A theory of intense-field dynamic alignment and high harmonic generation from coherently rotating molecules and interpretation of intense-field ultrafast pump-probe experiments

TL;DR

This paper develops a comprehensive analytical theory for intense-field dynamic alignment and high harmonic generation in coherently rotating molecules, successfully explaining experimental observations and predicting a 'magic' polarization angle linked to molecular orbital symmetry.

Contribution

It provides a fully developed analytical framework for HHG and dynamic alignment, connecting experimental data with molecular symmetry and predicting new phenomena.

Findings

Analytical results match experimental HHG data from nitrogen and oxygen.

Identification of a 'magic' polarization angle where harmonic modulation ceases.

Correlation between polarization angles and molecular orbital symmetry.

Abstract

A theory of ultra-fast pump-probe experiments proposed by us earlier [F.H.M. Faisal et al., Phys. Rev. Lett. 98, 143001 (2007) and F.H.M. Faisal and A. Abdurrouf, Phys. Rev. Lett. 100, 123005 (2008)] is developed here fully and applied to investigate the phenomena of dynamic alignment and high harmonic generation (HHG) from coherently rotating linear molecules. The theory provides essentially analytical results for the signals that allow us to investigate the simultaneous dependence of the HHG signals on the two externally available control parameters, namely, the relative angle between the polarizations, and the delay-time between the two pulses. It is applied to investigate the characteristics of high harmonic emission from nitrogen and oxygen molecules that have been observed experimentally in a number of laboratories. The results obtained both in the time-domain and in the frequency-domain are compared with the observed characteristics as well as directly with the data and are found to agree remarkably well. In addition we have predicted the existence of a "magic" polarization angle at which all modulations of the harmonic emission from nitrogen molecule changes to a steady emission at the harmonic frequency. Among other things we have also shown a correlation between the existence of the "magic" or critical polarization angles and the symmetry of the active molecular orbitals, that is deemed to be useful in connection with the "inverse problem" of molecular imaging from the HHG data.