Theory of ortho-para conversion in solid hydrogen at high densities

An expression is derived for the ortho-para conversion rate, as a function of density ρ, of solid ortho-H₂ in its rotationally disordered state. For ρ greater than about 1.1 times the zero-pressure density ρ₀, conversion is shown to result from a one-phonon process. This rate is proportional to (ρρ₀)^103-2γ, where γ is the Gr\"uneisen constant for solid H₂, times a phonon spectral function which depends on the density as a parameter. Using the data of Pedroni et al. for the density dependence of the conversion rate for 1.0≤ρρ₀≤1.7, we extract the phonon spectral function and compare it to a theoretical spectrum calculated in a simple harmonic approximation.