Researchers have proposed and employed several combinations of IR and optical spectroscopic techniques to study vibrational energy transfer in molecules.  Generally, such Raman interaction experiments are limited by time or excitation frequency resolution.  To this end, we introduce femtosecond nonlinear spectroscopies using new IR and visible pulse sequences; these include 2D IR-visible spectroscopy to study vibrational-electronic couplings and 2D IR-Raman spectroscopy to study anharmonic inter- and intramolecular vibrational couplings.  In these experiments, a mid-IR collinear pump pulse pair from a Mach-Zehnder interferometer excites a sample as in 2D Fourier transform (FT) spectroscopy; the sample is subsequently probed with a visible pulse or with a Raman pump-probe detection scheme analogous to that in femtosecond stimulated Raman spectroscopy.  During a single experiment, IR and Raman pump pulse chopping enables the isolation of various 3rd- and 5th-order signals.  From a symmetric scan of the delay between IR pump pulses, we generate two types of 2D FT spectra for an IR-probe delay:  2D IR-Raman (5th-order) spectra, which enable the measurement of cross terms between the IR dipole moment and the polarizability of a particular Raman mode; 2D IR-visible (3rd-order) spectra, which reveal IR and optical dipole moment cross terms for electronically resonant systems.  We use nonlinear response theory to calculate the expected 2D spectra from a model system that contains both an IR-active mode and a Raman-active mode and that is electronically nonresonant.  Combining 2D simulations with initial experimental data of neat solvents will help us characterize these new spectroscopies to determine their overall selection rules involving IR and Raman interactions.  The ability to probe vibronic couplings with high time and frequency resolutions should make these ultrafast spectroscopies invaluable to researchers studying correlated electronic and nuclear motions in chemistry, biology, and material science.

2D Fourier transform infrared-Raman and infrared-visible spectroscopies: Experiment and theory

248th National Meeting (2014)

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