MOLECULAR PHOTOIONIZATION CROSS-SECTIONS BY THE LOBATTO TECHNIQUE .1.VALENCE PHOTOIONIZATION

A method for the calculation of electronic continuum wave functions is presented which is based on the logarithmic derivative version of the Kohn (LDK) variational principle. The variational principle is cast i nto algebraic form by introducing a finite basis set that consists of spherical harmonic Gaussian-type functions (GTOs) and of Lobatto shape functions with the latter representing the translational part of the basis. A local effective potential which is obtained from density func tional theory results in fairly accurate photoionization cross section s. Also studied are asymptotic corrections to the effective potential for the photoelectron which, in many cases, lead to improved results. The Lobatto procedure is applied to the diatomics N-2 and CO and to be nzene which may be regarded as a prototype for larger non spherical sy mmetric systems for which the method is targeted. For the two diatomic s, results in excellent agreement with experiment have been found. For benzene the results are compared to those obtained by the Stieltjes-T chebychev (ST) imaging technique and by the continuum multiple scatter ing (CMS) method which both have been applied to similar effective loc al potentials. Comparison with the ST imaging technique shows that the LDK Lobatto (LDKL) method provides qualitatively similar results, but the LDKL cross sections are of higher resolution and allow a more det ailed analysis because of the explicit determination of the continuum wave function. For most of the valence orbitals of benzene the CMS met hod does not lead to satisfactory agreement with experiment due to the well-known deficiencies of this technique. The LDKL method implemente d with a combined basis set does not suffer from the limitations of th e ST and the CMS methods, but remains applicable to larger-size molecu les.