Time-resolved spontaneous Raman spectroscopy of infrared-multiphoton- excited SF6

Spontaneous Raman spectroscopy is used as a tool for studying the vibrational energy distribution of collisionless infrared-multiphoton-excited SF6. A collisionless increase in Stokes and anti-Stokes signals from the strong Raman-active1 mode is observed after infrared-multiphoton excitation by a high-power 500-ps CO2-laser pulse tuned to the infrared active 3 mode. Results are presented over a pressure range from 13 Pa (100 mTorr) to 270 Pa (2 Torr). The pressure dependence clearly proves that the increase does not depend on collisions. The effects are studied as a function of time and of the infrared energy fluence, infrared wavelength, and infrared pulse duration. The experimental data show that an intramolecular equilibrium of vibrational energy is established within the 20-ns time resolution of the experimental setup. The multiphoton excitation shows a red shift and intensity broadening. A comparison with results from photoacoustic measurements is made.