Inelastic Collisions in H2O+He Supersonic Jets by Raman Spectroscopy

Abstract

In order to study the H2O:He and H2O:H2O inelastic collisions at low temperature, several supersonic microjets of H2O+He mixtures, with H2O mole fractions from 1.4% to 33%, have been measured from a 350 micron nozzle at 362 K. Total stagnation pressure p0 ranged from 57 to 320 mbar. All these jets were checked to be free from H2O condensation, a must for the quantitative analysis of the collisional kinetics. The jets were probed by recording the Raman spectra of the Q-branch of the ¿1 symmetric stretching mode at 3657 cm-1 at a series of distances z along the jet axis. The primary experimental data are number densities n(z) and rotational populations PJ(z) which are then reduced to rotational TROT(z) and translational TTRA(z) temperatures. Number densities n were obtained by comparing the intensity of the 3657 cm-1 Raman band in the jet with that from a static sample at a known number density. It was found that the populations PJ of the lowest rotational energy levels very nearly obey a Boltzmann distribution for all our stagnation conditions. This enables an accurate determination of rotational temperatures TROT from the simulation of the observed Raman spectra, by using reference data from the literature [1]. Translational temperatures have been obtained from number densities and rotational temperatures by conservation of mass, momentum, and enthalpy along the jet [2]. From the analysis of the time evolution of the rotational populations by means of a kinetic Master Equation, we have determined the average rate coefficients, both for H2O:He and H2O:H2O collisions, for the 8 lowest levels of ortho-H2O at TTRA=100 K. Our results for H2O:He inelastic collisions will be compared with calculated [3, 4] state-to-state rate coefficients. [1] G. Avila, J. M. Fernández, G. Tejeda, and S. Montero, J. Molec. Spectrosc. 228, 38 (2004). [2] B. Maté, G. Tejeda, and S. Montero, J. Chem. Phys. 108, 2676 (1998). [3] S. Green, S. Maluendes, and A. D. McLean, Astrophys. J. Supp. Ser. 85, 181 (1993). [4] G. Tejeda, E. Moreno, E. Carmona-Novillo, M.I. Hernández, J.M. Fernández, S. Montero, to be published.