Methyl iodide photodissociation at 193 nm:: The I(2P1/2) quantum yield

Methyl iodide photolysis at 193 nm has been studied through probing the I(P-2(1/2)-P-2(3/2)) transition in the atomic iodine photofragment using diode laser spectroscopy. The I(P-2(1/2)) quantum yield has been determined through two different diode laser techniques and their compared. Frequency-modulated diode laser based absorption spectroscopy was used to extract nascent Doppler lineshapes from which an I(P-2(1/2)) quantum yield of unity is inferred. However when diode laser gain/absorption measurements were made, an I(P-2(1/2)) quantum yield of 0.68 +/- 0.04 was found. The reason for this discrepancy is shown to lie in the diode laser gain/absorption method. Molecular iodine is found to be formed during the experiment via atomic iodine recombination and then in turn dissociates to produce both I(P-2(1/2)) and I(P-2(3/2)), thus distorting the returned quantum yield. This conclusion is supported both by the reduction of the I(P-2(1/2)) quantum yield with number of photolysis laser shots when measured using this technique and by the presence of fluoresence which is shown to have excited-state lifetimes and quenching rates that are consistent with those previously measured for the D and D' states of molecular iodine.