Propagation of nonstationary electronic and nuclear states: attosecond dynamics in LiF

Rapid optical excitation of a molecule produces a nonstationary state localised in the Franck–Condon region. To move out of that region, one needs to propagate both the electronic and the nuclear state. We formulate the motion on a grid of nuclear coordinate. The coupling to the electric field is fully included in the Hamiltonian used for propagation. We use perturbation theory to analyse the results of dynamics from one grid point to another. The nonadiabatic coupling terms arise from propagating the electronic states. We apply the formalism to the simple case of a diatomic molecule in an approximate but accurate scheme that allows performing computation on a limited number of grid points. As the coherent dynamics unfolds, we expand the grid in the direction of the wave packet motion with the quantum chemical calculations of the electronic structure performed ‘on the fly’. The LiF molecule excited by a one-cycle IR pulse is used as a computational example. The 30-fs propagation through the crossing of the ionic and covalent states is overall adiabatic. The role of electron–nuclear coherences is emphasised.

对分子实施快速光激发，可生成局域于弗兰克-康登（Franck–Condon）区域的非定态。若要脱离该区域，需同时传播电子态与核态。我们基于核坐标网格构建了该运动的理论描述。传播所用的哈密顿（Hamiltonian）已完整纳入与电场的耦合项。我们采用微扰理论（perturbation theory），分析了单个网格点之间的动力学演化结果。非绝热耦合项源自电子态的传播过程。我们将该形式体系应用于双原子分子的简单案例，采用了一种近似但精准的计算方案，该方案可在有限网格点数下完成运算。随着相干动力学的展开，我们沿波包运动方向扩展网格，并同步开展电子结构的量子化学实时（on the fly）计算。我们以受单周期红外脉冲激发的氟化锂（LiF）分子作为计算示例。历经30飞秒的传播过程中，离子态与共价态的交叉区域整体呈现绝热特性。本研究着重强调了电子-核相干性的关键作用。