Femtosecond Stimulated Raman Scattering from Triplet Electronic States: Experimental and Theoretical Study of Resonance Enhancements

Femtosecond stimulated Raman scattering (FSRS) is a spectroscopic technique that probes the structural dynamics of molecules. The technique typically relies on an electronic resonance condition to increase signal strength or enhance species selectivity, giving a Raman enhancement that is vibrational-mode-specific and depends on the character of the resonant electronic state. The resonance condition is complicated for molecules already in an excited electronic state and also for systems where multiple electronic states are resonant or nearly resonant with the Raman excitation energy, both of which are often the case for FSRS. This paper examines the excitation wavelength dependence of the FSRS spectrum for the lowest triplet state of 2-phenylthiophene (PT). Except for an overall increase of the signal strength due to the resonance condition, the relative intensities of most Raman bands are relatively insensitive to the excitation wavelength, and the spectrum is remarkably similar to the calculated off-resonance spectrum obtained by neglecting the resonance condition. On the other hand, calculated resonance Raman spectra using a gradient approximation to simulate the resonance condition correctly predict the excitation wavelength dependence for a few modes but overestimate the relative enhancement of others. The weak wavelength dependence of the triplet spectrum of PT contrasts the case of the singlet FSRS spectrum for the same molecule. We attribute this discrepancy to a combination of homogeneous broadening, overlapping T-N <- T-1 transitions, and possibly vibronic coupling among states in the triplet manifold. This work highlights important limitations in using standard approaches to simulate excited-state resonance Raman spectra.