Solvent-dependent vibrational relaxation pathways after successive resonant IR excitation to υ = 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 υ = 2 to υ = 0 occurs predominantly via the υ = 1 state, with time constants of T12→1 = 10 ± 5 and T11→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 (T12→0 = 80 ± 20 ps), comparable to the energy lifetime of the first excited state (T11→0 = 125 ± 5 ps).