JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA.
J Chem Phys. 2019 Jan 28;150(4):044201. doi: 10.1063/1.5083050.
This work presents the first fully quantum-state-resolved measurements of a solute molecule evaporating from the gas-liquid interface in vacuum. Specifically, laser-induced fluorescence detection of NO(Π, v = 0, J) evaporating from an ∼5 mM NO-water solution provides a detailed characterization of the rotational and spin-orbit distributions emerging from a ⌀4-5 μm liquid microjet into vacuum. The internal-quantum-state populations are found to be well described by Boltzmann distributions, but corresponding to temperatures substantially colder (up to 50 K for rotational and 30 K for spin-orbit) than the water surface. The results therefore raise the intriguing possibility of non-equilibrium dynamics in the evaporation of dissolved gases at the vacuum-liquid-water interface. In order to best interpret these data, we use a model for evaporative cooling of the liquid microjet and develop a model for collisional cooling of the nascent NO evaporant in the expanding water vapor. In particular, the collisional-cooling model illustrates that, despite the 1/r drop-off in density near the microjet greatly reducing the probability of collisions in the expanding water vapor, even small inelastic cross sections (≲ 20 Å) could account for the experimentally observed temperature differences. The current results do not rule out the possibility of non-equilibrium evaporation dynamics, but certainly suggest that correct interpretation of liquid-microjet studies, even under conditions previously considered as "collision-free," may require more careful consideration of residual collisional dynamics.
这项工作首次实现了从真空气液界面蒸发的溶质分子的完全量子态分辨测量。具体来说，通过激光诱导荧光探测从约 5 mM 的 NO-水溶剂中蒸发的 NO(Π, v = 0, J)，提供了从 4-5 μm 液体微射流进入真空时出现的转动和自旋轨道分布的详细特征。发现内部量子态的分布很好地符合玻尔兹曼分布，但对应的温度明显低于（对于转动是 50 K，对于自旋轨道是 30 K）水表面。因此，这些结果提出了在真空-液体-水界面处溶解气体蒸发中存在非平衡动力学的有趣可能性。为了最好地解释这些数据，我们使用了一种用于液体微射流蒸发冷却的模型，并开发了一种用于初生 NO 蒸发物在膨胀水蒸气中碰撞冷却的模型。特别是，碰撞冷却模型表明，尽管在微射流附近密度呈 1/r 下降，大大降低了膨胀水蒸气中碰撞的概率，但即使小的非弹性横截面（≲20 Å）也可以解释实验观察到的温度差异。目前的结果并不排除非平衡蒸发动力学的可能性，但确实表明，即使在先前认为“无碰撞”的条件下，对液体微射流研究的正确解释，可能需要更仔细地考虑残留的碰撞动力学。
