Direct FTIR high resolution probe of small and medium size Ar n (CO 2 ) m van der Waals complexes formed in a slit supersonic expansion J. Thie ´vin a , Y. Cadudal a , R. Georges a, * , A.A. Vigasin b a Equipe Astrochimie Expe ´rimentale, UMR CNRS PALMS 6627, Universite ´ de Rennes 1, Rennes, France b Wave Research Center, General Physics Institute, Russian Academy of Sciences, Moscow, Russia Received 19 May 2006; in revised form 7 September 2006 Available online 26 September 2006 Abstract A slit nozzle continuous expansion of argon seeded with carbon dioxide is probed using a Bruker IFS 120 HR FTIR spectrometer operating at 0.05 cm 1 or 0.01 cm 1 spectral resolution. About 250 individual rovibrational transitions are assigned which belong to the asymmetric stretch of the CO 2 moiety in Ar–CO 2 and (CO 2 ) 2 complexes. This made it possible to refine the set of spectroscopic constants characterizing these van der Waals species. Analysis of the observed spectral features allowed for evaluation of the number densities of complexes formed in a jet. The manifold of rovibrational lines of van der Waals complexes (along with the monomer lines) sits on an unresolved pedestal, the shape of which varies dramatically as a function of reservoir pressure and initial CO 2 –Ar mixing ratios. Thor- ough examination of these variations allows for the observed features to be assigned to Ar n (CO 2 ) m clusters formed in CO 2 seeded Ar expansion. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Rovibrational spectra of van der Waals complexes; Weakly bound dimers; Mixed Ar–CO 2 clusters 1. Introduction Weakly bound complexes containing Ar atoms and car- bon dioxide molecules are amongst the van der Waals sys- tems, the most studied at both laboratory [1–7] and theoretical levels [8–11]. Much attention paid to the inves- tigation of these complexes is in part due to the fact that a relatively simple prototype system containing one atom and a linear molecule bears characteristics typical of many other small and medium size van der Waals aggregates. The study of the Ar–CO 2 complex allows in particular a detailed characterization of the anisotropy of intermolecu- lar interaction [8,9] and of the nonadditivity effect in ternary and higher order complexes [5,11] to be made. Various spectroscopic techniques have been used to characterize individual Ar n –(CO 2 ) m aggregates. Irrespec- tive of different detection schemes, the use of gas dynamics has long become indispensable to produce the complexes. An early microwave study [1] made it possible to character- ize the Ar–CO 2 dimers ground state spectroscopic con- stants and geometry. Later, infrared laser probing of the X–CO 2 (X = He, Ar, CO 2 ) dimers [2–7,12,13] have provid- ed a wealth of experimental information for the modelling of intermolecular potentials as well as for the deciphering of the vibrational excitation effect on the spectra and struc- ture. The breakthrough in the studies of the carbon dioxide dimer is marked by the establishment of its parallel sheared configuration following the detection of rotationally resolved IR dimer spectra [12,13] with the use of diode lasers or difference frequency laser sources. The use of high resolution infrared diode laser spectroscopy made it possi- ble to characterize carbon dioxide trimers which were 0022-2852/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.09.010 * Corresponding author. Fax: +33 02 23 23 67 86. E-mail address: robert.georges@univ-rennes1.fr (R. Georges). www.elsevier.com/locate/jms Journal of Molecular Spectroscopy 240 (2006) 141–152