Broadening and shifts of the lines of molecular ions by collisions with neutral perturbers

Abstract

We present a theoretical model for studying the shape of the rotational lines of molecular ions perturbed by collisions with neutral species. This model shows that the interaction between the ionic monopole and the multipole moments of the perturber has no role in the collisional relaxation of the lines. The effect of the ionic monopole is restricted to that of inducing a polarization of the neutral perturbers that can cause relaxation by interaction with the other permanent moments of the absorbing ion. We also present, for ${\mathrm{HCO}}^{+}$ perturbed by argon, an experimental study of the pressure broadening and shift of the rotational lines J=1←0, 2←1, and 4←3. The agreement between theory and experiment is satisfactory.