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 M_J = ±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 M_J = ±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) Ų 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.