New Experimental and Theoretical Results for Argon Broadening and Shift of HCO+ Rotational Lines

An experimental and theoretical study of the pressure broadening and the pressure shift of three HCO+ rotational lines (j=4-3, 5-4 and 6-5) perturbed by collisions with Ar is presented. The measurements are carried out at 77 K and are compared to close-coupling calculations performed on an accurate potential energy surface for the Ar-HCO+ interaction extending from small to very large separations between the ion and the perturber. For the pressure broadening, agreement between experiment and theory is satisfactory for both close-coupling and semiclassical calculations. For the pressure shift, however, close-coupling calculations are superior. The results agree with experiment in sign and order of magnitude, while semiclassical calculations are inaccurate for the shift of the presently studied lines because they neglect the contribution of strong collisions.