Photophysical processes in the vapour phase measured by the optic–acoustic effect. Part 7.—Vibrational–translational relaxation in I2, X state, and collisionally induced predissociation in the B state

Abstract

The spectrophone signal in gaseous I₂, at 0.3 Torr and 295 K, and following modulated excitation at 546 nm, is analysed in terms of contributions from thermal energy release and from particle density increase due to predissociation. The former contribution is shown to be dominant and allows the measurement of the vibrational–translational relaxational relaxation time in I₂(X state). The measurement is novel in that the excess translational energy produced by predissociation is frequency-monitored as it equilibrates with the thermally accessible vibrational levels. The extent of energy released is analysed to show that in the collisional predissociation process almost no energy (< 5%) is transferred to the vibration of the I₂ collision partner. There is no evidence for a significant yield of direct (Xâ†�B, I₂) intersystem crossing. “Hot” I atom excitation of I₂ vibrations is also discussed.