Knowledge Bank

Two-photon spectra for the lowest excited electronic state of NO have bee examined for measuring fluctuating local temperatures in gas flows seeded with NO[1]. Two-photon fluorescence excitation spectra were recorded for most of the $\gamma (0,0)$ band. Experimental line positions and relative rotational intensities are compared with calculated values using the rotational line-intensity factors of Halpern, et al[2]. Careful intensity measurements for selected lines in the $S11$ and satellite $R21$ branch verify the predicted intensities to with in a few percent. Preliminary measurements of the two-photon absorption cross-section have enabled us to evaluate the sensitivity of the two-photon technique for diagnostic applications. Our results indicate that a S/N ratio equal to the saturated value for single-photon excitation should be feasible with commercially available laser sources. Anomalous intensity behavior on several individual lines was observed in the $O12$ branches for the $\gamma (0,0)$ and $\gamma (0,1)$ bands. For example, the intensity of the $J\prime \prime =21/2$ line in the (0,0) band is significantly reduced while all other adjacent lines are of normal intensity . The magnitude of the intensity reduction was found to be dependent on the gas density and laser bandwidth. The maximum reduction from the theoretical intensity value was a factor of nearly two. Several experiments that explore various aspects of the phenomena will be described.