4 August 2000 Ž . Chemical Physics Letters 325 2000 648–654 www.elsevier.nlrlocatercplett Theoretical photoabsorption spectra of Ar q clusters n Nikos L. Doltsinis ) , Peter J. Knowles School of Chemistry, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 6 June 2000 Abstract q Ž . The photoabsorption spectra of selected Ar clusters n s7, 8, 17, 19, 23 have been investigated theoretically using an n extended Diatomics-in-Molecules approach including induced dipole – induced dipole and spin-orbit coupling interaction effects. Our calculations at 0 K confirm the experimentally observed spectral red-shift of the visible photoabsorption peak in w Ž . x the region 15 -n -20 Levinger et al., J. Chem. Phys. 89 1988 5654 . Furthermore, we have been able to reproduce the w Ž . x additional red-shift measured for 7 (n (9 Haberland et al., Phys. Rev. Lett. 67 1991 3290 by carrying out finite temperature Monte Carlo simulations. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Experimental photoabsorption spectra of Ar q n clusters provided the first direct information on the extent of charge delocalization in these systems. The observation of a photoabsorption spectrum of Ar q 3 which was distinctly different to that of Ar q meant 2 that the trimer is not built up from an Ar q subunit. 2 Measurements of the visible absorption bands of q wx larger Ar clusters by Levinger et al. 1 revealed a n close similarity between the Ar q spectrum and those 3 of Ar q with 4 ( n ( 15. It was therefore concluded n wx q 1 that Ar clusters in this size range consist of an n Ar q ionic core surrounded by neutral atoms. A 3 spectral red-shift of the visible absorption maxima was observed for n ) 15 suggesting a transition from a trimer to a tetramer core. In a similar experiment, wx however, Haberland and co-workers 2 found an ) Corresponding author. Present address: Department of Chem- istry, University of Cambridge, Lensfield Road, CB2 1EW, UK; e-mail: nikos@theor.ch.cam.ac.uk. additional red-shift in the region 7 ( n ( 9, which was again interpreted in terms of a change from an Ar q to an Ar q core. 3 4 Numerous theoretical studies have been devoted to resolving the geometric and electronic structure of Ar q clusters. The semiempirical Diatomics-in-Mole- n Ž . cules DIM method has proven extremely valuable for determining not only ground but also excited electronic state properties of Ar q . Not surprisingly, n theoretical photoabsorption spectra of Ar q have al- n most exclusively been calculated within the DIM or some related model. The first such investigation on clusters containing more than three atoms was car- wx ried out by Ikegami et al. 3 and revealed that a trimer ionic core for all clusters sizes can also lead to a spectral red-shift around n s 15. It was demon- strated that, in this case, the charge delocalization in the electronically excited state plays the crucial role in the determination of spectral properties. However, instead of the additional red-shift in the range 7 ( n ( 9 observed by Haberland, Ikegami and coworkers obtained a blue-shift for 4 ( n ( 11. A later attempt 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00745-4