Solvent-dependent vibrational relaxation pathways after successive resonant IR excitation to upsilon=2

With two subsequent resonant intense picosecond infrared pulses, we have succeeded in pumping a significant fraction of iodoform molecules in solution to the second vibrationally excited state of the C-H stretching mode. Transient populations of the vibrational levels are monitored with weak probe pulses. From these pump-pump-probe experiments, we find that the subsequent relaxation route depends critically on the solvent. In a strongly polar solvent (acetone) relaxation from nu = 2 to nu = 0 occurs predominantly via the nu = 1 state, with time constants of T-1(2-->1) = 10+/-5 and T-1(1-->0) = 60+/-5 ps, respectively. In contrast, in a less polar solvent (chloroform) direct decay to the ground state is observed, with a time constant (T-1(2-->0) = 80+/-20 ps), comparable to the energy lifetime of the first excited state (T-1(1-->0) = 125+/-5 ps).