Phonon-assisted ortho-para conversion in solid H-2 under pressure

A version of the Jacobi matrix method is employed to calculate, in the harm onic approximation, certain spectral densities which are involved in the ev aluation of conversion rates in solid hydrogen at elevated pressures. The e lements of the dynamical matrix used in spectral density calculations were reconstructed from measured (acoustic and Brillouin scattering) and calcula ted elastic moduli cu. Double conversion (i.e., the process when both inter acting ortho molecules become para) is shown to bring an appreciable contri bution (comparable to that of single conversion) at pressures above 250 MPa (2.5 kbar). Other factors that are important at high pressures have been t aken into account, in particular, the compression-related decrease in conve rsion energy caused by the increasing negative offset due to the electrosta tic quadrupole-quadrupole interaction between ortho molecules. The conversi on rates calculated as a function of pressure are in good agreement with ex periment at moderate pressures. (C) 2001 Elsevier Science BY. All rights re served.