Knowledge Bank

The $A2\Sigma +$ state of Oh has been excited by the simultaneous absorption of two photons in the 6200 {\AA} region. The pumping radiation, $\sim 50$ mJ per 10 nsec pulse from Nd: YAG-pumped dye laser, was tightly focussed into the burnt-gas region of an atmospheric pressure methane-air flame. The ultraviolet fluorescence was detected using either a spectrometer (30 {\AA} bandpass) or an interference filter for wavelength discrimination. The signal was found to be quadratic with laser intensity. Nearly 100 lines of the (0,0) band have been observed in an excitation scan; the experimental and calculated two-photon line strengths will be discussed. The two-photon cross section is small, estimated experimentally at $\sim 10-36$ $cm4$/watt.