Abstract
In this chapter, we discuss crossing between two or more Born–Oppenheimer states. We begin with wave packet motion which allows to introduce the classical limit for nuclear motion. The matrix elements of the nonadiabatic coupling can become very large or even divergent, whenever two electronic states come close. The “adiabatic to diabatic” transformation eliminates at least the singular parts of the derivative coupling. We derive the so-called diabatic Schrödinger equation and discuss the simplest case of a crossing between two states. For a Hamiltonian depending on only one nuclear coordinate, the transformation to a diabatic basis is possible and yields a diabatic coupling which is given by half the splitting of the adiabatic states. The semiclassical approximation makes use of narrow localized wavepackets and describes nuclear motion as a classical trajectory defined as the time-dependent average position. The famous Landau Zener model uses a linear approximation of the trajectory in the vicinity of the crossing point and obtains an explicit solution for the transition probability. If more coordinates are involved, conical intersections appear which are very important for ultrafast transitions. We discuss the linear vibronic coupling model for the dynamics in the vicinity of a conical intersection.
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Notes
- 1.
Without a magnetic field the electronic wavefunctions can be assumed to be real valued.
- 2.
In the following we make use of \(0=\frac{\partial }{\partial Q}UU^{-1}=\frac{\partial U}{\partial Q}U^{-1}+U\frac{\partial U^{-1}}{\partial Q}\) and \(0=\frac{\partial }{\partial Q}U^{-1}U=\frac{\partial U^{-1}}{\partial Q}U+U^{-1}\frac{\partial U}{\partial Q}\).
- 3.
In the language of gauge theories the substitution (21.23) is known as covariant gradient.
- 4.
This is in principle also the case for the Born–Oppenheimer approximation with only one term.
- 5.
The integrability condition (21.73) for the inverse rotation is fulfilled by construction.
- 6.
If the two states are of different symmetry then \(V=0\) and crossing is possible in one dimension.
- 7.
For instance by a series expansion.
- 8.
The energy at the intersection point is \(E(\mathbf {Q}^{0})=E^{a}(\mathbf {Q}^{0})=E^{d}(\mathbf {Q}^{0})\). Furthermore the sum of the diagonal elements (the trace) is invariant \(E_{1}^{a}+E_{2}^{a}=E_{1}^{d}+E_{2}^{d}=E_{1}+E_{2}\).
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Problems
Problems
21.1
Crude Adiabatic Model
Consider the crossing of two electronic states along a coordinate Q. As basis functions we use two coordinate independent electronic wavefunctions which diagonalize the Born–Oppenheimer Hamiltonian at the crossing point \(Q_{0}\)
Use the following ansatz functions
which can be written in more compact form
The Hamiltonian is partitioned as
Calculate the matrix elements of the Hamiltonian
where \(\chi (Q)\) and \(\zeta (Q)\) depend on the coordinate Q whereas the basis functions \(\varphi ^{1,2}\) do not. Chose \(\zeta (Q)\) such that \(T_{el}+V\) is diagonalized. Evaluate the nonadiabatic interaction terms at the crossing point \(Q_{0}\).
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Scherer, P.O.J., Fischer, S.F. (2017). Crossing of Two Electronic States. In: Theoretical Molecular Biophysics. Biological and Medical Physics, Biomedical Engineering. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-55671-9_21
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DOI: https://doi.org/10.1007/978-3-662-55671-9_21
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