BROADENING AND SHIFTS OF THE LINES OF MOLECULAR-IONS BY COLLISIONS WITH NEUTRAL PERTURBERS

We present a theoretical model for studying the shape of the rotationa l 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 restrict ed to that of inducing a polarization of the neutral perturbers that c an cause relaxation by interaction with the other permanent moments of the absorbing ion. We also present, for HCO+ perturbed by argon, an e xperimental study of the pressure broadening and shift of the rotation al lines J = 1 <-- 0, 2 <-- 1, and 4 <-- 3. The agreement between theo ry and experiment is satisfactory.