Vibrational dynamics of large hot molecules in the collisionless gas phase

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! 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 210% ~absorption increase! compared to the t 50 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. © 2002 American Institute of Physics. @DOI: 10.1063/1.1446850# JOURNAL OF CHEMICAL PHYSICS VOLUME 116, NUMBER 9 1 MARCH 2002