Mid-infrared spectra of methane dispersed in solid neon and argon

We have recorded, in absorption, infrared spectra of samples of methane, CH4 and CD4, dispersed at molar fraction 0.0001–0.005 in solid neon, in solid argon and in their equimolar solid mixture in the temperature range 3–21 K and in the spectral domain 5000–500 cm−1 at resolution 0.04–0.2 cm−1. We undertook quantitative fitting of the spectral profiles with components of gaussian and lorentzian shapes. Comparison of our spectra in regions of fundamental modes both ν3 and ν4 with published spectra of CH4 and CD4 in either crystalline para-dihydrogen or droplets of liquid helium indicates evidence for hindered rotation of CH4 molecules in Ar but not for rotation of CH4 or CD4 in Ne or in a mixture of Ne and Ar. For CD4 in solid Ar, the evidence for even hindered rotation is ambiguous. We make new assignments of lines to 13CH4 and 13CD4 in their environments in solid Ne and Ar.