Localisation effects on the Vibron Shifts in Helium-Hydrogen Mixtures

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 bonding1, or indirectly through increased local pressure2. Here, we demonstrate that the increase in the Raman frequency of the hydrogen molecule vibron is due to
the number of H2 molecules participating in the mode. There is no chemical bonding between He and H2, helium acts only to separate the molecules. The variety of possible environments for H2 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 H2-He mixture is primarily due to quantum nuclear effects, again the weak chemical bonding makes it a secondary effect.