We quantitatively determine cross sections for rotational decoherence from the decay of nonadiabatic laser-induced alignment in nitrogen and nitrogen-foreign gas mixtures in a temperature range between 80 K and room temperature. The cross section for rotational decoherence in pure nitrogen decreases from 102 A(2) at 80 K to 48 A(2) at 295 K, leading to long-lived coherences even at high temperatures. Comparison with the broadening of the transition lines of the Raman Q-branch reported in the literature shows that the decay of rotational coherence proceeds at the same rate as rotational depopulation. This is verified also for mixtures of nitrogen with hydrogen, helium, argon, and krypton. We discuss limits posed by a possible J-dependence of the cross sections and strategies for state resolved determination from the time-dependent alignment signal.

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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.