Abstract

The relative merits of five laser excitation schemes have been evaluated for photoionization spectrometry (PIS) measurements of NO in air. All five schemes utilize wavelengths near 215 nm, which correspond to excitation of rovibronic transitions in the Α <sup>2</sup>Σ<sup>+</sup> - Chi <sup>2</sup>Pi<sub>i</sub> (1,0) bands of NO. Photoionization of the excited NO molecules is accomplished by using wavelengths ranging from 215 to 1064 nm. Excitation spectra of the five PIS schemes reveal a significant enhancement when 355 nm radiation is used for photoionization. The enhancement is hypothesized to be related to a resonance or near-resonance ionization process for selected lines in the Α <sup>2</sup>Σ<sup>+</sup> - Chi <sup>2</sup>Pi<sub>1/2 </sub>band [specifically the R<sub>11</sub> + Q<sub>21</sub> (<i>J</i>" = 8.5-12.5) lines and the P<sub>21</sub> + Q<sub>11</sub> (<i>J</i>" = 9.5-10.5) lines] and the Α <sup>2</sup>Σ<sup>+</sup> - Chi <sup>2</sup>Pi<sub>3/2</sub> band [specifically the R<sub>12</sub> + Q <sub>22</sub> (<i>J</i>" = 8.5-12.5) lines and the P<sub>22</sub> + Q<sub>12</sub> (<i>J</i>" = 9.5-10.5) lines] of NO. Comparison measurements using all five schemes demonstrate that the highest sensitivity and highest signal-to-noise ratios are observed by using the 215 nm + 355 nm scheme, for which a limit of detection of 80 parts per trillion by volume for NO in air has been achieved.

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