Paterson Grant, Marinakis Sarantos, Costen Matthew L, McKendrick Kenneth G, Kłos Jacek, Toboła Robert
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom.
J Chem Phys. 2008 Aug 21;129(7):074304. doi: 10.1063/1.2967861.
The depolarization of OH(X (2)Pi(3/2),v=0,J=1.5-6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A (2)Sigma(+)-X (2)Pi(1,0) band. The evolution of the ground-state OH(X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A (2)Sigma(+)-X (2)Pi(0,0) band. The PS signal decay rate constant k(PS) decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured k(PS) equals the sum k(RET)+k(Lambda)+k(dep), where k(RET), k(Lambda), and k(dep) are the rate constants for rotational energy transfer, Lambda-doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH(X) angular momentum, as specified), respectively. Values of k(dep) can be extracted from the measured k(PS) with prior knowledge of k(RET) and k(Lambda). Because k(RET) and k(Lambda) were not previously available for collisions of Ar with OH(X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that k(dep) is systematically markedly higher for alignment than for orientation for OH(X)+Ar but much more weakly so for OH(X)+He. Calculated k(RET) and k(Lambda) values at 298.15 K are consistent with a substantial contribution from k(dep) for OH(X)+Ar but not for OH(X)+He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH(X)-He PES of Lee et al. [J. Chem. Phys. 113, 5736 (2000)] to reproduce k(RET)+k(Lambda) accurately.
利用双色偏振光谱（PS）研究了在热条件（298K）下，OH（X²Π₃/₂，v = 0，J = 1.5 - 6.5，e）转动角动量（RAM）与He和Ar碰撞时的去极化情况。在非对角OH A²Σ⁺ - X²Π₁,₀能带分别使用圆偏振或线偏振脉冲激发，实现了OH RAM的取向或排列。专门使用一个独立的、调谐到对角OH A²Σ⁺ - X²Π₀,₀能带的线偏振脉冲，探测基态OH（X）RAM极化的演化。对于OH（X）+Ar，PS信号衰减速率常数k(PS)随转动量子数增加而减小，但对于OH（X）+He并非单调变化。测得的k(PS)等于k(RET)+k(Λ)+k(dep)之和，其中k(RET)、k(Λ)和k(dep)分别是转动能量转移、Λ双重态变化碰撞以及（如所指定的OH（X）角动量取向或排列的）转动弹性去极化的速率常数。k(dep)的值可以在已知k(RET)和k(Λ)的情况下，从测得的k(PS)中提取。由于之前没有Ar与OH（X，v = 0）碰撞的k(RET)和k(Λ)数据，我们在本文首次提出的一个新的势能面（PES）上进行了精确的、完全量子力学的散射计算。原始实验结果表明，对于OH（X）+Ar，排列时的k(dep)系统地明显高于取向时的k(dep)，但对于OH（X）+He则弱得多。在298.15K时计算得到的k(RET)和k(Λ)值与k(dep)对OH（X）+Ar有显著贡献但对OH（X）+He没有显著贡献一致。这可能表明吸引力在弹性去极化中的作用。实验结果对Lee等人[《化学物理杂志》113, 5736 (2000)]最新的从头算OH（X）-He PES准确再现k(RET)+k(Λ)的能力提供了一个非常灵敏的检验。
