Vibronic and vibrational coherence and relaxation dynamics of molecules in condensed phases

In this work, the effects of vibronic coupling competing with vibrational and/or vibronic relaxation on the femto-second pump-probe stimulated emission spectra of molecules in condensed phases are investigated theoretically. The femto-second pump-probe stimulated emission spectra are simulated as a function of temperature, excitation wavelength, and the energy gap and vibronic coupling constant between the relevant electronic states, We find that the vibronic transition depends on the excitation condition whether excitation occurs at higher or lower than the most probable transition level. Our simulation shows that even in non-equilibrium conditions a turn-over feature can be seen as a function of the energy gap between the vibronic states. We also analyze the fs pump-probe experimental data of 1,1'3,3,3'3'-hexamethylindotricarbocyanine iodide (HITCI) in solutions. We find that vibrational coherence and its relaxation is the major contribution in the early stage after excitation.