Photoionization cross sections calculation with mixed L2 basis set: STOs plus B-Splines. Results for N2 and C2H2 by KM-RPA method

A new mixed L2 basis set, built from multicenter STOs and single-center radial B-Splines, is proposed to evaluate the differential photoionization cross section of molecules. As a first comparison we report the results for nitrogen and acetylene molecules, whose photoionization cross sections, since they have been the object of very numerous studies both experimentally and theoretically, represent a reliable test. The theoretical approach utilized to evaluate the continuum is based upon the K-Matrix (KM) determination in the interacting channels random phase approximation framework (IC-RPA) and the results obtained with the new basis set are in good agreement with the experiment and with the best calculations available. Thus the mixed basis employed appears very adequate to describe the electronic continuum and the present calculations are helpful in the assignment of some fine details in the low energy portion of the spectrum. In the high energy region, although the rough behaviour agrees with the experiment, the limitations of the KM-RPA method are such as to not account for a more detailed description of the spectrum.