Low-frequency Raman excitations in phase I of solid H2:: Role of crystal fields -: art. no. 064304

Low-frequency Raman spectra (up to 800 cm(-1)) measured below 18 K are reported for the entire pressure range of phase I of para-hydrogen doped with low fractions of the ortho-modification. In addition to bands corresponding to the E-2g optical phonon and the J=2<--0 rotational transitions, we observed a weak band attributed as S-0(1). These spectra are employed to deduce information on the interactions that play a decisive role in bringing about the transition to the broken symmetry phase Il. A new theoretical analysis is presented to understand the rotational Raman excitations and the role of various pairwise anisotropic interactions between H-2 molecules, in order to explain the pressure dependence of the S-0(0) and S-0(1) bands, we infer that a fourth-harmonic crystal field is present. The frequencies of the S-0(0) and S-0(1) bands are used to reconstruct the respective crystal held parameter.