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

Laser-induced fluorescence detection of the hydroxyl radical (OH) was used to measure the rotational temperature and the absolute gas density along the centreline of a continuous supersonic free-jet expansion. Measurement of the lifetime of the electronically excited state of OH with position in the expansion enabled the observed signal to be corrected for the effects of collisional quenching. Shock structures were observed with steep gradients in both temperature and density. The visualisation enabled fluorescence lifetimes taken inside the supersonic region to be used to obtain the rate constant for the quenching of OH(A(2) Sigma(+), upsilon' = 0) by air at 26 +/- 4 K. A value of 2.56 +/- 0.40 x 10(-10) molecule(-1)cm(3)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(A(2) Sigma(+)). A detailed 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.