Characterization of ion-molecule reactions and rotational relaxation in a free jet expansion.

Abstract

Our group has recently developed and characterized a novel free jet flow reactor in which molecular reaction dynamics are studied in the c::old c::ore of a pulsed free jet expansion. Extremely low translational temperatures, often less than 1 K, are obtained with no condensation problems that exist in cryogenic cooling techniques. The reaction is initiated in the expansion and the species are monitored in the frame of the flowing free jet as a function of time. Kinetic information is obtained from a temporal profile of the mass and density distributions in the expansion. A free jet expansion is not at thermal equilibrium. The consequence of thermal anisotropy between different degrees of freedom must be addressed when properly analyzing free jet kinetic data. A detailed kinetic treatment has been developed which, within the accurate flow model of the jet adopted, rigorously accounts for the thermal anisotropy in the expansion. Approximations to the convoluted exact expressions are then made to aid in experimental application. Astrophysica1ly important bimolecular reactions of C⁺ were measured. The rate coefficients are reported and compared to current capture models. For the reactions with only two exothermic channels. branching ratios are determined. To understand the realistic flow dynamics present in our free jet flow reactor a solution to the Boltzmann Equation for a multi-component atomic expansion was derived. Both velocity and temperature slip are naturally incorporated into the model. To better understand the internal cooling in molecular expansions. rotational state population distributions were obtained in the core of a free jet for both pure and mixed mixtures of N₂ by means of 2+2 REMPI. Spectral fitting shows no evidence for non-Boltzmann behaviour in the rotational populations. The experimental results fit well to a solution of the generalized Boltzmann equation.