Raman signal from a hindered hydrogen rotor

Peter I. C. Cooke, Ioan B. Magdǎu, Miriam Peña-Alvarez, Veronika Afonina, Philip Dalladay-Simpson, Xiao-Di Liu, Ross T. Howie, Eugene Gregoryanz, Graeme J. Ackland

Research output: Contribution to journalArticlepeer-review

Abstract

We present a method for calculation of Raman modes of the quantum solid phase I hydrogen and deuterium. We use the mean-field assumption that the quantized excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarization. We show that in the free rotor limit, the H2 and D2 frequencies differ by a factor of 2, which evolves toward √2 as the modes acquire librational character due to stronger interactions. The ratio overshoots √2 if anharmonic terms weaken the harmonic potential. We also use density functional theory and molecular dynamics to calculate the E2g optical phonon frequency and the Raman linewidths. The molecular dynamics shows that the molecules are not free rotors except at very low pressure and high temperature, and become like oscillators as phase II is approached. We fit the interaction strengths to experimental frequencies, but good agreement for intensities requires us to also include strong preferred orientation and stimulated Raman effects between S0(1) and S0(0) contributions. The experimental Raman spectrum for phase II cannot be reproduced, suggesting that the mean-field assumption is invalid in that case.
Original languageEnglish
Article number064102
JournalPhysical Review B
Volume102
Issue number6
DOIs
Publication statusPublished - 1 Aug 2020

Keywords

  • cond-mat.mtrl-sci

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