Photofragmentation-laser induced fluorescence: a new method for detecting atmospheric trace gases

A new method for the in situ detection of nonfluorescing molecular species is proposed: photofragmentation-laser induced fluorescence (PF-LIF). In this approach, the species to be detected is first laser photolyzedat a wavelength λ₁producing one or more vibrationally excited photofragments. Before vibrational relaxation occurs, one of these photofragments is pumped into a bonding excited state by a second laserpulse centered at wavelength λ₂. Fluorescence is sampled at a wavelength λ₃where λ₃ < λ₂ and λ₁. Thispumping configuration thus permits massive discrimination against Rayleigh and Raman scattering as wellas white noise fluorescence from the laser wavelengths λ₁ and λ₂. The technique should be both highly sensitiveand selective for numerous atmospheric trace gases. Specific sampling schemes for detecting NO₂, NO₃and HNO₂ are proposed. Various noise sources and chemical interferences are discussed. Specificcalculations that estimate the sensitivity of the PF-LIF system for detecting NO2, NO3, and HNO2 are given.