Ka Being J, Geva Eitan
Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA.
J Phys Chem A. 2006 Aug 10;110(31):9555-67. doi: 10.1021/jp062363c.
Vibrational energy relaxation (VER) of polyatomic, as opposed to diatomic, molecules can occur via different, often solvent assisted, intramolecular and/or intermolecular pathways. In this paper, we apply the linearized semiclassical (LSC) method for calculating VER rates in the prototypical case of a rigid, symmetrical and linear triatomic molecule (A-B-A) in a monatomic liquid. Starting at the first excited state of either the symmetric or asymmetric stretches, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR). The VER rate constants for the various pathways are calculated within the framework of the Landau-Teller formalism, where they are expressed in terms of two-time quantum-mechanical correlation functions. The latter are calculated by the LHA-LSC method, which puts them in a "Wignerized" form, and employs a local harmonic approximation (LHA) in order to compute the necessary multidimensional Wigner integrals. Results are reported for the LHL/Ar model of Deng and Stratt [J. Chem. Phys. 2002, 117, 1735], as well as for CO(2) in liquid argon and in liquid neon. The LHA-LSC method is shown to give rise to significantly faster VER and IVR rates in comparison to the classical treatment, particularly at lower temperatures. We also find that the type and extent of the quantum rate enhancement is strongly dependent on the particular VER pathway. Finally, we find that the classical and semiclassical treatments can give rise to opposite trends when it comes to the dependence of the VER rates on the solvent.
与双原子分子不同，多原子分子的振动能量弛豫（VER）可以通过不同的、通常是溶剂辅助的分子内和/或分子间途径发生。在本文中，我们应用线性化半经典（LSC）方法来计算刚性、对称且线性的三原子分子（A - B - A）在单原子液体中的典型情况下的VER速率。从对称或不对称伸缩的第一激发态开始，VER可以直接弛豫到基态，也可以通过分子内振动弛豫（IVR）间接发生。各种途径的VER速率常数是在Landau - Teller形式体系的框架内计算的，在该体系中它们用双时量子力学关联函数来表示。后者通过LHA - LSC方法计算，该方法将它们转化为“维格纳化”形式，并采用局部谐波近似（LHA）来计算必要的多维维格纳积分。报道了Deng和Stratt [《化学物理杂志》2002年，117卷，1735页]的LHL/Ar模型以及二氧化碳在液态氩和液态氖中的结果。结果表明，与经典处理相比，LHA - LSC方法能产生显著更快的VER和IVR速率，特别是在较低温度下。我们还发现量子速率增强的类型和程度强烈依赖于特定的VER途径。最后，我们发现当涉及到VER速率对溶剂的依赖性时，经典处理和半经典处理可能会产生相反的趋势。
