Visualisation of a supersonic free-jet expansion using laser-induced fluorescence spectroscopy: Application to the measurement of rate constants at ultralow temperatures

2 Σ⁺, v^′=0) by air at 26±4 K. A value of 2.56±0.40×10^-10 molecule^-1 cm³ s^-1 was obtained, a factor of more than four higher than at room temperature, and consistent with attractive forces dominating the quenching of OH(A²Σ⁺). A d etailed computational fluid dynamics (CFD) simulation of the supersonic free-jet expansion was performed, providing a two-dimensional visualisation of temperature and density variations throughout the expansion. The CFD calculations reproduced the salient features of the experimental temperature and density profiles along the centreline. Comparison between experiment and computation has allowed validation of CFD codes. Received: 31 January 1997/Revised version: 24 March 1997