# Quantum mechanical study of high-order harmonic generation from HeH$^{2+}$ molecule in homogeneous and plasmonic-enhanced laser Fields

## TL;DR

This study theoretically explores high-order harmonic generation from HeH$^{2+}$ molecules in different initial states under homogeneous and plasmonic-enhanced laser fields, revealing orientation effects and the influence of plasmonic fields on HHG yield and cutoff.

## Contribution

It introduces a detailed analysis of HHG from HeH$^{2+}$ in various initial states and the impact of plasmonic-enhanced fields on HHG spectra, including cutoff extension.

## Key findings

- Enhanced HHG yield in the first excited state due to orientation effects.
- Plasmonic fields increase HHG cutoff and yield when polarized normal to the molecular axis.
- Longer excited state lifetime contributes to higher harmonic emission.

## Abstract

High-order harmonic generation (HHG) from molecular ion HeH$^{2+}$ in two initial states of $1s\sigma$ (ground state) and $2p\sigma$ (first excited state) is investigated theoretically, in homogeneous and plasmonic-enhanced laser fields. The electron ionization and the enhancement of HHG yield in ground and first excited states, as the initial states, is studied and a strong orientation effect for ionization of electron in $2p\sigma$ state is discovered, that it can be used to reach a longer lifetime for the electron in the first excited state, which is responsible for the enhancement in HHG yield, during the high harmonic emission. In order to investigate effect of plasmonic field on the cutoff position of HHG spectra, first the enhancement of laser field in a nano-antennae is calculated and then the interaction of the enhanced field with HeH$^{2+}$ molecule is studied by solving the time-dependent Schr\"{o}dinger equation. According to the results, when HeH$^{2+}$ molecule, in the initial state of $2p\sigma$, is irradiated by plasmonic polarized laser field, with the polarization normal to the molecular axis, the HHG yield and cutoff position enhance while the excited state has a long lifetime during the HHG process.