Dépolarisation de la lumière diffusée par des molécules anisotropes : dépendance avec la densité

Abstract

2014 Trappeniers et coll. [1] calculent le taux de dépolarisation de la lumière diffusée dans l'approximation D.I.D. et font l'hypothèse de l'orientation aléatoire des axes intermoléculaires des molécules en cours de collisions. Cette hypothèse n'est satisfaisante qu'à très basse densité [2, 3]. Nous tenons compte des corrélations d'orientations des axes intermoléculaires et donnons une expression du taux de dépolarisation pour des molécules faiblement anisotropes à densités modérées. L'accord avec les mesures (N2, C2H4) [4] est satisfaisant. Abstract. 2014 Trappeniers et al. [1], using a D.I.D. approximation, have given an expression of the depolarization of scattered light. They assume that the orientations of the intermolecular axes of pairs of colliding molecules are independent. Except at very low density, this hypothesis is not satisfactory [2, 3]. In this paper we take into account the correlations of the orientation of these intermolecular axes and we give a theoretical expression valid for weakly anisotropic molecules at moderate densities. The results obtained are in good agreement with expérimental data (N2, C2H4) [4].

Coherent light scattering by interacting anisotropic molecules with variable dipolar polarizability

“…These cross-contributions vanish always for the molecular S-model, the only non-zero one being the purely fluctuational contribution I A 1 I2 ) [6,8,11] ] usually taken into account in the spectral theories [15][16][17][18][19][20]. But it should be kept in mind that contributions of the same order as I A 12 > give non-zero cross-terms [7,8,11,13] ( Aa A2 + Ai Ao ) in the S-model. It is noteworthy that, in the molecular S-model, the factor causing the Lorentz-Lorenz function to be density-dependent is precisely the second-order change in molecular polarizability A 2 ) [4,5], whereas the first-approximation variation A 1 ) gives non-zero contributions but in the G-model [5, 23, Above, a(i)t] is the linear polarizability tensor of the i-th isolated molecule.…”

“…In their theoretical description of the spectral distribution of light scattered by anisotropic molecules, some authors [15][16][17][18][19][20][21][22] [6][7][8][9] and, in particular, recent papers [14][15][16][17][18] are restricted to calculations of the first order perturbations A 1. This, however, may lack justification when applying the G-model since, here, the crossterm contributions Ag A 1 + A1* Ao ) can be nonzero [6,9,[11][12][13][14].…”

“…With regard to the transformation (9), the devia- (2) The scattering constants (6) and (7) Applying (10), we write the constant (7) in the form of a sum of incoherent (i = j) and coherent (i == j) anisotropic scattering of the zeroth approximation :…”

“…distinct molecular models : (i) the S-model (specific model), considering the anisotropic molecules as correlated radially but having statistically independent orientations [4,5,7,8], and (ii) the (general) G-model, considering both the positions and orientations of the anisotropic molecules as statistically dependent [9][10][11][12][13]. On the latter G-model, analytical and numerical calculations have been successfully carried out to the end for the radial-angular correlations with regard to the two limiting cases of (i) weak angular (tensorial) interactions, dealt with as a perturbation to the radial interactions [6,8,11 ], and (ii) strong angular interactions, expressed in terms of two-parameter Langevin functions [12][13][14].…”

“…Equally fertile was the idea put forward by Yvon [3] concerning the role, in propagation and scattering of light, of statistical translational fluctuations of atoms, correlated by dipolar forces into two-and three-body assemblages. The Yvon model of dipole-induced dipole interaction (DID) was subsequently extended to the case of intrinsically anisotropic molecules and applied to calculate the LorentzLorenz function (or molecular optical refraction) [4,5] as well as the second virial coefficient of isotropic and anisotropic light scattering by compressed gases [6,7].…”