Absolute photoionization cross sections and resonance structure of doubly ionized silicon in the region of the [Formula Presented] threshold: Experiment and theory

We present the absolute photoionization cross section of doubly ionized silicon as a function of photon energy. These were obtained by merging a [Formula Presented] ion beam generated in an electron cyclotron resonance source with monochromatized synchrotron radiation from an undulator. The photoion yield measurements were carried out in the photon energy range between 95 eV and 170 eV, i.e., the region corresponding to the excitation followed by the ionization (threshold [Formula Presented] of an inner-subshell [Formula Presented] electron. Resonance structure due to [Formula Presented] excitation in the [Formula Presented] metastable state was also observed with its contribution to the total cross section not exceeding 3%. Calculation of the [Formula Presented] photoionization continuum cross section as a function of photon energy was carried out using the relativistic random-phase approximation (RRPA) and agreed very well with the corresponding measurements. The resonance structure in the [Formula Presented] cross section below the [Formula Presented] threshold was found to be in good agreement with the multiconfiguration atomic structure calculations of Sayyad et al. [J. Phys. B 28, 1715 (1995)], while the corresponding RRPA-RMQDT (relativistic multi-channel quantum-defect theory) calculations proved less successful. © 2003 The American Physical Society.