Diode laser based studies of the UV photolysis of molecular iodine

Abstract

The photolysis of molecular iodine at 193 and 248 nm has been studied by diode laser based frequency-modulated (FM) absorption spectroscopy with detection of the nascent iodine photofragment via the I(²P_1/2−²P_3/2) transition at 1.315 μm. Use of narrow band radiation enables nascent measurements with sufficient speed resolution to allow both the character of the initial electronic transition and speed of the fragments to be determined. The time dependence of the integrated area of the measured Doppler profiles has been used to determine both the I* quantum yield and the collisional electronic quenching rate constant of I* (I* = ²P_1/2) by I₂. These values are also determined using the diode laser gain versus absorption technique. In the 248 nm case an I* quantum yield of 0.45 ± 0.04 and 0.42 ± 0.04 is found by each method, respectively, and an electronic quenching rate constant of (3.6 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, consistent with literature, is determined. The form of the nascent Doppler profile indicates that excitation to a Ω = 1 state dominates, with subsequent dissociation to I(²P_1/2) + I(²P_3/2), in keeping with assignment of the upper state as 1441 ³Σ⁺(1_u). The deviation from Φ(I*) = 0.5 can be attributed to a contribution from the 1441 ³Σ⁺(0_u⁻) state which dissociates to two ground state iodine atoms. 193 nm excitation exhibits more complicated dynamics and kinetics, including a pressure dependent I* quantum yield. At the low pressures, <200 mTorr, used in the FM Doppler measurements there are two speed components to the profile suggesting multiple dissociation pathways. Firstly, fast iodine fragments indicate single photon absorption in a parallel transition followed by direct dissociation. Secondly, a slower speed component can be attributed to iodine atoms formed after radiative transfer to unbound levels in the ground state and the a′ surface. Investigation of the changes in Doppler profiles with time suggest that some of the a′ surface population may transfer to the bound B(³Π_0u⁺) state which then in turn undergoes collision-induced predissociation to produce I atoms on the ground electronic state.