Role of phonon coupling in the broadening of rotational lines of diatomics trapped in rare gas matrices. A model calculation for HCl in Ar

The near infrared spectra of HCl trapped in pure and N2-doped Ar matrices at low temperature are reported. In pure Ar, a hindered rotational spectrum is obtained and the R(0) line exhibits a strong temperature dependence. The ’’Q’’ feature observed in the N2-doped experiment is rather insensitive to temperature. The temperature dependence of the rotational line is interpreted on the basis of a rotational-phonon coupling model. The absorption coefficient is calculated within the framework of the linear response theory applied to the case of a diatomic molecule embedded in a linear chain of matrix atoms. The calculated full-width at half-maximum is in good agreement with the observed trend.