NEW METHOD FOR MEASURING LOW NO CONCENTRATIONS USING LASER-INDUCED 2-PHOTON IONIZATION

A new method based on laser induced two photon ionization was develope d for detection of low levels of NO concentration in the troposphere. A frequency-doubled pulsed dye laser operating at near 226 nm was used to photoionize NO by a (1 + 1) resonance enhanced two photon ionizati on via its A (2) Sigma <-- X (2) Pi(0,0) band. NO ions are detected by an electron multiplier. Air gas samples are injected through a pulsed nozzle into a vacuum chamber. Cooling of rovibrational levels during the expansion process allows better selectivity for NO detection from other nitrocompounds. Measurement conditions were optimized as a funct ion of X/D for the nozzle (X is the distance from the nozzle orifice t o the ionization area, and D is the diameter of the nozzle orifice), l aser power, and electron multiplier voltage. For the first time, selec tivities for NO measurement, which were obtained from ion intensity ra tios of NO and other nitrocompounds such as NO2 at the same concentrat ions, are discussed. A selectivity of 45 for NO vs NO2 was obtained us ing a 44 mu J pulse energy and an X/D of 120 for the nozzle. The curre nt detection limit determined for NO was 16 pptv with an integration t ime of 1 min and a pulse energy of 44 mu J. (C) 1997 American Institut e of Physics.