Solvent-dependent vibrational relaxation pathways after successive resonant IR excitation to v=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 v = 2 to v = 0 occurs predominantly via the v = 1 state, with time constants of T₁^2Æ1 = 10±5 and T₁^1Æ0 = 60±5 ps, respectively. In contrast, in a less polar solvent (chloroform) direct decay to the ground state is observed, with a time constant (T₁^2Æ0 = 80±20 ps), comparable to the energy lifetime of the first excited state (T₁^1Æ0 = 125±5 ps).