Quantum Reorientational Excitations in the Raman Spectrum of Hydrogen

TL;DR

This study investigates low-frequency Raman peaks in hydrogen under high pressure and low temperature, attributing them to reorientational excitations and analyzing their behavior across different phases and conditions.

Contribution

It provides new insights into reorientational excitations in hydrogen, linking Raman spectral features to intermolecular interactions and phase transitions.

Findings

Reorientational transitions follow pressure, temperature, and ortho-H2 concentration.

A broad continuum appears at higher temperatures due to higher J-states.

In phase III, the continuum is quenched, revealing optical phonons.

Abstract

Low-frequency Raman peaks, below 250 cm-1, are observed in hydrogen between 2-174 GPa and 13-300 K. The origin of these features is attributed to reorientational transitions (DeltaJ = 0; Q0-branch), which shift from the Rayleigh line as anisotropic intermolecular interactions lift the mJ degeneracy. This family of excitations closely follows the behavior of the S0-branches, sharing their dependence on pressure, temperature, and ortho-H2 concentration. Above 65 K, spectra corrected by the Bose-Einstein population factor reveal a broad continuum arising from populated higher J-states and increased ortho-para disorder. Upon entering phase III, where molecular rotation is inhibited, this continuum is quenched, giving way to well-established optical phonons. Below 25 K, equilibrated samples demonstrate a fine structure from isolated and pair excitations from impurity ortho-H2 molecules in a parahydrogen lattice, the latter a sensitive probe to anisotropic intermolecular interactions relevant to the quantum modeling of solid H2.