Sequential two-photon-laser-induced fluorescence: a new method for detecting atmospheric trace levels of NO

Sequential two-photon-laser-induced fluorescence (TP-LIF) in NO has been achieved by using 226-nm radiation to excite the X(2)Pi ? A(2)Sigma transition followed by excitation of the A(2)Sigma ? D(2)Sigma transition using the fundamental output of a Nd:YAG laser at 1064 nm. The resulting fluorescence was monitored at wavelengths as low as 187 nm (D(2)Sigma ? X(2)Pi transition), thus permitting major discrimination against conventional noise sources as encountered in single-photon-laser-induced fluorescence (SP-LIF) systems. Also, since the 226-nm radiation used in the TPLIF method is generated in the same manner as in the SP-LIF technique (i.e., the sum-frequency mixing of 288-with 1064-nm radiation), the two-photon sequential-pumping scheme requires no major extra optical hardware. Although the final optimization of the TP-LIF NO method has not yet been completed, extrapolations based on currently measured signal and noise levels suggest a detection sensitivity level of 3 parts in 10(13) (0.3 pptv) under atmospheric conditions. This would represent approximately a 25-fold increase in sensitivity over the SP-LIF NO technique.