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. In a recent article, the authors 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. They demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for ionized molecular hydrogen and its isotopes in full dimensionality. By means of a simple analytical model, they 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. The 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.
Even harmonic generation in isotropic media of dissociating homonuclear molecules
R. E. F. Silva and P. Rivière and F. Morales and O. Smirnova and M. Ivanov and F. Martín
Scientific Reports, 6, 32653, 2016 (September 6, 2016)