Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N$_{2}$ molecules}

TL;DR

This study investigates how laser intensity and molecular alignment affect high-order harmonic generation in N₂ molecules, revealing orbital contributions and shape resonance effects through experiments and simulations.

Contribution

It provides the first combined experimental and theoretical analysis of intensity-dependent orbital contributions and shape resonance in HHG of aligned N₂ molecules.

Findings

HHG spectral minima shift and disappear with increasing laser intensity

Inner orbital contributions influence the HHG spectra

Shape resonance is predicted to exist and varies with laser alignment angle

Abstract

We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N$_2$ molecules using a 1200-nm intense laser field when the generating pulse is perpendicular to the aligning one. With increasing laser intensity, the minimum in the HHG spectra first shifts its position and then disappears. Theoretical simulations including the macroscopic propagation effects in the medium reproduce these observations and the disappearance of the minimum is attributed to the additional contribution of HHG from inner orbitals. We also predict that the well-known shape resonance in the photoionization spectra of N$_2$ should exist in the HHG spectra. It is most clearly seen when the generating laser is parallel to the aligning one, and disappears gradually as the angle between the two lasers increases. No clear evidence of this shape resonance has been reported so far when using lasers with different wavelengths. Further experimentation is needed to draw conclusions.