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Time resolved dynamics of phonons and rotons in solid parahydrogen
Falk Königsmann1, Nikolaus Schwentner, David T Anderson
1Institut für Experimentalphysik, Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Germany.
Physical Chemistry Chemical Physics : PCCP
|September 12, 2013
Summary
Femtosecond optical Kerr effect spectroscopy reveals long-lived roton coherences in solid para-hydrogen (pH2). These dynamics are influenced by molecular interactions and impurities, offering insights into quantum states in solid hydrogen.
Area of Science:
- Condensed Matter Physics
- Quantum Dynamics
- Spectroscopy
Background:
- Understanding quantum phenomena in solid hydrogen is crucial for fundamental physics.
- Rotational dynamics and phonon interactions govern the properties of solid hydrogen.
- Previous studies lacked detailed insights into roton lifetimes and phase coherence.
Purpose of the Study:
- To investigate the energetics and lifetimes of transverse optical phonons and J=2 rotons in solid para-hydrogen (pH2).
- To analyze the influence of molecular interactions and impurities on roton dynamics.
- To elucidate the mechanisms of dephasing in different phases of hydrogen.
Main Methods:
- Femtosecond optical Kerr effect (OKE) spectroscopy was employed for time- and wavelength-resolved pump-probe measurements.
- Measurements were conducted on pH2 in gas, liquid, and solid phases up to 300 ps delay times.
- OKE spectroscopy was also performed on normal-hydrogen (nH2) solids and pH2 with ortho-hydrogen (oH2) impurities.
Main Results:
- In solid pH2, long-lived roton coherences with T2 lifetimes up to 132 ps were observed for J=2 roton substates.
- Normal-hydrogen solids exhibited overdamped librational excitations decaying within 3 ps, unlike pH2.
- Gaseous pH2 dephasing implies a collision cross section consistent with elastic resonant collisions, a mechanism proposed for liquid and solid phases.
Conclusions:
- Solid para-hydrogen supports long-lived rotational coherences (rotons), indicating quasi-free molecular rotation.
- Ortho-hydrogen impurities significantly alter roton dynamics, suggesting localized effects.
- Elastic resonant collisions are identified as a key mechanism for roton dephasing across different hydrogen phases.
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