Slow intermolecular redistribution of vibrational energy in SF6 gas excited by a tea CO2 laser

Infra-red fluorescence (IRF) spectra of SF₆ excited by the 944.2 cm^-1 line of a pulsed CO₂ laser were observed at various times after the time of the laser excitation. Each spectrum showed a strong IRF peak of the v₃ mode which was red shifted relative to the room temperature fundamental (948 cm^-1) by an amount which depended, apart from the level of excitation, on the different times employed. For a strong excitation with 〈n〉 ≈ 11 photons absorbed per molecule, a significant decrease of red shift versus time was observed, indicating mainly excitation losses by IRF emission. For weak excitation with 〈n〉 ≈ 1.4, almost an constant red shift versus time is observed. This result, and the previous finding that at weak excitation a nonthermal energy distribution in the ensemble of molecules exists, leads to the conclusion that intermolecular redistribution of vibrational energy in SF₆ is slow, and does not exceed the observed fluorescence duration (～1 ms).