Measurement of the ortho-to-para hydrogen conversion in the high-pressure compound Ar(H 2 ) 2

Ortho-para hydrogen conversion kinetics has been measured in the stoichiometric Ar(H 2 ) 2 compound, at pressures from 3 to 35 GPa, by analysing the intensity of the rotational Raman transitions. The validity of this method to measure ortho-para composition for high-pressure solids is discussed, and verified for the pressures used in this work. The conversion rate increases with pressure of almost a factor of 50 in the investigated pressure range, but always remains lower than that measured for pure solid hydrogen at the same pressure. The different crystal structure, in particular the lower number of nearest neighbours, plays a significant role in this difference.