Kinetic Mass Spectrometric Study of the Flash Photolysis of NO₂

Abstract

WE have re-examined the flash photolysis of NO₂ using a flash photolysis system coupled to an EAI ‘Quad 200’ series mass spectrometer. Initial experiments indicated that efficient electrical and optical screening must be provided between the flash lamp and the mass spectrometer chamber. A reactor with an aluminium outer case and base-plates was used to eliminate interference by the flash with the operation of the spectrometer and with the size of the background peaks m/e 17, 18, 28 and 44 resulting from desorption from the chamber walls. The nipple which protruded into the ion source was fitted with a molybdenum disk with a 50 µm aperture supplied by Aeon Laboratories. We calculated that in this way more than 80 % of the reactants passed through the aperture directly into the ionization chamber and the spectra obtained were representative at all times of the composition of the gases inside the reactor. The outside wall of the reactor consisted of a glass tube 2.5 cm in diameter, 10 cm long, carrying a B24 conical socket joint and fastened with ‘Araldite’ into the base plate. A quartz flash lamp filled with 15 Torr of argon fitted coaxially inside the reactor. A 0.001 µF capacitor was connected across the leads to the flash lamp to eliminate interference from silent discharge during the charging of the main capacitor. The half-width duration of the flash was about 40 µs. The mass spectrometer signal was either displayed on an oscilloscope and photographed or recorded by an ultraviolet oscillograph.