Conclusions

In summary, we have conducted six-wave mixing experiments with I3- using deep UV

laser pulses generated in a two-color filament. The experiments demonstrate that high-quality signals are readily obtained with the data acquisition rate and sensitivity afforded by combining a background-free laser beam geometry with spectral interferometry. Signals were examined in two different representations. One representation simply carried out a 2D Fourier transform with respect to the two experimentally controlled delay times using the wavelength-integrated signal field. The pattern of resonances in the 2D spectrum is well-described with response functions based on a cumulant expansion. The second representation involves three dimensions: the first experimentally controlled delay time, ₁; the Fourier transform of the second delay time, ₂; the detection frequency, _t. Correlation spectra associated with ₂ and _t reveal that both the detection frequency and signal magnitude oscillate in ₁ due to wavepacket motions in the symmetric stretching coordinate. These oscillations encode information about anharmonic

motions in the ground state (which are weak in I3-), and the dynamic energy gap between

electronic states. It will be interesting to examine systems with multiple Franck-Condon active modes using this technique. Pulses with broader bandwidths may also enhance sensitivity by increasing the oscillation amplitude in _t .

The analysis presented in Section 4.5 suggests that cascaded third-order nonlinearities, which significantly challenged fifth-order off-resonant Raman spectroscopies,24,25,54-57 do not dominate the fifth-order resonance Raman response of I3-. This interpretation of the response is

supported by four aspects of the measured signals: the line shapes, the signal phase, concentration dependence, and the relative magnitudes of third and fifth-order signals.

Consistency between these fairly unrelated aspects of the signals constitutes strong evidence that the direct fifth-order signal field is much larger than the cascaded signal field. The model calculations presented in Section 4.5.2 suggest that the cascaded signal contribution to the peak at ₁=₂=112 cm-1 is more than three orders of magnitude smaller than that associated with the direct fifth-order response. It should be emphasized that this analysis pertains only to the signal component that oscillates at the fundamental mode frequency in both dimensions. In other words, the mode displacement controls how the cascaded signal intensity is distributed between (vibrationally) coherent and incoherent signal components (see Section 4.5.6). For this reason, it is presently unclear if this method can be successfully applied to larger molecules, which

generally possess smaller mode displacements (i.e. I3- has an extraordinarily large mode

displacement). Alternate laser beam geometries may be considered for such cases.24

Applications of 2D electronic spectroscopy in the deep UV are challenged by dispersion management and the large off-resonant response of the sample medium near time-zero.21,22,59-

63,66-68

The present work shows that multi-dimensional resonance Raman experiments are largely immune to these technical limitations. Most importantly, vibrational dephasing is much slower than electronic dephasing, so the coherence spike associated with the region of laser pulse overlap is not as problematic. Regarding dispersion management, low-frequency (<1000 cm-1) vibrational resonances are readily probed with manageable laser bandwidths. For example, the (functionally important) modes localized to disulphide bridges in proteins are found below 500 cm-1 and may be excellent targets for six-wave mixing experiments that employ 200-nm laser pulses.102 Higher-frequency vibrational resonances (>1000 cm-1) may be probed without extraordinary resources by incorporating pairs of laser pulses with spectra shifted by amounts equal to the mode frequencies of interest. Such a two-color approach would circumvent the need for sub-20-fs pulses.

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CHAPTER 5: ELUCIDATION OF REACTIVE WAVEPACKETS BY TWO-