# The Observed Vibron Shift in Helium-Hydrogen Mixtures are due to Quantum Nuclear and Localisation Effects, not bonding

## TL;DR

This study shows that the observed vibron shift in helium-hydrogen mixtures is caused by quantum nuclear effects and local environment variations, not chemical bonding, as demonstrated through DFT simulations.

## Contribution

The paper reveals that the vibron frequency increase is due to environmental effects and quantum nuclear phenomena, not He-H chemical bonding, challenging previous assumptions.

## Key findings

- Vibron shift is due to the number of H₂ molecules in the mode.
- No chemical bonding exists between He and H₂.
- Quantum nuclear effects dominate the pressure in mixtures.

## Abstract

The vibrational frequency of hydrogen molecules has been observed to increase strongly with He concentration in helium hydrogen fluid mixtures. This has been associated with He-H interactions, either directly through chemical bonding, or indirectly through increased local pressure. Here, we demonstrate that the increase in the Raman frequency of the hydrogen molecule vibron is due to the number of H$_2$ molecules participating in the mode. There is no chemical bonding between He and H$_2$, helium acts only to separate the molecules. The variety of possible environments for H$_2$ gives rise to many Raman active modes, which causes broadening the vibron band. As the Raman active modes tend to be the lower frequency vibrons, these effects work together to produce the majority of the shift seen in experiment. We used Density Functional Theory (DFT) methods in both solid and fluid phases to demonstrate this effect. DFT also reveals that the pressure in these H$_2$-He mixture is primarily due to quantum nuclear effects, again the weak chemical bonding makes it a secondary effect.