Bargheer M, Cohen A, Gerber R B, Gühr M, Korolkov M V, Manz J, Niv M Y, Schröder M, Schwentner N
Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
J Phys Chem A. 2007 Sep 27;111(38):9573-85. doi: 10.1021/jp071771t. Epub 2007 Jul 13.
Three approaches are combined to study the electronic states' dynamics in the photodissociation of F(2) and ClF in solid argon. These include (a) semiclassical surface-hopping simulations of the nonadiabatic processes involved. These simulations are carried out for the F(2) molecule in a slab of 255 argon atoms with periodic boundary conditions at the ends. The full manifold of 36 electronic states relevant to the process is included. (b) The second approach involves quantum mechanical reduced-dimensionality models for the initial processes induced by a pump laser pulse, which involve wavepacket propagation for the preoriented ClF in the frozen argon lattice and incorporate the important electronic states. The focus is on the study of quantum coherence effects. (c) The final approach is femtosecond laser pump-probe experiments for ClF in Ar. The combined results for the different systems shed light on general properties of the nonadiabatic processes involved, including the singlet to triplet and intertriplet transition dynamics. The main findings are (1) that the system remains in the initially excited-state only for a very brief, subpicosecond, time period. Thereafter, most of the population is transferred by nonadiabatic transitions to other states, with different time constants depending on the systems. (2) Another finding is that the dynamics is selective with regard to the electronic quantum numbers, including the Lambda and Omega quantum numbers, and the spin of the states. (3) The semiclassical simulations show that prior to the first "collision" of the photodissociated F atom with an Ar atom, the argon atoms can be held frozen, without affecting the process. This justifies the rigid-lattice reduced-dimensionality quantum model for a brief initial time interval. (4) Finally, degeneracies between triplets and singlets are fairly localized, but intertriplet degeneracies and near degeneracies can span an extensive range. The importance of quantum effects in photochemistry of matrix-isolated molecules is discussed in light of the results.
三种方法相结合来研究固态氩中F₂和ClF光解离过程中电子态的动力学。这些方法包括：(a) 对所涉及的非绝热过程进行半经典表面跳跃模拟。这些模拟是在一个包含255个氩原子的平板中对F₂分子进行的，两端采用周期性边界条件。该过程相关的36个电子态的完整流形都被考虑在内。(b) 第二种方法涉及用于泵浦激光脉冲引发的初始过程的量子力学低维模型，该模型包括在冻结的氩晶格中对预取向的ClF进行波包传播，并纳入重要的电子态。重点是研究量子相干效应。(c) 最后一种方法是对Ar中的ClF进行飞秒激光泵浦 - 探测实验。不同系统的综合结果揭示了所涉及的非绝热过程的一般性质，包括单重态到三重态以及三重态间的跃迁动力学。主要发现如下：(1) 系统仅在非常短暂的亚皮秒时间段内保持在初始激发态。此后，大部分粒子通过非绝热跃迁转移到其他态，转移时间常数因系统而异。(2) 另一个发现是，动力学对电子量子数具有选择性，包括Λ和Ω量子数以及态的自旋。(3) 半经典模拟表明，在光解离的F原子与Ar原子首次“碰撞”之前，氩原子可以保持冻结状态，而不影响该过程。这证明了在初始短暂时间间隔内采用刚性晶格低维量子模型的合理性。(4) 最后，三重态和单重态之间的简并相当局域化，但三重态间的简并和近简并可以跨越很广的范围。根据这些结果讨论了量子效应在基质隔离分子光化学中的重要性。
