Raman signal from a hindered hydrogen rotor
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 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.
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].Attached Files
Published - PhysRevB.102.064102.pdf
Submitted - 2004.04449.pdf
Supplemental Material - Supp.pdf
Files
Additional details
- Alternative title
- The raman signal of a hindered rotor
- Eprint ID
- 104779
- Resolver ID
- CaltechAUTHORS:20200806-132312028
- 695527
- European Research Council (ERC)
- Royal Society
- P022561
- Engineering and Physical Sciences Research Council (EPSRC)
- Created
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2020-08-06Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field