Infrared resonant desorption of butane from Al2O3(112̄0): Evidence for an ordered adlayer from vibrational mode selectivity

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Abstract
The resonant desorption of butane from Al2O3(112̄0) was studied using tunable infrared radiation from an infrared free electron laser. Resonant desorption was induced by exciting C–H stretching vibrations in butane adlayers at coverages of θ=1–7 ML of Al2O3(112̄0) at 90 K. The resonant desorption spectrum did not duplicate the infrared absorption spectrum of solid butane. In contrast, the asymmetric C–H stretches produced a greater desorption yield than expected in comparison to the symmetric C–H stretches. The greater desorption efficiency of the asymmetric C–H stretches was attributed to the orientation of butane molecules in an ordered adlayer on Al2O3(112̄0). The proposed orientation in this adlayer is butane in an all‐trans configuration with the carbon backbone perpendicular to the surface. The resonant desorption yield increased approximately linearly with laser energy and surface coverage. Isotope experiments were also performed with equal amounts of butane‐h10 and butane‐d10 on the surface. Equivalent amounts of protonated and deuterated species were observed to desorb after excitation of butane‐h10. The results were all consistent with a thermal mechanism for the resonant desorption of butane from Al2O3(112̄0). In this thermal mechanism, vibrational energy deposited in the C–H stretching vibrations is thermalized quickly and leads to resonant heating of the butane adlayer and subsequent thermal desorption.