S. Hogiu et al., Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer, J CHEM PHYS, 113(4), 2000, pp. 1587-1594
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].