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

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbonliquid, squalane, were investigated and compared to the results of an earlier study on analogousoxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto asqualane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as afunction of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCltime-of-flight ~from the surface to the detector! distributions revealed thermal and hyperthermalinteraction channels, in analogy to the dynamical behavior of the O and OH signals observed in theprevious study. The thermal HCl product may arise from two mechanisms: ~i! desorption of trappedHCl 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 onlycome from desorption of trapped OH. The hyperthermal HCl signal is the result of an Eley-Ridealdirect-abstraction mechanism. An analogous reaction gave rise to hyperthermal OH products inO-atom reactions with squalane. The HCl and OH hyperthermal products were scatteredpreferentially in the specular direction in the laboratory frame. However, a kinematic analysisshowed a propensity for scattering sideways or even backwards~with respect to the direction of theincident atomic beam! in the center-of-mass ~c.m.! frame. The kinematic analysis also allowed thedetermination of the effective mass of the surface collision partner, the c.m. collision energy, and thefinal c.m. translational energy. Similar c.m. scattering dynamics have been observed in gas-phasereactions of O and Cl atoms with saturated hydrocarbons