J. Phys. B: At. Mol. Opt. Phys. 29 (1996) L687–L692. Printed in the UK LETTER TO THE EDITOR Characteristic features of the 3p absorption spectra of free iron-group elements due to the duplicity of the ‘inner-valence’ 3d electrons. Application to Mn 2+ V K Dolmatov† The Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, UK Received 10 June 1996 Abstract. The 3p absorption spectrum of Mn 2+ (3d 56 S) over the 3p → 4s and 3p → 3d resonance regions is theoretically studied for the first time. The calculations were performed within the ‘spin-polarized’ random-phase approximation with exchange. The ordering of these resonances is found to be opposite to that in Cr, Cr + and Mn + . As a result, the 3p absorption of Mn 2+ differs markedly from that of Mn + . The ‘permutation’ of the 3p → 4s and 3p → 3d resonances is proved to be inevitable for a 3d element with the increase in its ionic charge. This arises from the fact that the 3d electrons of iron-group elements are valence by energy but inner by spatial distribution. The duplicity of the 3d electrons is also found to result in additional characteristic features of the 3p absorption spectra, such as, at first glance, the ‘inconsistent’ energy ordering of the 3p → 4s and 3p → 3d resonance structures in neutral and low charged elements and the insensitivity of the 3p → 3d transition energy to the degree of ionization of a given element from the iron-group. The 3p absorption spectra of free iron-group elements have attracted much attention because they are dominated by a giant 3p → 3d autoionizing structure which, in turn, determines the whole behaviour of a near 3p threshold photoabsorption (see the review paper of Sonntag and Zimmermann 1992 and a relatively recent original paper of Whitfield et al 1994, both full of references). To date the most detailed data have been obtained for the 3p absorption of Cr and Cr + (Mansfield 1977, Bruhn et al 1982, Meyer et al 1986, Costello et al 1991), Mn and Mn + (Connerade et al 1976, Schmidt et al 1985, Costello et al 1991, Whitfield et al 1994), as well as laser-excited Cr ∗ (3d 5 4p 17 P 4 ) (Dohrmann et al 1995). In the present letter the 3p → 4s and 3p → 3d autoionizing resonances in the 3p absorption spectrum of a higher charged Mn 2+ (3d 56 S) ion are discussed for the first time. The ordering of these resonances with increasing photon energy proved to be opposite to those in Cr, Cr + and Mn + in that the 3p → 3d resonance structure comes first, and on its high-frequency shoulder a structure caused by the 3p → 4s autoionization appears. The interaction between the 3p → 3d and 3p → 4s resonances through electron correlation turns out to qualitatively depend on whether the 3p → 4s resonance is on the low- or high-frequency shoulder of the 3p → 3d resonance structure, for which reason the 3p absorption spectrum of Mn 2+ is found to differ markedly from that of Mn + . The calculations were performed within the ‘spin-polarized’ random-phase approximation with exchange (SP † Permanent address: S V Starodubtsev Physical-Technical Institute of Academy of Sciences of Republic of Uzbekistan, Mavlyanova Str. 2, 700084 Tashkent, Uzbekistan. E-mail address: valeri@vkd.silk.glas.apc.org 0953-4075/96/190687+06$19.50 c  1996 IOP Publishing Ltd L687