Dynamics of the reaction of O(3P) atoms with alkylthiol self-assembled monolayers

We have studied the dynamics of the reactions of O(3P) atoms with alkylthiol self-assembled monolayers (SAMs). Superthermal O(3P) atoms, with a fairly broad distribution of laboratory-frame kinetic energies (mean = 16 kJ mol-1, fwhm = 26 kJ mol-1),were generated by 355 nm photolysis of NO2 introduced at a low pressure above the SAM surface. Nascent OH v' = 0 products were detected by laser-induced fluorescence. SAMs of two different alkyl chain lengths, C6 and C18, were studied. The existence of SAM layers, and their robustness under our experimental conditions during the relevant measurement period, were confirmed by scanning-tunneling microscopy (STM). Reaction at the SAM surface was verified as the authentic source of the hydroxyl radicals using a perdeuterated C6D13-SAM sample. The OH appearance profiles as a function of photolysis-probe delay, and the rotational-state distributions at their peaks, were compared with those of liquid squalane (C30H62, 2,6,10,15,19,23- hexamethyltetracosane). The reactivity of the SAMs and of squalane was found to be comparable. We conclude that the O(3P) atoms must be able to access the more reactive secondary hydrogen atoms along the alkyl chains of the SAMs. We find no perceptible differences in reactivity or product energy disposal between the two SAM chain lengths. Both produce a substantial fraction of the OH with relatively high velocities, which must result from direct, impulsive reaction. There is also a slower component, with velocities consistent with a thermal, ng-desorption mechanism. The proportion of this component appears to be lower for SAMs than for qualane. This would be compatible with the expected greater smoothness of the SAM surface at the molecular scale. We find little evidence for significant rotational excitation of the OH products, although the details of any correlation between translational and rotational energy release require further investigation. We compare our results with the limited available prior theoretical modeling of O(3P) + SAM systems. © 2009 American Chemical Society.