Non-exponential relaxation of a single quantum vibrational excitation of alarge molecule in collision free gas phase at elevated temperature

The intramolecular vibrational dynamics of the asymmetric CO stretch of W(C O)(6) (similar to 2000 cm(-1)) have been studied in the collision free gas phase at 326 K using ps mid-infrared pump-probe spectroscopy. The pump-prob e decay is a tri-exponential with components 140 ps, 1.28 ns, and > 100 ns. The middle component is the population relaxation into the collection of l ow frequency intramolecular modes. This relaxation heats the low frequency modes causing a spectral shift that gives rise to the long component. The f ast component, which is described theoretically, is caused by the dynamics of the superposition state composed of thermally populated low frequency mo des. (C) 1999 Elsevier Science B.V. All rights reserved.