Mesure du taux de dépolarisation dans l'azote comprimé et évaluation théorique des différents processus qui y contribuent

Abstract

Le taux de dépolarisation de la lumière diffusée par l'azote gazeux a été mesuré à 310 K et à des pressions allant jusqu'à 440 bars : la diffusion propre et la diffusion collisionnelle (calculée dans l'approximation D.I.D.) contribuent aux intensités dépolarisées. Le calcul de Cabannes prévoit la diffusion propre par des molécules aléatoirement orientées. On montre que les corrections les plus importantes viennent de la diffusion collisionnelle et des interférences entre diffusions propre et collisionnelle. Ce dernier terme, habituellement négligé, est beaucoup plus important que celui correspondant aux interférences entre diffusions propres par des molécules différentes (corrélations d'orientation). L'accord de ces calculs avec l'expérience est satisfaisant

Calculations of second light-scattering virial coefficients of linear and quasi-linear molecules

“…which follows from the probability in equation (10). However, before such averaging can be carried out, it is necessary to eliminate from the right hand side of equation (33) the redundant interaction parameters l} 2), l~2), AI, and A2 using equations (26)(27)(28)(29).…”

“…The presence of B in equation (2) [8], and may be the reason that very few measurements of Bp for anisotropic molecules have been undertaken. Nitrogen, which has a small B value of only -4-2 x 10 .6 m 3 tool -1 at 300 K, and which exhibits a Gp nearly five times this value, has been recognized as a favourable molecule to investigate experimentally [8,9] and theoretically [10,11]. Berrue, Chave, Dumon, and Thibeau [10,11] present expressions for the Gp contribution to Bp which include angular correlation and collision-induced anisotropy, but in their evaluation of the latter they neglect the polarizability anisotropy in successive dipole-induced-dipole interactions.…”

“…The probability P('r) that molecule 1 has a neighbour in dT at -r is related to the intermolecular potential energy U12(~-) by [13] N P(T) = ~mm exp {-U12(T)/kT} (10) with ~ the integral over the orientational coordinates of the neighbouring molecule. Confining ourselves to dipole-dipole interactions in the differential polarizability, we write where [14] (11) (12) is the well-known second-order T-tensor used to express the electric field of a point dipole at a point R~ from its centre in the simple form E~ = T,~#~.…”

Raman bands of fluid N2

“…As is apparent, the model underestimates the higher density value, whereas the value at 270 amg is fairly well reproduced. Indeed this model has been found to yield a satisfactory density evolution of the second moment [21,22] and of the depolarization ratio [38] up to the critical density.…”

“…From table 2 we note that ~'20 and r2~ decrease with increasing the density, indicating that a cancellation occurs due to many body effects on the pair distribution function. A very simple model [38,22] leads to an exponential decay of r3o and the predicted values are reported in parentheses. As is apparent, the model underestimates the higher density value, whereas the value at 270 amg is fairly well reproduced.…”