Theoretical description of adiabatic laser alignment and mixed-field orientation: the need for a non-adiabatic model

TL;DR

This paper develops a non-adiabatic theoretical model to accurately describe laser alignment and mixed-field orientation of asymmetric top molecules, highlighting the limitations of purely adiabatic approaches in reproducing experimental results.

Contribution

The authors introduce a diabatic model that accounts for non-adiabatic population transfer, improving the understanding of molecular orientation dynamics under combined laser and electrostatic fields.

Findings

Pure alignment matches adiabatic simulations.

Non-adiabatic effects are crucial for accurate orientation modeling.

Diabatic model aligns well with experimental data.

Abstract

We present a theoretical study of recent laser-alignment and mixed-field-orientation experiments of asymmetric top molecules. In these experiments, pendular states were created using linearly polarized strong ac electric fields from pulsed lasers in combination with weak electrostatic fields. We compare the outcome of our calculations with experimental results obtained for the prototypical large molecule benzonitrile (C$_7$H$_5$N) [J.L. Hansen et al, Phys. Rev. A, 83, 023406 (2011)] and explore the directional properties of the molecular ensemble for several field configurations, i.e., for various field strengths and angles between ac and dc fields. For perpendicular fields one obtains pure alignment, which is well reproduced by the simulations. For tilted fields, we show that a fully adiabatic description of the process does not reproduce the experimentally observed orientation, and it is mandatory to use a diabatic model for population transfer between rotational states. We develop such a model and compare its outcome to the experimental data confirming the importance of non-adiabatic processes in the field-dressed molecular dynamics.