MOLECULAR PHYSICS, 1990, VOL. 71, No. 5, 1043-1054 Calculated spectra for the N2-Ar van der Waais complex By ANDRI~S GARCIA AYLLON and JESdIS SANTAMARIA Departamento de Quimica-Fisica, Faculdad de CC. Quimicas, Universidad Complutense, 28040 Madrid, Spain STEVEN MILLERt and JONATHAN TENNYSONt Department of Physics and Astronomy, University College London, London WC1E 6BT, U.K. (Received 4 May 1990; accepted 22 May 1990) Calculations are presented that estimate the energies of all the truly bound states of the N2-Ar van der Waais complex using a semi-empirical potential- energy surface. An effective dipole surface is constructed by comparison with the infrared spectra of McKellar. These spectra are reproduced by convoluting the 30000 strongest transitions with suitable temperature- and pressure-dependent line profiles. These synthetic spectra reproduce the observed experimental features, supporting and extending McKeilar's assignments. Some of the peaks are shifted to higher frequency, suggestingthat the bending potential is slightly too stiff. The corresponding 'pure' (far-infrared) spectra of the van der Waais complex are also synthesized.These do not show the low-frequency features seen in the infrared spectra. 1. Introduction van der Waals systems, particularly atom-diatom complexes, have been well studied in recent years [1, 2]. For these weakly bound complexes, unlike chemically bound species, extensive potential-energy surfaces have been obtained for a number of systems by direct comparison with spectroscopic and other properties of the system [3, 4]. Although it is well known that the spectra contain information on the system not only from the transition frequencies but also from the transition intensities [5, 6], to our knowledge no attempt has yet been made to use this extra information. Henderson and Ewing [7] obtained infrared spectra of the N2-Ar van der Waals complex by studying the simultaneous excitation of the normally infrared-inactive N 2 fundamental and various modes of the complex. Recently McKellar [8] has signifi- cantly improved these spectra and resolved considerable structure, which he ascribes to the van der Waals complex. These spectra were recorded at relatively high tem- peratures in the range 77-90 K and at several pressures. The spectra of the van der Waals complex appear as structure on the diffuse collision-induced fundamental of N:. It is these spectra that we analyse in this work. The bound (and quasibound) rovibrational levels of N2-Ar have been studied by Brocks and van der Avoird (BvdA) [9, 10]. BvdA used the semi-empirical surface of Candori et al. [11], obtained from a multiproperty fit, to calculate transition frequen- cies and hence obtain simulated spectra. BvdA's spectra suffer from two deficiencies. First, they only considered a few rotationally excited states of the complex (J ~< 3), t BITNET/EARN addresses respectively: SM7 and JT at UKACRL. 0026-8976]90 $3.00 9 1990 Taylor & Francis Ltd