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