Vibrational dynamics of excited states probed by fs/ps CARS: Simulations and applications to ultrafast charge transfer dynamics

The subpicosecond time evolution of a push-pull stilbene, 4-dimethylamino-4'-nitrostilbene (DANS) following 400 nm excitation in acetonitrile is observed with vibrational resolution using the recently-developed fs/ps CARS probe technique. Raman-active modes in the fingerprint region of the vibrational spectrum are observed to evolve, both in intensity and in frequency. The evolution of the DANS vibrational spectrum in the nitro symmetric stretch region is probed, exhibiting a frequency upshift on a 300 fs timescale. These findings agree well with timescales observed in transient absorption/gain experiments and support the formation of an intramolecular charge transfer (ICT) state on a subpicosecond timescale in acetonitrile that precedes the fast (∼12 ps) internal conversion timescale observed in prior studies. Simulations of fs/ps CARS technique help in understanding the experimental transition from time resolved CARS to fs/ps CARS. Simulations indicate an increase in frequency resolution of modes as well as direct measurement of the vibrational modes.