Laser Photolytic Fragmentation-Fluorescence Spectrometric Determination of Nitromethane

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 A²Δ-X²II transition. A limit of detection for nitromethane of 1.6 × 10⁸ molecules is reported, corresponding to an interrogated sample volume of 0.0375 cm³ and a chamber pressure of 1.3 × 10⁻⁷ Torr (i.e., a concentration of 4.2 × 10⁹ molecules/cm³). 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/cm²).