Photoabsorption for helium, lithium, and beryllium atoms in the random-phase approximation with exchange

The photoionization cross sections and the oscillator strengths for helium, lithium, and beryllium atoms are calculated in the framework of the random-phase approximation with exchange. The energy-level shift for discrete transitions is taken into account consistently in this approximation. The results are compared with other many-body calculations and with the experimental data. The comparison shows that the random-phase approximation with exchange can even be used for systems with a small number of particles.