Theory and ab initio calculations of 2p photoabsorption spectra:: The lowest Rydberg resonances in HCl

We present a theory for 2p core excitation spectra of linear molecules that explicitly includes the spin-orbit splitting of the 2p core orbitals as well as all nonrelativistic effects. This is applied to the absorption spectrum of HCl at the 2p --> 6 sigma(*), 4s, 4p, 3d, and 5s resonance energies. All input data for the spectrum are obtained from ab initio calculations. These are (i) the nonrelativistic energies and transition probabilities of the contributing states, (ii) the spin-orbit coupling parameter, and (iii) the total Auger transition rates of the core excited states. The prior theoretical finding that the total Auger decay rate depends substantially on the core hole orientation is supported by a comparison of the theoretical and experimental absorption spectra. Furthermore, the preferred orientation of the core excited HCl molecule with respect to the molecular bond axis is discussed. General considerations about intensity ratios between corresponding states with 2p(3/2) and 2p(1/2) holes are given on the basis of the theoretical framework. (C) 1999 American Institute of Physics. [S0021-9606(99)30438-4].