Photoinitiated Reactions of H Atoms with CO2: OH(v = 0) Rotational Excitation from the 239‐nm Photolysis of CO2HI Complexes

Photoinitiated H + CO2 → OH + CO reactions are discussed, with emphasis on reactions in CO2HI complexes. Under single‐collision bulk conditions, reaction probability rises with collision energy by two orders of magnitude throughout the range 10300–19000 cm−1 (ΔH = 8960 cm−1). Modest probabilities at collision energies well above threshold are interpreted as due to the inability of the heavier nuclei to move fast enough to trap the H atom on the HOCO potential surface. The pronounced increase in reaction probability with collision energy can be due to impact‐induced distortion of the CO2 frame, localizing the H atom in a shallow region of the HOCO potential surface long enough for the heavier nuclei to move toward the HOCO equilibrium geometry, thus capturing the H atom. Measurements of nascent OH(v = 0) R, T excitations indicate a significant bias toward product translation and away from OH rotation at the highest collision energies. OH LIF spectra taken at different collision energies provide a map of nascent OH(v = 0) rotational excitation for different values of E†, the HOCO† energy in excess of the OH + CO product channel. With CO2HI complexes, pairwise I‐H and H‐CO2 repulsions before HOCO† is formed increase the I‐HOCO† speed at the expense of HOCO† internal excitation. It is pointed out that with CO2HBr, the Br atom is 3.6 Å from the C atom along a line perpendicular to the CO2 axis, with the H atom localized near one of the O atoms. CO2HI is expected to be qualitatively similar. The OH(v = 0) rotational distribution obtained using 239‐nm photolysis of CO2HI complexes differs markedly from that obtained under single‐collision conditions at the same photolysis wavelength, the former being colder and qualitatively distinct from any of the OH(v = 0) distributions obtained at a single collision energy. The OH(v = 0) rotational distribution obtained using CO2HI complexes can be reconciled with a bimodal P(E†) distribution (e.g., ∼ 30% at E† ∼ 800 cm−1 and ∼ 70% at E† ∼ 6000 cm−1). The 6000‐cm−1 component is attributed to the squeezed‐atom effect (E† = 7880 cm−1 for single‐collision conditions at the same photolysis wavelength), while the origin of the other component is uncertain. It may derive from (i) mechanisms that produce HOCO† with low E† values, (ii) mechanisms that relax HOCO† and/or OH such as interactions with the nearby I atom and (iii) higher‐than‐binary complexes.