SUBPICOSECOND VIBRATIONAL-RELAXATION OF SKELETAL MODES IN POLYATOMIC-MOLECULES

Vibrational modes in the electronic ground state between 1300 and 1800 cm-1 are resonantly excited by tunable, ultrashort infrared pulses and the change of the population is monitored by a second (visible) pulse promoting the vibrationally excited molecules to a fluorescing electronic state. The fluorescence signal gives information on the lifetime of the individual vibrations. Data are presented for a solution of coumarin 6 in C2Cl4, where population lifetimes of several tenths of a picosecond for skeletal modes and of 2 ps for the C = O stretching mode are observed. The intermolecular transfer of vibrational excess energy to the solvent occurs with a time constant of 7 ps.