Even harmonic generation in isotropic media of dissociating homonuclear molecules

TL;DR

This paper demonstrates that in isotropic dissociating homonuclear molecules, the interplay of electronic and nuclear dynamics can produce even harmonics, challenging the standard rule that only odd harmonics are generated.

Contribution

It reveals how light-induced charge localization in dissociating molecules leads to even harmonic generation, extending nonlinear optics understanding.

Findings

Even harmonics can dominate in dissociating homonuclear molecules.

The effect depends on laser pulse duration and molecular dissociation dynamics.

A simple model explains the physical origin of the even harmonics.

Abstract

Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schr\"odinger equation for $H$$_2$$^+$ and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.