Comparative dynamics of Cl(2P) and O(3P) interactions with a liquid hydrocarbon surface

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight ~from the surface to the detector! distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: ~i! desorption of trapped HCl product and ~ii! reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl signal is the result of an Eley–Rideal direct-abstraction mechanism. An analogous reaction gave rise to hyperthermal OH products in O-atom reactions with squalane. The HCl and OH hyperthermal products were scattered preferentially in the specular direction in the laboratory frame. However, a kinematic analysis showed a propensity for scattering sideways or even backwards ~with respect to the direction of the incident atomic beam! in the center-of-mass ~c.m.! frame. The kinematic analysis also allowed the determination of the effective mass of the surface collision partner, the c.m. collision energy, and the final c.m. translational energy. Similar c.m. scattering dynamics have been observed in gas-phase reactions of O and Cl atoms with saturated hydrocarbons.