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

The intramolecular vibrational dynamics of the asymmetric CO stretch of W(CO)6 (∼2000 cm−1) have been studied in the collision free gas phase at 326 K using ps mid-infrared pump–probe spectroscopy. The pump–probe 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 low frequency intramolecular modes. This relaxation heats the low frequency modes causing a spectral shift that gives rise to the long component. The fast component, which is described theoretically, is caused by the dynamics of the superposition state composed of thermally populated low frequency modes.