Photoinitiated Reactions of H Atoms with CO 2 : OH( v = 0) Rotational Excitation from the 239‐nm Photolysis of CO 2 HI Complexes

Photoinitiated H + CO 2 → OH + CO reactions are discussed, with emphasis on reactions in CO 2 HI 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 CO 2 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 CO 2 HI complexes, pairwise I‐H and H‐CO 2 repulsions before HOCO † is formed increase the I‐HOCO † speed at the expense of HOCO † internal excitation. It is pointed out that with CO 2 HBr, the Br atom is 3.6 Å from the C atom along a line perpendicular to the CO 2 axis, with the H atom localized near one of the O atoms. CO 2 HI is expected to be qualitatively similar. The OH( v = 0) rotational distribution obtained using 239‐nm photolysis of CO 2 HI 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 CO 2 HI 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.