Ortho-para conversion rate in solid hydrogen under pressure

We report new measurements of the ortho-para conversion rate in solid H₂ at 4.2 K for densities ?₀ ? ? ? 1.7?₀, where ?₀ is the density at zero pressure. The conversion rate shows evidence of two maxima, at ～ 1.3?₀ and at ～ 1.5?₀ respectively. A qualitative discussion of this behavior is presented in terms of the theory of phonon creation during the conversion, and this discussion shows the consistency between the observed maxima and the shape of the phonon spectrum.