Vibrational dynamics of large hot molecules in the collisionless gas phase

Abstract

Infrared ps pump-probe experiments are presented for the P, Q, and R rotational branches of the asymmetric CO stretching mode of tungsten hexacarbonyl (1997 cm−1) in the collisionless gas phase. The pump-probe decays are tri-exponentials (140 ps, 1.3 ns, and >100 ns) in contrast to single exponential decays observed in supercritical fluids and liquid solvents. The 1.3 ns decay component is the vibrational energy relaxation (VER) time. The long component occurs following intramolecular VER into a distribution of low-frequency modes. After VER is complete, the R signal is 48%, the Q signal is 29%, and the P signal is −10% (absorption increase) compared to the t=0 signal. These long-lived signals result from an increase in the occupation numbers of low-frequency modes (internal heating) that causes a shift of the vibrational spectrum. The fastest decay is produced by spectral diffusion. The spectrally narrow pump pulse burns a hole in the inhomogeneous ground state spectrum and generates a narrow spectral population in the excited state (excited state peak). The inhomogeneity arises from the distribution of occupation numbers of the low-frequency modes that produces different combination band spectral shifts. Spectral diffusion is caused by the time evolution of the complex low-frequency thermal vibrational wave packet. Two possible models for the spectral diffusion are evaluated, one in which spectral diffusion reduces the pump-probe signal by hole filling and broadening of the excited state peak, and one in which the spectral diffusion only broadens the excited state peak.