Abstract
An experimental system has been developed wherein a pulsed laser beam photofragments molecules of a gaseous sample introduced into a vacuum chamber. Fluorescence of the fragments so produced is observed. Studies of emission by photoproducts of the excimer laser photolysis of nitromethane at 193 nm are reported here. The principal fragment emission occurs at 431 nm, due to the CH A<sup>2</sup>Δ-X<sup>2</sup>Π transition. A limit of detection for nitromethane of 1.6 × 10<sup>8</sup> molecules is reported, corresponding to an interrogated sample volume of 0.0375 cm<sup>3</sup> and a chamber pressure of 1.3 × 10<sup>−7</sup> Torr (i.e., a concentration of 4.2 × 10<sup>9</sup> molecules/cm<sup>3</sup>). The linear dynamic range extends over 3.5 decades in nitromethane pressure. The fragment fluorescence measurements can be made with excellent precision (relative standard deviation of less than 4%). The CH fragment fluorescence signal is linear in the first power of laser fluence over the range of 60 to 135 mJ/pulse (0.8-1.8 J/cm<sup>2</sup>).
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