The vibrational Jahn–Teller effect in E⊗e systems

Nonadiabatic effects in electronic and nuclear dynamics

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.

An extended E⊗e Jahn-Teller Hamiltonian for large-amplitude motion: Application to vibrational conical intersections in CH3SH and CH3OH

An extended E⊗e Jahn-Teller Hamiltonian is presented for the case where the (slow) nuclear motion extends far from the symmetry point and may be described approximately as motion on a sphere. Rather than the traditional power series expansion in the displacement from the C_3v symmetry point, an expansion in the spherical harmonics is employed. Application is made to the vibrational Jahn-Teller effect in CH₃XH, with X = S, O, where the equilibrium CXH angles are 83° and 72°, respectively. In addition to the symmetry-required conical intersection (CI) at the C_3v symmetry point, ab initio calculations reveal sets of six symmetry-allowed vibrational CIs in each molecule. The CIs for each molecule are arranged differently in the large-amplitude space, and that difference is reflected in the infrared spectra. The CIs in CH₃SH are found in both eclipsed and staggered geometries, whereas those for CH₃OH are found only in the eclipsed geometry near the torsional saddle point. This difference between the two molecules is reflected in the respective high-resolution spectra in the CH stretch fundamental region.