Temperature dependence of OH yield, translational energy, and vibrational branching in the reaction of O(³P)(g) with liquid squalane

The relative reactivity of the liquid surface of a long-chain, partially branched hydrocarbon (squalane, C₃₀H₆₂) with gas-phase O(³P) atoms has been measured as a function of liquid temperature. The O(³P) atoms were generated with a superthermal velocity distribution by 355 nm photolysis of NO₂. Laser-induced fluorescence was used to detect the relative branching into specific OH product vibrational states. The yield of OH(v'=0) proves significantly less dependent on liquid surface temperature than the yield of OH(v'=1). Time-of-flight measurements of the escaping OH provide partially resolved product translational energy distributions. These profiles also differ between OH vibrational states. OH(v'=1) shows overall longer arrival times, but with a clear trend toward earlier times as the surface temperature is increased. OH(v'=0) shows little detectable variation of the distribution of arrival times over the range of temperatures investigated (263-333 K). We discuss the interpretation of these findings, taking account of earlier experimental work, which has indicated significant contributions from distinct "direct" and "trapping-desorption" reaction mechanisms, and new molecular dynamics simulations of the surface structure. There are a number of factors that may contribute, including both energetic and structural effects. It is not possible on the basis of the current evidence to discriminate conclusively between them. Nevertheless, we conclude, on balance, that structural effects may well be the more important. In particular, higher temperatures are predicted to promote more open structures. We speculate that this may enable more OH(v'=1) to escape before it is either vibrationally relaxed or, less probably, undergoes vibrationally enhanced reaction to produce H₂O. © 2007 American Chemical Society.