Time resolved dynamics of phonons and rotons in solid parahydrogen

Abstract

We use femtosecond optical Kerr effect (OKE) spectroscopy to perform time- and wavelength-resolved pump-probe measurements on the energetics and lifetimes of transverse optical phonons and J = 2 rotons in solid para-hydrogen (pH2). By systematically studying the OKE spectroscopy of pH2 in the gas, liquid, and solid phases for delay times up to 300 ps, we can disentangle homodyne and heterodyne contributions in the solid to the signal that results from the slow phonon (900 fs) and fast roton (94 fs) dynamics. In solid pH2 at 8.5 K, the energies of the J = 2 Raman-active rotons are measured to be 351.98(8) cm(-1), 353.99(8) cm(-1), and 356.00(8) cm(-1) corresponding to the crystal field split MJ = ±1, ±2, and 0 substates. Consistent with the picture of quasi-free molecular rotation within the solid, we observe long-lived roton coherences with T2 lifetimes of 132(5), 114(5), and 82(5) ps for the MJ = ±1, ±2, and 0 substates. In contrast, similar measurements on normal-hydrogen (nH2) solids which nominally contain 75% ortho-hydrogen (oH2) and 25% pH2 molecules display qualitatively different roton dynamics; no persistent roton excitations are observed but rather overdamped librational excitations that decay within 3 ps. The measured low temperature T2 dephasing time of gaseous pH2 implies a collision cross section of 2.08(10) Å(2) which is close to the theoretical value for so-called elastic resonant collisions whereby rotational energy is exchanged between the two colliding partners, but the sum of the rotational energies is preserved. We argue that this same collisional process also determines the T2 dephasing lifetimes in the liquid and solid phases. Finally, OKE spectroscopy on pH2 solids with oH2 concentrations of 1-3% shows evidence for extremely long-lived rotational coherences which likely correspond to J = 2 rotons that are pinned next to single oH2 impurities within the pH2 solid.

Excitation of rotons in parahydrogen crystals: The laser-induced-molecular-alignment mechanism

“…A closer examination with higher resolution revealed a splitting into |M| = 0, 1, 2-state substructure, where three components with 93.7, 94.2, and 94.8 fs oscillation periods can be resolved along with their mutual quantum beats. 29,30 The refined experiments also reported extended dephasing times T 2 on the order of 100 ps -a value which is controlled by the purity with respect to orthohydrogen concentration. 30,31 The extent of delocalization for the roton-type excitation in the solid parahydrogen is another interesting point accessible to evaluation by the impurity dependence.…”

“…29,30 The refined experiments also reported extended dephasing times T 2 on the order of 100 ps -a value which is controlled by the purity with respect to orthohydrogen concentration. 30,31 The extent of delocalization for the roton-type excitation in the solid parahydrogen is another interesting point accessible to evaluation by the impurity dependence. 30 Here, we report quantum simulations together with an illustrative experimental signal.…”

“…Theoretical foundations of solid pH 2 are well known 28 and rotational Raman spectra of the solid have also been thoroughly considered. [32][33][34] Although the transverse optical phonon part of the signal is a very prominent feature in the experiments, 27,29,30 it is left out from the discussion by assuming negligible interplay of phonon and roton dynamics. These elementary excitations have a tenfold energy mismatch and a fivefold difference in dephasing time.…”

“…These elementary excitations have a tenfold energy mismatch and a fivefold difference in dephasing time. 30 Phonon sidebands are only a very weak feature in rotational Raman spectra, and rotational excitations are strongly decoupled from intramolecular vibrations, too. 35 Furthermore, roton lifetimes are independent of temperature, which eliminates possible influence of lattice vibrations.…”

“…30,31 The extent of delocalization for the roton-type excitation in the solid parahydrogen is another interesting point accessible to evaluation by the impurity dependence. 30 Here, we report quantum simulations together with an illustrative experimental signal.…”