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Raman signal from a hindered hydrogen rotor

Cooke, Peter I. C. and Magdău, Ioan B. and Peña-Alvarez, Miriam and Afonina, Veronika and Dalladay-Simpson, Philip and Liu, Xiao-Di and Howie, Ross T. and Gregoryanz, Eugene and Ackland, Graeme J. (2020) Raman signal from a hindered hydrogen rotor. Physical Review B, 102 (6). Art. No. 064102. ISSN 2469-9950.

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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 E_(2g) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Cooke, Peter I. C.0000-0002-9977-9194
Magdău, Ioan B.0000-0002-3963-5076
Peña-Alvarez, Miriam0000-0001-7056-7158
Liu, Xiao-Di0000-0002-0067-7244
Gregoryanz, Eugene0000-0001-9080-8756
Ackland, Graeme J.0000-0002-1205-7675
Alternate Title:The raman signal of a hindered rotor
Additional Information:© 2020 American Physical Society. Received 23 July 2019; accepted 16 April 2020; published 5 August 2020. M.P.-A., G.J.A., and E.G. acknowledge the support of the European Research Council Grant Hecate Reference No. 695527. G.J.A. acknowledges a Royal Society Wolfson fellowship. EPSRC funded studentships for P.I.C.C., I.B.M., V.A., and computing time (UKCP Grant No. P022561). We would like to thank A. Dhingra for discussions about this work as part of her Masters thesis. We thank G. Collins for drawing our attention to the microwave data [34,35].
Funding AgencyGrant Number
European Research Council (ERC)695527
Engineering and Physical Sciences Research Council (EPSRC)P022561
Issue or Number:6
Record Number:CaltechAUTHORS:20200806-132312028
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104779
Deposited By: Tony Diaz
Deposited On:06 Aug 2020 20:38
Last Modified:06 Aug 2020 20:38

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