Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer

Vibrational relaxation in the electronic ground state initiated by intramolecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. Measurements were carried out with B-30 dissolved in slowly as well as in rapidly relaxing solvents. We observed a risetime of the Raman band with the highest frequency near 1600 cm⁻¹ which is close to the b-ET time ${\tau }_{\mathrm{b-ET}}$ of B-30. For B-30 dissolved in propylene carbonate ${\mathrm{(\tau }}_{\mathrm{b-ET}}\text{∼1\hspace{0.05em}}\mathrm{ps}\mathrm{),}$ the population of this mode exhibits a rise time of 1 ps whereas vibrational populations between 400 and 1400 cm⁻¹ increase substantially slower. In contrast, in glycerol triacetin ${\mathrm{(\tau }}_{\mathrm{b-ET}}\text{∼3.5\hspace{0.05em}}\mathrm{ps})$ and in ethanol ${\mathrm{(\tau }}_{\mathrm{b-ET}}\text{∼6\hspace{0.05em}}\mathrm{ps})$ rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-ET is favored for modes with the highest frequencies and high Franck–Condon factors. Later on, indirect channels of population due to vibrational energy redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical excitation.