Inelastic Collisions of N2, H2, and H2+He Mixtures in Supersonic Jets by Raman Spectroscopy

6 pages, 4 figures.-- PACS nrs: 31.70.Hq; 33.20.Fb; 34.50.Ez; 47.40.Ki; 47.45.Ab; 47.60.Kz.We present a detailed study of inelastic collisions at low temperature in several supersonic jets of N2, H2, and H2+He mixtures using different nozzles and stagnation conditions. Absolute number density and rotational population data of unprecedented accuracy are measured along the jet axis by Raman spectroscopy with high spatial resolution (<5μm) and high-sensitivity (<1 photon/sec). The experimental data are interpreted by means of a master ecuation describing the time evolution of the rotational populations in terms of the state-to-state rate coefficients derived from high-level quantum calculations. This combination of experimental and calculated data leads to a detailed understanding of the underlying physics, consistent with the assumed isentropic behaviour. The breakdown of rotational-translational thermal equilibrium, and its space-time evolution along the jet axis are accounted for by the microscopic (state-to-state rate coefficients) and macroscopic (flow velocity, number density, temperatures) physical quantities. A highly consistent picture, free from any additional parameters, bridges this way the microscopic and macroscopic approaches to fluid dynamics along the jet axis.This work has been supported by the Spanish Ministerio de Educación y Ciencia, research Projets Nos. FIS2004-02576, HF2004-232, ESP2004-21060, and by The Consejería de Educación de la Comunidad de Madrd, Project No. S-0505/ESP/0237 (ASTROCAM).Peer reviewe