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

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.

我们描述了分子散射产物的实空间成像，该成像应用于羟基自由基与低蒸气压液体表面的碰撞。由于技术原因，平均实验室坐标系动能为29.5 kJ/mol的OD脉冲分子束被导向全氟聚醚、角鲨烷和角鲨烯等代表性液体不断更新的表面。激光诱导荧光（LIF）由整形为平面片的脉冲激光激发，激光调谐到OD A - X波段中选定的振转跃迁。在被外置相机捕捉之前，LIF发射光被成像并增强。图像序列使得能够观察到入射分子团以及OD分子散射羽流的演变。这些结果证实了先前对散射OD的内部态分布及其在不同液体表面上的不同存活概率的观察结果。对角度分布的新测量发现它们都很宽且大致对称，与入射角无关。这被解释为意味着高度的原子尺度粗糙度，而非主要的捕获 - 解吸机制，因为观察到了其他脉冲散射的特征；这些特征包括OD旋转激发程度、超热速度以及速度与散射角度的相关性。这种方法在相关的气 - 面散射实验中具有相当大的应用潜力。它不受一些涉及带电粒子检测的其他成像方法的困难影响，并能够对垂直于表面的散射平面的空间扩展区域进行成像。