The HeH⁺ isotopologues in intense asymmetric laser fields

Abstract

In this thesis, the effects of intense laser pulses on molecular dynamics are studied at the example of the helium hydride molecular ion HeH⁺. It serves as a benchmark system for asymmetric molecules because it has features such as an asymmetric mass distribution, a permanent dipole moment and a rich structure of electronic levels while still being relatively simple and easy to model. In order to carry out numerical simulations in high accuracy, we first develop a reduced-dimensional model system for HeH⁺ that still reproduces crucial real-world data. This quantum-mechanical non-Born-Oppenheimer model is then used in time-dependent Schrödinger-equation calculations to study various effects:

The ionization (electron removal) and subsequent dissociation of HeH⁺ are studied in laser fields of 800 nm and 400 nm. Enhanced ionization at a certain internuclear distance as well as excitation of vibrational motion—if possible—have significant effects on the molecular dynamics and the ionization probability.

Breaking the molecule into He + H⁺ (ground-state dissociation) or He⁺ + H⁺ + 𝑒¯ (ionization) are two prototypical, very simple chemical reactions. By means of collinearly polarized two-color fields which have a spatial asymmetry, we show that it is possible to switch from one fragmentation channel to the other one just using the relative two-color delay as a control knob.

Finally, molecular dynamics depends on the choice of isotopologue, i. e. the nuclear masses. The reduced mass of the nuclei has an obvious and very important effect on the time scale of vibrational motion and the available HeH⁺ isotopologues allow to study this. However, also the mass distribution within the molecule influences the dynamics, especially in molecular ions where the dipole moment depends on the nuclear center of mass.

The quantum-mechanical non-Born-Oppenheimer calculations are supported by a multi-level Born-Oppenheimer model and by classical-trajectory calculations where appropriate. We compare some of our results to measurement data and suggest feasible experimental parameters for other of our findings where there are no measured results yet.