Activation of weak IR fundamentals of two species of astrochemical interest in the T_d point group - The importance of amorphous ices

New measurements are reported on the weak ν₁ and ν₂ fundamentals of frozen CH₄, a solid of considerable astrochemical interest. Infrared spectra in the ν₁ and ν₂ regions are presented for three CH₄-ice phases at 10-30 K with new absorption coefficients and band strengths to quantify the results. In contrast to the situation with the two crystalline phases of CH₄, both ν₁ and ν₂ were seen clearly in methane's amorphous phase. To support our CH₄ work, we also present new results for NH₄SH, a component of Jupiter's atmosphere, showing that the ν₂ vibration of NH₄⁺ undergoes a dramatic loss of intensity during an amorphous-to-crystalline phase transition, but is regenerated in equally-dramatic fashion by radiation-induced amorphization of the sample. Results are compared to work recently published in this journal and elsewhere.