Methane clathrate hydrate infrared spectrum II. Near-infrared overtones, combination modes and cages assignments

Context. Recently, we recorded the infrared spectrum of the methane clathrate hydrate stretching mode at low temperature, a caged compound of possible interest for solar system studies as well as interstellar ice mantles. Aims. We provide a practical infrared spectroscopic identification for methane clathrate hydrate to examine its astrophysical presence or absence. We investigate the crystal field induced shifts, and assign the different transitions to the different encaged molecules environments in this clathrate hydrate. Methods. A methane clathrate crystal is produced in an infrared transmitting moderate-pressure closed cell. Using Fourier transform infrared (FTIR) spectroscopy, the overtones (3v(4), 2v(3)) and combination modes (v(2) + 2v(4), v(1) + v(4), v(3) + v(4), v(2) + v(3), v(3) + 2v(4), 2v(2) + 2v(4), v(2) + v(3) + v(4)) falling in the 6000-3000 cm(-1) (similar to 1.65-3.4 mu m) and their temperature behaviour are investigated. In addition, non-astrophysical CH(4)/CF(4) gas mixtures are used to build clathrates with different methane large and small cage occupancies to help in assignments. Results. Combination modes show the two distinct cages and the quasi-free rotor low temperature ro-vibrational structure expected for methane clathrate hydrates. A comparison with the pure phase I is performed. Implications for methane clathrate hydrate detection are clearly identified. Conclusions. Solid methane actual remote observations of solar system objects surfaces do not display the clathrate hydrates' specific shift and occupancy signatures. Observationnally, a search for their infrared spectroscopic specific signatures should be performed, focusing on thermodynamically favourable objects like trans- neptunian objects (TNOs) or recently exposed (e.g. fresh impact) planet (or their satellites) surfaces. On the modeling side, efforts must be undertaken to progressively implement clathrate formation kinetics.