Sauer Petra, Allen Roland E
Department of Physics, Texas A&M University, College Station, Texas 77843, USA.
J Phys Chem A. 2008 Nov 6;112(44):11142-52. doi: 10.1021/jp801347z. Epub 2008 Oct 11.
When a molecule is subjected to a short intense laser pulse, the ensuing dynamical processes depend qualitatively on the pulse parameters, including duration, frequency, and fluence. Here we report studies of cis to trans photoisomerization of azobenzene following femtosecond-scale laser pulses which are relatively short (10 fs) or long (100 fs) and which have a central frequency matched to either the first excited state (S1, or HOMO to LUMO in a molecular orbital picture) or the second (S2, or HOMO-1 to LUMO). The results presented here demonstrate that photoisomerization involves a rather intricate sequence of connected steps, with the nuclear and electronic degrees of freedom inextricably coupled. One important feature is the de-excitation required for the molecule to achieve its new ground-state after isomerization. If the primary excitation is to S1, then we find that only a single HOMO/LUMO avoided crossing is required and that this crossing occurs halfway along a rotational pathway involving the central CNNC dihedral angle. If the primary excitation is to S2, then the same HOMO/LUMO avoided crossing is observed, but it must be preceded by another avoided crossing that permits transfer of holes from the HOMO-1 to the HOMO, so that the HOMO is then able to accept electrons from the LUMO. We find that this earlier crossing can occur in either of two geometries, one near the cis configuration and the other near the trans. The fact that S2 (pi pi*) isomerization requires two steps may be related to the fact that isomerization yields are smaller for this (UV) excitation than for the S1 (n pi*, visible-light) excitation.
当一个分子受到短时间的强激光脉冲作用时，随之而来的动力学过程在性质上取决于脉冲参数，包括持续时间、频率和能量密度。在此，我们报告了对飞秒级激光脉冲作用下偶氮苯顺反异构化的研究，这些脉冲相对较短（10飞秒）或较长（100飞秒），且中心频率与第一激发态（S1，在分子轨道图中为最高占据分子轨道（HOMO）到最低未占据分子轨道（LUMO））或第二激发态（S2，或HOMO - 1到LUMO）相匹配。此处呈现的结果表明，光异构化涉及一系列相当复杂的相互关联步骤，其中核自由度和电子自由度紧密耦合。一个重要特征是分子在异构化后达到新基态所需的去激发过程。如果初始激发到S1，那么我们发现只需要一个单一的HOMO/LUMO避免交叉，并且这个交叉发生在涉及中心CNNC二面角的旋转路径的中途。如果初始激发到S2，那么会观察到相同的HOMO/LUMO避免交叉，但在此之前必须有另一个避免交叉，该交叉允许空穴从HOMO - 1转移到HOMO，这样HOMO随后就能从LUMO接受电子。我们发现这个较早的交叉可以在两种几何构型中的任何一种中发生，一种接近顺式构型，另一种接近反式构型。S2（ππ*）异构化需要两个步骤这一事实可能与以下事实有关：对于这种（紫外）激发，异构化产率比S1（nπ*，可见光）激发时要小。
