I* (5 2P1/2) quenching by hydrogen halides

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Abstract
The total quenching rates of electronically excited iodine (I*,5 2P1/2) by HF, HCl, HBr, and HI have been measured. Two techniques were used to observe the temporal behavior of the I*: (1) absorption of the 206.2 nm resonance line and (2) I* emission at 1.315 μm. Close agreement between the two methods supports the modified form of the Beer–Lambert relation for data analysis in the absorption technique. Rate constants for quenching by HF, HCl, HBr, and HI are 3±1×10−12, 6.5±0.9×10−15, 1.58±0.07×10−13, and 5±1×10−14 cm3 molecule−1 sec−1, respectively. I* quenching by HF and HCl is shown to be consistent with the efficient electronic to vibrational (E–V) energy transfer mechanism known to exist for Br* (4 2P1/2) quenching by hydrogen halides. Quenching by HBr and HI is too rapid to be consistent with the E–V argument and is probably dominated by reactive channels.