Real-space laser-induced fluorescence imaging applied to gas-liquid interfacial scattering

We describe the real-space imaging of the products of molecular scattering, applied to collisions of hydroxyl radicals with low-vapor-pressure-liquid surfaces. A pulsed molecular beam of OD (for technical reasons) with a mean laboratory-frame kinetic energy of 29.5 kJ mol(-1) was directed at continually refreshed surfaces of the representative liquids perfluoropolyether, squalane, and squalene. Laser-induced fluorescence (LIF) was excited by pulsed laser light shaped into a planar sheet, tuned to selected rovibronic transitions in the OD A-X band. The LIF emission was imaged and intensified before being captured by an external camera. Sequences of images allowed the evolution of the incident packet and scattered plumes of OD molecules to be observed. The results confirm previous observations of the internal-state distributions of the scattered OD and its differential survival probability on different liquid surfaces. New measurements of the angular distributions found them all to be broad and approximately symmetric, independent of the angle of incidence. This is interpreted as implying a high degree of atomic-scale roughness, rather than a predominant trapping-desorption mechanism, because of the other observed signatures of impulsive scattering; these include the degree of OD rotational excitation, superthermal speeds, and the correlation of speed with scattering angle. This approach has considerable potential to be applied in related gas-surface scattering experiments. It is immune from the difficulties of some other imaging methods that involve charged-particle detection and allows a spatially extended region of the scattering plane perpendicular to the surface to be imaged.