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.