Pure Even Harmonic Generation from Oriented CO in Linearly Polarized Laser Fields

TL;DR

This paper demonstrates the generation of pure even harmonics from oriented CO molecules in linearly polarized laser fields, revealing a new dipole acceleration mechanism related to permanent dipole moments.

Contribution

It is the first to theoretically show pure even harmonic generation from oriented molecules, expanding understanding of harmonic generation mechanisms.

Findings

Pure even harmonics are generated when molecular axis is perpendicular to laser polarization.

The phenomenon is linked to dipole acceleration from permanent dipole moments.

Applicable to systems with permanent dipole moments, including crystals and polyatomic molecules.

Abstract

The first high harmonic spectrum, containing only the odd orders, was observed in experiments 30 years ago (1987). However, a spectrum containing pure even harmonics has never been observed. We investigate the generation of pure even harmonics from oriented CO molecules in linearly polarized laser fields employing the time-dependent density-functional theory. We find that the even harmonics, with no odd orders, are generated with the polarization perpendicular to the laser polarization, when the molecular axis of CO is perpendicular to the laser polarization. Generation of pure even harmonics reveals a type of dipole acceleration originating from the permanent dipole moment. This phenomenon exists in all system with permanent dipole moments, including bulk crystal and polyatomic molecules.