Chemical Physics 115 (1987) 43-55 North-Holland, Amsterdam 43 zyxwvutsr A SELF-CONSISTENT REACTION FIELD APPROACH TO LIQUID PHOTOIONIZATION zyxwvutsrqp Hans AGREN, Carmen MEDINA LLANOS Institute of Quantum Chemistry University of Uppsala, S-75120 Uppsala, Sweden and Kurt V. MIKKELSEN ’ Institute of Chemistv, Aarhus University, DK-8000 Aarhus C, Denmark Received 13 November 1986; in final form 23 February 1987 A cluster-dielectric model with a self-consistent reaction field interaction is proposed and evaluated for liquid photioniza- tion. The model is numerically applied to solvent binding energy shifts for some cations and anions in aqueous solutions. The role of medium interaction with the ion or with the ion dressed with its first solvation shell cluster is investigated by means of a self-consistent Hartree-Fock method employing a reaction field Fock operator. Total binding energy shifts and orbital energy shifts are obtained as functions of the dielectric constant. It is shown that for the range of values of the dielectric constant corresponding to the most common solvents, variation in the shifts results from structural effects rather than from pure dielectric effects. It is found that the Born approximation, which is poor for the prediction of the BE shift for the pure ion, works well for cations dressed with its first solvation shell cluster. For anions the model with only one solvation shell in addition to the dielectric is not as appropriate as for cations, which can be argued from comparison of present data with experimental and simulation data. The relation between the present method and a previously devised statistical method is established. 1. Introduction Electron spectroscopy has recently been estab- lished as a new technique for the study of liquid samples and solutions [l-3], and has now pro- vided a large body of data of solvation shifts of electron binding energies. The outstanding fea- tures of this technique for the study of liquids and solutions can be derived from the vertical nature of photoionization. This implies that only the elec- tronic interactions with the solvent are included in the solvation shift, while the reorientational re- sponse upon ionization is not. One may therefore gain insight into various contributions to absolute solvation energies. With certain assumptions also ’ Present address: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60201, USA the electronic contributions to the solvation en- ergy can be decomposed by means of a combined use of binding energy and Auger energy shifts [3,4], i.e. by studying the differential shifts be- tween first and second ionization steps. Previous theoretical investigations of liquid photoionization have used dielectric models [5], cluster calculations [6] and statistical model calcu- lations [4,7] which employ information from stat- istical simulations. These methods explore the solvation effects on binding energies from rather different starting points; each of them possessing inherent merits and limitations with respect to the treatment of short-range, long-range and struct- ural effects. In the present work we propose a self-consistent cluster-dielectric model approach. The sample is divided into a solute species (ion, atom or molecule) where the ionization takes place; 0301-0104/ 87/ $03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)