Collision Induced Far Infrared Absorption of Solutions of CH4 and CD4 in Liquid Argon

Abstract

The far infrared spectra of CH4 and CD4 dissolved in liquid argon were recorded to probe the dynamics of this liquid state system. Broad bands devoid of any fine structure were observed. Methane does not possess a permanent dipole moment and the observed absorption results from a dipole induced by the octopolar field of the molecule during a collision. For octopole induced rotational transitions the selection rules are ΔJ=0–3. Assuming free rotor energy levels and line strength factors, and a modified Lorentzian band shape, synthetic spectra were generated which agree quite well with the experimental data. The integrated absorption coefficient is much smaller than that predicted for binary collisions. This is expected since binary interactions in the liquid state cannot be isolated from higher order interactions with neighboring atoms. The dipole moment induced during a collision is partially cancelled by many-body interactions so the absorption coefficient in the liquid state is diminished, compared to the low density gas. Time correlation functions were determined from the far infrared data as well as from near infrared and Raman spectra of methane and compared. The quantum mechanical free rotor time correlation functions were calculated and found to agree with the experimental results for times shorter than liquid state collision times. Since the far infrared correlation function is due to an induced dipole moment, it is damped accordingly. We estimate that the duration of collision is ≈ 2 × 10−13sec. The good agreement of the synthetic spectra and free rotor correlation functions with experiment indicate a barrier to rotation that is comparable to the magnitude of rotational energies observed. Hindered rotation in liquids is also discussed in terms of collisional effects.