Permanent and interaction-induced far-infrared spectra of CO in dense Ar: a molecular dynamics approach

Permanent and electric multipole induced contributions to the far-infrared absorption spectrum of CO in Ar for dense gas and liquid densities have been calculated by means of classical molecular dynamics simulations. The comparison of the simulated spectra with experiments let us to give an estimation of some multipole moments of CO: quadrupole, octupole and hexadecapole. Although the experimental profiles lack of a fine rotational structure, the results of the simulations and their comparison with previous theoretical works, indicate that at low temperatures, the consideration of a quantum time correlation function for the dipole moment is necessary to get a good agreement with experiments. Finally, it is shown that the permanent-induced and induced-induced cross terms in the absorption coefficient, that usually are difficult to calculate from a theoretical viewpoint, are not very relevant for this system.