Density narrowing effect in the collisional cluster scattering of the light by gases Victor Teboul a, * , Yves Le Duff b a Departement de Physique, Laboratoire POMA, Unite Mixte de Recherche CNRS 6136, Universite ´dÕAngers, 2 boulevard Lavoisier, FR-49045 Angers Cedex, France b LRP, 5 rue de la Rouvraie, 49080 Bouchemaine, France Received 18 January 2005; in final form 26 January 2005 Abstract The spectral intensities of a collisional cluster scattering band in CF 4 gas have been computed using molecular dynamics. Several densities from 20 to 269 Amagat as well as several models of interaction induced polarizability and of intermolecular potential have been studied. A qualitative agreement with experimental results has been obtained showing the narrowing effect of the gas density on the two-body line shape of the collisional cluster Raman band. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Molecular interactions influence the scattering spectra of any media. Different aspects of this subject have been discussed at a workshop that held at Banff (Canada) in 1993 [1]. Recently it has been shown that a new type of bands is present in the scattering spectra of high pressure gases [2,3] that can be attributable to a double incoherent scattering process. These bands do not belong to the Raman scattering spectrum of a molecule of the gas but are specific of temporary clusters that are generated dur- ing collisions or more generally during molecular interac- tion processes. In the first studies of these collisional cluster (CC) bands it was observed that their line width decreases when the gas density goes up [2]. The origin of this effect was not identified. In this Letter, we study this density narrowing effect and use molecular dynamics simulations to calculate the spectral intensity of these collisional cluster scatter- ing bands for gaseous tetrafluoromethane at several densities. We consider the two-molecule clusters of tetrafluoromethane (CF 4 ) that are temporary created during interaction processes in a CF 4 gas at room tem- perature. In particular, we study the cluster scattering band observed at twice the Raman frequency of the m 1 allowed vibrational mode of CF 4 . The Raman shift for this cluster band is about 1816.3 cm 1 at 269 Amagat. Several models of interaction induced polarizability anisotropy as well as intermolecular potential are used. 2. Theoretical considerations In dense media, interaction induced light scattering (IILS) arise from changes of molecular polarizabilities on account of molecular interaction processes. If evi- dences of three-body irreducible contributions have been reported [1], contributions from pairs of molecules in interaction are preponderant [4] in IILS. However interferences between the electric fields scattered by pairs of molecules in interaction induce two-body, three-body and four-body contributions, leading to cancellation effects [1,5] at high densities. 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.01.090 * Corresponding author. Fax: +33 2 41 73 52 16. E-mail addresses: victor.teboul@univ-angers.fr (V. Teboul), yleduff@libertysurf.fr (Y. Le Duff). www.elsevier.com/locate/cplett Chemical Physics Letters 404 (2005) 199–205