Direct Determination of Threshold Energies for the Reactions of Hydrogen Atoms with N2O and with CO2

Abstract

IT has been realized for some time that hydrogen atoms produced by photolysis of the hydrogen halides or of certain other hydrides possess kinetic energy in excess of that corresponding to thermal equilibrium1–5. This arises because the energy of the quantum absorbed is larger than the minimum required to disrupt the molecule into stationary atoms. For example, the dissociation energy of HI is 296 kJ moles−1 (ref. 6) and its first absorption maximum occurs at 220 nm7, corresponding to an energy of 539 kJ moles−1. As a result of conservation of momentum, most of the excess energy appears as translational energy of the hydrogen atom. Such hydrogen atoms are often described as “hot” and their energy is manifested in enhanced reactivity. Their reactions with HI1–3, H2S4, halogens1–4, D2 and several deuterated hydrocarbons (or the corresponding reactions of D or T with H2 or hydrocarbons)8–13 have been demonstrated, and in some cases the variation of reaction probability with wavelength of photolysis has been measured10–12.

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OLDERSHAW, G., PORTER, D. Direct Determination of Threshold Energies for the Reactions of Hydrogen Atoms with N2O and with CO2. Nature 223, 490–491 (1969). https://doi.org/10.1038/223490a0

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