Laser‐Induced Fluorescence: A Method to Measure the Internal State Distribution of Reaction Products

passes through the reaction zone. As the wave­length of the dye laser is scanned, the BaO molecules are excited to the (v', 1') level whenever the laser wavelength coincides with an allowed v"i"-+v'i' transition in the A-X band system. The resulting fluorescence emitted at right angles to the laser beam is detected by a fast-response photomultiplier. The laser pulse triggers a boxcar integrator (Princeton Applied Research or Keithley Instruments) whose electronic gate (window) is maintained open from the time of the laser pulse to 2-5 times the BaO A state lifetime. This analog device permits us to average the signals resulting from many laser pulses. The scattered light from the laser beam is rejected either with a sharp cutoff Corning filter or by delaying by typically 20 nsec the opening of the electronic gate. The BaO fluorescence rate versus laser wavelength is shown in Fig.