Femtosecond stimulated Raman spectroscopy is extended to probe ground state anti-Stokes vibrational features. Off resonance, negative anti-Stokes features are seen that are the mirror image of the positive Stokes side spectra. On resonance, the observed dispersive lineshapes are dramatically dependent on the frequencies of the picosecond pump and femtosecond probe pulses used to generate the stimulated Raman spectra. These observations are explained by the contributions of the inverse Raman and hot luminescence four-wave mixing processes discussed by Sun [J. Chem. Phys. 128, 144114 (2008)], which contribute to the overall femtosecond stimulated Raman signal.
Bibliographical noteFunding Information:
We thank Jocelyn Cox and Fleming Crim for sharing their unpublished FSRS results and Soo-Ying Lee for many valuable discussions. This work was supported by the Mathies Royalty Fund.
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