Electronicab initioquantum-defect theory. Low-resolutionH2photoionization spectrum

This paper presents a global ab initio approach, called the electronic ab initio quantum-defect theory (EAQDT), to molecular photoionization spectrum calculations. This method first obtains the ab initio electronic parameters by the hereafter developed electronic iterative-variational eigenchannel approach (EIVEA). Then these electronic parameters are used in the calculation of the final state and the corresponding photoionization cross section by the multichannel quantum-defect theory (MQDT) [Jungen and Atabek, J. Chem. Phys. 66, 5584 (1977)]. In EIVEA, an elaborate $N$-electron correlated final-state wave function is introduced. This wave function allows us to take into account the ionic-core relaxation, many ionic cores and electronic autoionizing interactions. In calculating the transition probability we also use an electron-correlated initial state. Compared with the standard MQDT, the EAQDT is free from the use of semiempirical parameters and therefore has a wider applicability. This method is applied, as a test, to the low-resolution photoionization spectrum of ${\mathrm{H}}_2$. In the energy range between threshold and 23 eV studied here, where electronic autoionization does not take place, we obtain a theoretical cross section which bears comparison with the experimental one above 17.5 eV.