Theoretical studies of high-harmonic generation: Effects of symmetry, degeneracy and orientation

TL;DR

This paper uses quantum mechanical models to study how molecular symmetry, degeneracy, and orientation influence high-harmonic generation in polyatomic molecules, revealing effects on harmonic yield and potential for UV attosecond pulse production.

Contribution

It provides a detailed numerical analysis of high-harmonic generation considering symmetry, degeneracy, and orientation effects in polyatomic molecules, highlighting the role of orbital contributions.

Findings

Orbital symmetry affects harmonic yield in ethylene.

Degenerate orbitals contribute coherently to high-harmonic signals in methane.

Molecular orientation influences the generation of high-frequency harmonics in propane and butane.

Abstract

Using a quantum mechanical three-step model we present numerical calculations on the high-harmonic generation from four polyatomic molecules. Ethylene (C$_2$H$_4$) serves as an example where orbital symmetry directly affects the harmonic yield. We treat the case of methane (CH$_4$) to address the high-harmonic generation resulting from a molecule with degenerate orbitals. To this end we illustrate how the single orbital contributions show up in the total high-harmonic signal. This example illustrates the importance of adding coherently amplitude contributions from the individual degenerate orbitals. Finally, we study the high-harmonic generation from propane (C$_3$H$_8$) and butane (C$_4$H$_{10}$). These two molecules, being extended and far from spherical in structure, produce harmonics with non-trivial orientational dependencies. In particular, propane can be oriented so that very high-frequency harmonics are favorized, and thus the molecule contains prospects for the generation of UV attosecond pulses.