Molecular hydrogen in graphite: A path-integral simulation

TL;DR

This study uses path-integral molecular dynamics to explore the behavior, vibrational properties, and isotope effects of molecular hydrogen within graphite layers across a range of temperatures.

Contribution

It provides detailed finite-temperature insights into H_2 in graphite, including vibrational frequencies and orientation dynamics, using a tight-binding potential fitted to DFT calculations.

Findings

H_2 prefers a parallel orientation to graphite sheets at low energy.

Vibrational frequency of H_2 in graphite at 300 K is 3916 cm-1, lower than isolated H_2.

Isotope effects show D_2 has a vibrational frequency of 2816 cm-1.

Abstract

Molecular hydrogen in the bulk of graphite has been studied by path-integral molecular dynamics simulations. Finite-temperature properties of H_2 molecules adsorbed between graphite layers were analyzed in the temperature range from 300 to 900 K. The interatomic interactions were modeled by a tight-binding potential fitted to density-functional calculations. In the lowest-energy position, an H_2 molecule is found to be disposed parallel to the sheets plane. At finite temperatures, the molecule explores other orientations, but its rotation is partially hindered by the adjacent graphite layers. Vibrational frequencies were obtained from a linear-response approach, based on correlations of atom displacements. For the stretching vibration of the molecule, we find at 300 K a frequency omega_s = 3916 cm-1, more than 100 cm-1 lower than the frequency corresponding to an isolated H_2 molecule. Isotope effects have been studied by considering also deuterium and tritium molecules. For D_2 in graphite we obtained omega_s = 2816 cm-1}, i.e., an isotopic ratio omega_s(H) / omega_s(D) = 1.39.