Marinakis Sarantos, Paterson Grant, Kłos Jacek, Costen Matthew L, McKendrick Kenneth G
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK.
Phys Chem Chem Phys. 2007 Aug 21;9(31):4414-26. doi: 10.1039/b703909c. Epub 2007 Jun 1.
One-colour polarization spectroscopy (PS) on the OH A (2)Sigma(+)- X (2)Pi(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X (2)Pi) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X (2)Pi(3/2), J = 1.5-6.5, e states. The PS signal decay rate constant, k(PS), is sensitive to all processes that remove population and destroy polarization. The contribution to k(PS) from pure (elastic) alignment depolarization within the initial level, k(DEP), can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), k(RET), and for Lambda-doublet changing, k(Lambda), collisions from k(PS). Literature values of k(RET) and k(Lambda) are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of k(RET) and k(Lambda) on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for k(DEP) from this subtraction for He are found to be modest, around 0.4 x 10(-10) cm(3) s(-1), whereas for Ar k(DEP) is found to range between 0.6 +/- 0.2 x 10(-10) cm(3) s(-1) and 1.7 +/- 0.3 x 10(-10) cm(3) s(-1), comparable to total population removal rate constants. The differences between k(DEP) for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of k(DEP) may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions.
利用OH A (2)Σ⁺ - X (2)Π(0,0)带的单色偏振光谱（PS）来测量基态OH(X (2)Π)在与He和Ar碰撞中整体转动角动量取向的消除情况。在不同碰撞体分压下的准一级PS信号衰减被用于确定X (2)Π(3/2)、J = 1.5 - 6.5、e态的二级衰减速率常数。PS信号衰减速率常数k(PS)对所有去除粒子数并破坏偏振的过程都很敏感。通过从k(PS)中减去独立测量或预测的总转动能量转移（RET）速率常数k(RET)与Λ - 双重态变化速率常数k(Λ)的碰撞之和，可以提取初始能级内纯（弹性）取向退极化对k(PS)的贡献k(DEP)。k(RET)和k(Λ)的文献值可从He和Ar的实验以及仅针对Ar的量子散射计算中获得。因此，我们还给出了基于Lee等人[《化学物理杂志》2000年，113卷，5736页]最新的从头算OH(X)-He势能面进行的k(RET)和k(Λ)的全新、精确、完全量子力学计算结果。对于He，通过这种减法得到的k(DEP)结果适中，约为0.4×10⁻¹⁰ cm³ s⁻¹，而对于Ar，k(DEP)在0.6±0.2×10⁻¹⁰ cm³ s⁻¹至1.7±0.3×10⁻¹⁰ cm³ s⁻¹之间，与总粒子数去除速率常数相当。两个碰撞体的k(DEP)差异很可能是由于Ar存在比He更深的吸引阱。k(DEP)的测量可能提供一种有用的新工具，它比转动状态变化碰撞对分子间势能远程部分的形式更敏感。
