Astrochemists have long recognized that many astronomical ices exist in an amorphous phase, and not a crystalline one, in both the interstellar medium and the outer solar system. However, amorphous ices of small molecules can present special problems in their formation and characterization and, in fact, it is only in the past year that the IR spectrum of amorphous C2H2 has been published and quantified (Hudson et al., Icarus, 2014). We recently have extended this work to include solid methane, finding that nearly all published IR spectra of CH4 ices at 10 - 30 K are for a crystalline phase, not the amorphous one sometimes claimed or implied. As a terrestrial bonus from these studies of an astronomical ice, we find that the IR spectrum of amorphous methane clearly shows the two weak, and nominally IR-forbidden, fundamental vibrational transitions of the molecule. We have used the IR spectra of our ices to calculate the optical constants, absorption coefficients, and band strengths of all four vibrational fundamentals of amorphous CH4 for the first time. Key results are available in electronic form at http://science.gsfc.nasa.gov/691/cosmicice/constants.html. This work is funded by NASA’s Outer Planets Research program and the NASA Astrobiology Institute.

Activation of two weak IR fundamentals of solid methane: The importance of amorphous ices

250th National Meeting (2015)

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