The Cl + H₂ → HCl + H Reaction Induced by IR + UV Irradiation of Cl₂ in Solid para-H₂: experiment

Abstract

We report IR + UV coirradiation photolysis experiments conducted on Cl2-doped para-hydrogen (p-H2) crystals at 1.8 K, using pulsed 355 nm UV radiation and cw broad-band near-IR light from a FTIR tungsten source. The amount of HCl photoproduct is monitored using FTIR spectroscopy as a function of the IR + UV exposure time. Detailed analysis of the HCl growth kinetics reveals that the reaction Cl + H2(v=1,J=0) → HCl + H is playing a significant (15%) role in the in situ photochemistry. In contrast, UV-only photolysis experiments conducted under similar conditions produce almost exclusively (99%) isolated Cl atom photofragments, indicating the reaction Cl + H2(v=0,J=0) → HCl + H is not readily occurring. This combination of photolysis experiments confirms that under these conditions, the Cl + H2 reaction probability increases by a factor greater than 25 for Cl atom reactions with H2(v=1) versus H2(v=0). These results are therefore consistent with the expectation that vibrational excitation of the H2 reagent lowers the reaction threshold and increases the reaction cross section for the Cl + H2 reaction. These experimental studies were motivated by and are compared to the quantum model simulations reported by Korolkov, Manz, and Schild in the accompanying paper.