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 electric discharge of a supersonic expansion of BrCN seeded in He was used to generate a pulsed molecular beam of rotationally cold CN(X²Σ⁺) radicals, with a mean laboratory-frame 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(A²Π-X²Σ⁺) (2,0) band was used to measure Doppler lineshapes for individual CN rotational states as a function of time after the DC discharge pulse. This enabled the characterization of both the incident molecular beam and 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.