Velocity and force correlations in H2–He mixtures

The fundamental vibrational band of the pressure-induced infrared spectrum of hydrogen in room-temperature helium gas (compressed to twice the density of the normal liquid) is analyzed to determine the force autocorrelation function and, hence, the velocity autocorrelation function and the mean square displacement of a hydrogen molecule as a function of time. The initial curvature of the force autocorrelation function, extrapolated to zero density, yields a value 0.087 for the ratio ρ/σ where ρ is the range of the repulsive core of the intermolecular potential and σ is the diameter of this core. Moment relations, which enable one to determine the parameters in a model introduced recently by Van Kranen-donk, are derived for the force autocorrelation function.