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 77K 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 in-accurate for the shift of the presently studied lines because they neglect the contribution of strong collisions.