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

Vibrational relaxation in the electronic ground state initiated by intramol ecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by pic osecond time-resolved anti-Stokes Raman spectroscopy. Measurements were car ried out with B-30 dissolved in slowly as well as in rapidly relaxing solve nts. We observed a risetime of the Raman band with the highest frequency ne ar 1600 cm-1 which is close to the b-ET time tau(b-ET) of B-30. For B-30 di ssolved in propylene carbonate (tau(b-ET)similar to 1 ps), the population o f this mode exhibits a rise time of 1 ps whereas vibrational populations be tween 400 and 1400 cm-1 increase substantially slower. In contrast, in glyc erol triacetin (tau(b-ET)similar to 3.5 ps) and in ethanol (tau(b-ET)simila r to 6 ps) rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-E T is favored for modes with the highest frequencies and high Franck-Condon factors. Later on, indirect channels of population due to vibrational energ y redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical ex citation. (C) 2000 American Institute of Physics. [S0021-9606(00)50328-6].