1 Introduction

Ionization potentials for 2s and 2p subshells of the third row elements range respectively from 75.161 and 21.581 eV for atomic sodium up to 281.80 and 250.21 eV for argon (Indelicato et al. 1998). In some peculiar astrophysical situations, such as planetary nebula haloes or in the vicinity of soft X-ray sources, where densities and temperatures are low, a few hundred eV photons are often present and give rise, through photoionization and consecutive Auger cascades, to multiply ionized atoms. Other cases are early-type stars producing shock X-rays transferred by stellar wind, and stellar sources located near normal galaxies surrounded by a local interstellar medium. Generally the ionized atoms are excited and consequently distinct groups of UV lines are emitted.

The consequences of inner-shell photoionization followed by Auger decays has been often considered in the equations of balance of successive ionization stages for optically thin plasma in the vicinity of a point source of X-rays. For example Weisheit (1974) treated the case of silicon in an interstellar medium undergoing soft-X-ray irradiation. Most of the silicon is of course Si II, however, Si III, Si IV and Si V are also strongly affected. The interest in those studies was focused on the double ionization of the target; single inner-shell photoionization followed by radiationless decay was considered. No attention was however payed to the details of the ionization process, such as shake and double shake processes, Auger shake processes, and direct production of lines issued from successive ionization stages.

In our previous work (Kochur et al. 2001) we addressed the case of the L-shell photoionization of atomic Mg. While 2p-ionization leads to the Mg III ground term, for 2s-ionization the production of the Mg III 2p⁵3s ¹P $^{{\rm o}}$ term is favored, giving rise to the 231.73 Å line.

We now proceed to the case of Al where the presence of 3s and 3p electrons makes the decay processes more complex. We consider line emission during the cascading decays of the following 2s- and 2p-hole states:

1.

Single inner-shell vacancy states produced by single photoionization, here 2s2p⁶3s²3p and 2s²2p⁵3s²3p.

2.

The states with one outer electron (n = 3) either shaken up (SU) or shaken off (SO) through a monopole mechanism (Carlson & Nestor 1973). Monopole processes are those additional excitations/ionizations that occur without change of the orbital angular momentum quantum number. Here, for example, upon 2s-ionization: 2s2p⁶3s² and 3s3p (SO), 2s2p⁶3s²4p and 3s3p4s (SU). We also consider the states produced by double monopole shake processes (dS), when two outermost-shell electrons are either shaken up or off simultaneously.

3.

The states produced through dipole excitations i.e. conjugate shake up (cSU) processes (Badnell et al. 1997). Of these, the strongest are the cSU without change of the principal quantum number n, i.e. 3p $\rightarrow$3d and 3s $\rightarrow$3p excitations which, for example for 2p-photoionization, lead to formation of the states 2p⁵3s3p² and 2p⁵3s²3d. The cSU processes may be understood as being due to the collisional effect induced by an outgoing photoelectron.

In the following discussion, shake process probabilities upon 2s- or 2p-ionization are given as ratios to respective single process probabilities. Photon energies, both for 2s and 2p vacancy creation, are given in dimensionless units of X, defined as the ratio of the incident photon energy to the 2s ionization potential.

For compactness, we shall identify, for example, the SU 3p $\rightarrow$4p upon 2s ionization by the notation 2s/SU 3p $\rightarrow$4p. In the same manner, we shall abbreviate the conjugate shake up processes, for example, 2p/cSU3p $\rightarrow$3d. Double shake processes are referred to as, for example, 2s/dS 3s3p $\rightarrow \{n^{\prime },\varepsilon ^{\prime }\}$s, $\{n^{\prime\prime },\varepsilon ^{\prime \prime }\}$p.

The 1s² is not mentioned hereafter when identifying configurations.