Characterization of supersonic beams of polyatomic molecules

Abstract

Supersonic free jets and beams have been characterized in terms of the terminal Mach number, velocity spread, translational temperature and the average value of specific heat ratio (γ) pertinent to supersonic expansion. It has been found that for a polyatomic molecule, the average value of γ, used to describe the flow, depends upon the stagnation conditions. Vibrational relaxation of SF 6 as a representative of polyatomic molecules has been investigated using the “sudden freeze” model of Knuth and the relaxation rate equation model of Quah. Though the “sudden freeze” model predicts the experimental terminal vibrational temperature of Luijks et al., it overestimates cooling close to the nozzle exit. Quah's model shows that the vibrational relaxation cross section (σ v ) increases with decrease in temperature, contrary to the prediction of the Landau—Teller relaxation mechanism. This has been attributed to enhanced efficiency of vibrational relaxation at lower temperature due to facile exchange of internal energy with translational energy through the formation of quasi-resonant and van der Waals complexes.