Perkins Bradford G, Häber Thomas, Nesbitt David J
JILA, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440, USA.
J Phys Chem B. 2005 Sep 1;109(34):16396-405. doi: 10.1021/jp0511404.
An apparatus for detailed study of quantum state-resolved inelastic energy transfer dynamics at the gas-liquid interface is described. The approach relies on supersonic jet-cooled molecular beams impinging on a continuously renewable liquid surface in a vacuum and exploits sub-Doppler high-resolution laser absorption methods to probe rotational, vibrational, and translational distributions in the scattered flux. First results are presented for skimmed beams of jet-cooled CO(2) (T(beam) approximately 15 K) colliding at normal incidence with a liquid perfluoropolyether (PFPE) surface at E(inc) = 10.6(8) kcal/mol. The experiment uses a tunable Pb-salt diode laser for direct absorption on the CO(2) nu(3) asymmetric stretch. Measured rotational distributions in both 00(0)0 and 01(1)0 vibrational manifolds indicate CO(2) inelastically scatters from the liquid surface into a clearly non-Boltzmann distribution, revealing nonequilibrium dynamics with average rotational energies in excess of the liquid (T(s) = 300 K). Furthermore, high-resolution analysis of the absorption profiles reveals that Doppler widths correspond to temperatures significantly warmer than T(s) and increase systematically with the J rotational state. These rotational and translational distributions are consistent with two distinct gas-liquid collision pathways: (i) a T approximately 300 K component due to trapping-desorption (TD) and (ii) a much hotter distribution (T approximately 750 K) due to "prompt" impulsive scattering (IS) from the gas-liquid interface. By way of contrast, vibrational populations in the CO(2) bending mode are inefficiently excited by scattering from the liquid, presumably reflecting much slower T-V collisional energy transfer rates.
描述了一种用于详细研究气液界面处量子态分辨非弹性能量转移动力学的装置。该方法依赖于超声速喷射冷却分子束撞击真空中连续可再生的液体表面，并利用亚多普勒高分辨率激光吸收方法探测散射通量中的转动、振动和平动分布。给出了喷射冷却的CO₂（束流温度约15K）掠射束在E(inc)=10.6(8)kcal/mol下垂直入射与全氟聚醚（PFPE）液体表面碰撞的初步结果。该实验使用可调谐铅盐二极管激光器对CO₂的ν₃不对称伸缩振动进行直接吸收。在00(0)0和01(1)0振动分支中测得的转动分布表明，CO₂从液体表面发生非弹性散射，形成明显的非玻尔兹曼分布，揭示了平均转动能量超过液体（T(s)=300K）的非平衡动力学。此外，对吸收谱的高分辨率分析表明，多普勒宽度对应的温度明显高于T(s)，并随J转动态系统地增加。这些转动和平动分布与两种不同的气液碰撞途径一致：(i) 由于俘获-解吸（TD）产生的约300K的组分，以及 (ii) 由于气液界面的“即时”脉冲散射（IS）产生的温度高得多的分布（T约750K）。相比之下，CO₂弯曲模式中的振动布居通过与液体的散射激发效率较低，这大概反映了T-V碰撞能量转移速率要慢得多。
