wave mixing signal fields challenge the application of 2D Raman spectroscopy under off-resonant conditions. Cascades were ruled out in our previous all-UV 2DRR study of triiodide using control experiments based on the signal phase, concentration dependence of the signal intensity, and the relative phases of the vibrations detected in four and six-wave mixing signals.26 The direct 2DRR response should be even more dominant in the present study, because lower-frequency laser beams are employed. Moreover, the direct response is favored in the present experiments for the same reasons discussed at length in Reference 26. In the Supplementary Material, we demonstrate that the sign of the 2DRR response is consistent with the direct fifth-order nonlinearity rather than a cascade.38