Inelastic Scattering of CN Radicals at the Gas-Liquid Interface Probed by Frequency-Modulated Absorption Spectroscopy

We describe the results of the first application of frequency-modulated (FM) absorption spectroscopy to molecular scattering dynamics at a gas-liquid interface. A pulsed direct-current (dc) electric discharge of a supersonic expansion of BrCN seeded in He was used to generate a pulsed molecular beam of rotationally cold CN(X-2 Sigma(+)) radicals with a mean laboratoryframe kinetic energy of 43.5 kJ mol-1. The molecular beam was directed at normal incidence onto a continually refreshed perfluoropolyether liquid surface. FM absorption spectroscopy on the CN(A(2)Pi-X-2 Sigma(+)) (2,0) band was used to measure Doppler line shapes for individual CN rotational states as a function of time after the dc discharge pulse. This enabled characterization of both the incident molecular beam and the inelastically surface-scattered CN rotational and translational energy distributions. The surface-scattered CN rotational distribution is well characterized by a single temperature of 850 +/- 130 K. The translational distributions perpendicular to the surface normal are non-Maxwellian and are substantially superthermal. We interpret these observations as the result of impulsive scattering being the dominant mechanism, similar to our previous independent measurements of OD inelastic scattering at liquid surfaces. Within the current limitations of signal to noise, no clear evidence for a discrete component from thermal desorption is observed, in contrast to previous literature measurements of NO and CO2 scattering at perfluoropolyether surfaces.