Physics Express Letters: http://www.iop.org/PEL J. Phys.: Condens. Matter 7 (1995) L299–L305 LETTER TO THE EDITOR A theoretical and experimental investigation of the electronic structure of α-Fe 2 O 3 thin films Lidia Armelao†, Marco Bettinelli‡, Maurizio Casarin†, Gaetano Granozzi†, Eugenio Tondello† and Andrea Vittadini§ † Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universit` a di Padova, Padova, Italy ‡ Istituto Policattedra, Universit` a di Verona, Facolt` a di Scienze, Verona, Italy § Istituto di Chimica e Tecnologie Inorganiche dei Materiali Avanzati CNR, Padova, Italy Received 14 March 1995, in final form 31 March 1995 Abstract. Ground and excited states of α-Fe 2 O 3 have been investigated by determining the spin- polarized wavefunctions and eigenvalues of an embedded Fe 2 O 12− 9 cluster using the discrete variational X α method. The computed transition energies compare reasonably well with the recorded experimental spectrum of high-purity α-Fe 2 O 3 thin films obtained by the sol–gel technique. The theoretical data herein reported predict a very high valence–conduction band gap incompatible with the experimental outcomes, which were routinely interpreted as originated by an interband transition. In contrast to this, the lowest-energy optical transitions have a charge transfer nature, involving excitation of electrons from the occupied O 2p-based spin down levels to the empty Fe atom-like spin down orbitals. Iron (III) oxide thin films are particularly appealing for experimental and theoretical investigations in view of their technological applications. Actually, they can be used as catalysts in dehydrogenation reactions [1], magnetic devices [2], temperature and water sensors and optical filters [3, 4]. Furthermore, the applicability of Fe 2 O 3 thin films in the relatively new field of non-linear optics has been suggested [5]. The UV–VIS absorption spectra of ferric oxides have received great attention in the past in order to understand the role of the ground and excited electronic states in determining their electronic properties. Unfortunately, in the case of measurements carried out on bulk samples, a very strong absorption starting at about 2.45 eV obscures a large section of the UV–VIS spectrum, preventing any analysis of the energy region above such an edge. As as consequence, in these kinds of sample, the only bands for which an assignment can be attempted are the very weak ones lying at about 1.85 eV, which, on the basis of crystal field calculations [6, 7], are assigned as a whole to the formally doubly forbidden d–d electronic transitions within the d 5 configuration of the iron (III) ion. In a recent contribution, some of us [8] investigated by means of x-ray photoelectron (XPS), UV–VIS and M¨ ossbauer spectroscopies high-purity Fe 2 O 3 thin films prepared by the sol–gel technique. In detail, XPS measurements revealed that iron is only present (within the sensitivity of the technique, <1%) in the ferric form and that films without any carbon contamination are obtained at temperatures above 400 ◦ C. The absence of any absorption in the near-IR range confirmed the absence of the ferrous species for all the heat treatment temperatures. Furthermore, M¨ ossbauer spectroscopy detected exclusively the presence of iron (III) ions with an octahedral environment in the amorphous phase as well as in 0953-8984/95/230299+07$19.50 c  1995 IOP Publishing Ltd L299