Abstract
Due to their very nature, ultrafast phenomena are often accompanied by the occurrence of nonadiabatic effects. From a theoretical perspective, the treatment of nonadiabatic processes makes it necessary to go beyond the (quasi) static picture provided by the time-independent Schrödinger equation within the Born-Oppenheimer approximation and to find ways to tackle instead the full time-dependent electronic and nuclear quantum problem. In this review, we give an overview of different nonadiabatic processes that manifest themselves in electronic and nuclear dynamics ranging from the nonadiabatic phenomena taking place during tunnel ionization of atoms in strong laser fields to the radiationless relaxation through conical intersections and the nonadiabatic coupling of vibrational modes and discuss the computational approaches that have been developed to describe such phenomena. These methods range from the full solution of the combined nuclear-electronic quantum problem to a hierarchy of semiclassical approaches and even purely classical frameworks. The power of these simulation tools is illustrated by representative applications and the direct confrontation with experimental measurements performed in the National Centre of Competence for Molecular Ultrafast Science and Technology.
Highlights
We will consider a process to be adiabatic if an applied time-dependent change is so slow that all the characteristic degrees of freedom of the system have enough time to adapt at every instant, i.e., the system reaches its equilibrium at all times and can be described with a single eigenstate instead of a superposition
By diagonalizing the potential energy matrix in the diabatic representation, one obtains the adiabatic representation, in which the molecular Hamiltonian is given by where E^ EðQÞ diagðE1ðQÞ; ...; ESðQÞÞ is a diagonal matrix of adiabatic potential energy surfaces and F^ FðQÞ stands for nonadiabatic couplings, which arise from the dependence of the adiabatic electronic basis states on nuclear coordinates
The XUV pump has the effect of bringing the system on the ground or on one of the electronically excited states of the cation: using tabulated values138,139 for the cross sections of ethylene for monochromatic light, we estimated that 95% of the electronic state population is confined to the cation ground state and the first three excited states described by the spectroscopic states X~2B3u; A~2B3g; B~2Ag, and C~2B2u [Fig. 10(d)]
Summary
The terms adiabatic and nonadiabatic, respectively, are used in many different contexts. The adiabatic regime is approached for wele ) wnuc and DE ) 0 Under these conditions, the electronic and nuclear degrees of freedom can be fully separated and the time-evolution of the system can be described by solving the time-independent Schr€odinger equation for every given set of fixed nuclei (clamped nuclei approximation) and the system evolves adiabatically on a single potential energy surface (PES) [Born-Oppenheimer (BO) approximation]. We give a short summary of different quantum mechanical methods that are able to describe nonadiabatic effects starting with solutions of the full time-dependent electron-nuclear quantum problem through exact nonadiabatic quantum dynamics using the split-operator method necessarily limited to small systems in the gas phase, to different semiclassical methods (meanfield Ehrenfest dynamics and fewest switches surface hopping) to multisurface adiabatic reactive molecular dynamics (MS-ARMD) based on classical force fields (FF). For a more extensive review, see, e.g., Refs. 9, 10, 12, and 13
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