Studies of the spectral dependence of photoionization spectrometry measurements of NO in air

The relative merits of five laser excitation schemes have been evaluated fo r photoionization spectrometry (PIS) measurements of NO in air, All five sc hemes utilize wavelengths near 215 nm, which correspond to excitation of ro vibronic transitions in the A (2)Sigma(+)-X (2)Pi(i) (1,0) bands of NO. Pho toionization of the excited NO molecules is accomplished by using wavelengt hs ranging from 215 to 1064 nm. Excitation spectra of the five PIS schemes reveal a significant enhancement when 355 nm radiation is used for photoion ization. The enhancement is hypothesized to be related to a resonance or ne ar-resonance ionization process for selected lints in the A (2)Sigma(+)-X ( 2)Pi(1/2) band [specifically the R-11 + Q(21) (J " = 8.5-12.5) lines and th e P-21 + Q(11) (J " = 9.5-10.5) lines] and the A (2)Sigma(+)-X (2)Pi(3/2) b and [specifically the R-12 + Q(22) (J " = 8.5-12.5) lines and the P-22 + Q( 12) (J " = 9.5-10.5) lines] of NO. Comparison measurements using all five s chemes 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 ach ieved.