IR absorption of SF6 excited up to the dissociation threshold

Abstract

SF₆ was pumped by a P20 TEA CO₂ laser up to an average energy of 10 000 cm⁻¹ in the absence of collisions and up to 20 000 cm⁻¹ with a certain collisional relaxation. A second CO₂ laser, very much attenuated, was used to determine absorption cross sections for frequencies from 915 to 985 cm⁻¹. No coherent effects and no collisionless relaxations were found, in contrast to measurements with a cw laser as a probe. The ν₃ band continuously shifts to longer wavelengths, but the shift of the ν₂+ν₆ band is 10 to 20 times smaller. Many more rotational states of the vibrational ground state are depopulated than expected. To explain it we suggest direct two‐photon excitation, for which independent evidence is also presented. Many peaks and holes were found, part of which we assign to high states. Evidence for inhomogeneity of these structures is found by comparison with single laser absorption spectra. Collisional relaxation of unidentified nature and with a time constant of 48±10 ns mbar generates new spectra, indicating large nonequilibrium rotational populations.