Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II

Nuclear-spin conversion analysis of ν2 + ν4 combination band of crystalline methane in phase II

We conducted reflection-absorption infrared spectroscopy in an ultrahigh vacuum and observed the ν₂ + ν₄ combination band of crystalline methane in phase II, where the rotating and librating species coexist. We analyzed the time- and temperature-dependence of the spectrum due to the nuclear-spin conversion of methane to update the assignment of the rotational and librational structure of this band. The conversion analysis performed in the present work will also be applicable to the detailed assignments of overtones and other combination bands.

Infrared Spectroscopic Study of Solid Methane: Nuclear Spin Conversion of Stable and Metastable Phases

Infrared spectroscopy was employed to study thin films of solid methane at low temperatures. We report new measurements of temporal changes of infrared spectra of methane ice in the ν₃ and ν₄ bands due to nuclear spin conversion upon rapid cooling from 30 to 6.0-11.0 K. The relaxation rates of the nuclear spin were found to be a function of temperature. The activation energy associated with the relaxation has been determined over an extended temperature range. We also found a new metastable phase of methane ice upon deposition at T < 7 K. After the deposition at 6 K and annealed to a higher temperature, a phase transition from the metastable phase to a stable crystalline phase takes place. We found that the relaxation has different activation energies below and above 8.5 K. From a quantitative analysis of the ν₃ and ν₄ IR bands, we suggest that the metastable phase is a crystalline phase with a degree of orientational disorder between the two known stable solid phases.