Experimental and Theoretical Study of Helium Broadening and Shift of HCO+ Rotational Lines

An experimental and theoretical study of pressure broadening and pressure shift of HCO+ rotational lines perturbed by collisions with He is presented. Results are reported from measurements at 88 K for the lines j=4 !3, 5 !4 and 6 !5 with frequencies ranging from 0.35 to 0.54 THz. Using a new CCSD(T)/aug-ccpVQZ potential energy surface for the He-HCO+ interaction, the collisional line shape parameters are studied from fully quantum and semiclassical calculations. Results from the quantum treatment are in satisfactory agreement with experiments whereas the semiclassical approach can lead to appreciable differences. A study of the dependence of line width G and shift s as a function of the translational energy shows the presence of quantum oscillations. Calculations on a previous Hartree-Fock-based potential energy surface lead to quite similar results for the collisional line shape parameters. Using a simplified version of the potential morphing method it is found that the line width G is particularly sensitive to the long-range part of the potential energy surface. This also explains the success of the first line-broadening calculations which date back to the 1950s.