ELECTRONICALLY EXCITED HALOGEN ATOMS AND SPIN ORBIT RELAXATION
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Date
1966
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Ohio State University
Abstract
The optical metastability arising from the slow electric dipole forbidden transition $X n^{2}P_{1/2}\to X n^{2}P_{3/2}+h\nu$ facilitates the direct study of electronically excited bromine and iodine atoms, $Br(4^{2}P_{^{1}/_{2}})$ and $I(5^{2}P_{^{1}/_{2}})$ and the atoms in their ground states, by kinetic spectroscopy in absorption in the vacuum ultraviolet. The electronically excited atoms are generated by the flash photolysis of a number of simple bromides and iodides of which $CF_{3}Br$ and $CF_{3}I$ are the most convenient sources. Kinetic measurements on the spin orbit relaxation of $Br(4^{2}P_{^{1}/_{2}})$ and $I(5^{2}P_{^{3}/_{2}})$ in the presence of a large number of gases have been carried out in times during which the contribution to the rate from slow atomic recombination is negligible. The relatively large electronic energies of these states cannot be totally transferred to translation, and when the effect of inert gases can be investigated in the absence of significant collisional deactivation by other gases present, the decay rate is determined primarily by that of diffusion of the excited atoms to the walls of the reaction vessel. The presence of strong chemical interaction dominates the overall rate of collisional quenching although the effect of a small energy discrepancy for transfer to vibration is significant in some cases. There is no general correlation between the quenching efficiency and some simple physical parameter of the colliding molecule such as ionisation potential, boiling point, bond dissociation energy or molecular complexity. In the absence of strong chemical interaction, the rate at which electronic energy is transferred appears to be dependent on the low probability of a multivibrational transition in the colliding molecule. The rate of spin orbit relaxation is faster for the more weakly coupled atom, namely, $Br(4^{4}P_{^{1}/_{2}})$.
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Author Institution: Department of Physical, Chemistry, University of Cambridge