Beswick J Alberto, Zare Richard N
Laboratoire Collisions, Agregats, Reactivite, UMR 5589 du CNRS IRSAMC, Universite Paul Sabatier, 31062 Toulouse, France.
J Chem Phys. 2008 Oct 28;129(16):164315. doi: 10.1063/1.3000581.
Quantum and quasiclassical expressions for the angular distribution of photofragments from an initially polarized precursor molecule are compared under the conditions of a one-photon electric dipole transition to a repulsive state followed by prompt axial recoil into two separating fragments. The treatment is most readily applicable to diatomic molecules, but it is more general than that. It is shown that when the rotational and electronic angular momentum J(i) and its projection along the body-fixed z axis Omega(i) are well defined in the initial state, the quantum and quasiclassical expressions are identical for any initial polarization of the molecule prior to photolysis and for all values of J(i) and Omega(i). For the particular case of an mid R:J(i)Omega(i)M(i) selected state this is in agreement with a previous result [T. Seideman, Chem. Phys. Lett. 253, 279 (1996)]. Moreover, the quasiclassical expression is still a good approximation even when the initial state is a coherent superposition of mid R:J(i),Omega(i),M(i) levels for the same Omega(i). This near identity still pertains even when Omega(i) is not well defined for a parallel transition (DeltaOmega=0) but fails for a perpendicular transition (DeltaOmega=+/-1) if the initial state is in a coherent superposition of Omega(i) states differing by +/-2. These conclusions apply to preparation schemes employing optical excitation, static inhomogeneous and/or homogeneous electric and/or magnetic fields, as well as to molecules physisorbed on solids or clusters. We discuss the importance of these results in the interpretation of photofragment distributions when some other angular momenta are involved, such as electronic angular momentum, with and without nuclear spin, coupled to molecular rotation, asymmetric top rotational angular momentum, or internal vibrational angular momentum in polyatomics.
在单光子电偶极跃迁到排斥态，随后迅速轴向反冲为两个分离碎片的条件下，比较了来自初始极化前体分子的光碎片角分布的量子和准经典表达式。该处理方法最容易应用于双原子分子，但比这更具普遍性。结果表明，当初始态中转动角动量和电子角动量J(i)及其沿分子固定z轴的投影Ω(i)定义明确时，对于光解前分子的任何初始极化以及J(i)和Ω(i)的所有值，量子和准经典表达式都是相同的。对于中R:J(i)Ω(i)M(i)选定态的特殊情况，这与先前的结果一致 [T. Seideman, Chem. Phys. Lett. 253, 279 (1996)]。此外，即使初始态是具有相同Ω(i)的中R:J(i)、Ω(i)、M(i)能级的相干叠加，准经典表达式仍然是一个很好的近似。即使对于平行跃迁（ΔΩ = ０），Ω(i)定义不明确时，这种近似仍然成立，但如果初始态是相差±2的Ω(i)态的相干叠加，则对于垂直跃迁（ΔΩ = ±1）不成立。这些结论适用于采用光激发、静态非均匀和/或均匀电场和/或磁场的制备方案，以及吸附在固体或团簇上的分子。我们讨论了这些结果在解释光碎片分布时的重要性，此时涉及一些其他角动量，如电子角动量（有或没有核自旋）与分子转动、不对称陀螺转动角动量或多原子分子的内振动角动量耦合。
