Single laser detection of CO and OH via laser-induced fluorescence

Two-photon laser-induced fluorescence detection of carbon monoxide with excitation in the Fourth Positive System near 280 nm is demonstrated in carbon monoxide/nitrogen mixtures at ambient conditions and in a methane/air Bunsen flame. Fully resolved rotational spectra are presented for the A–X (5,0) and (4,0) bands near 279 and 284 nm, respectively. Energy transfer from excited molecular nitrogen to carbon monoxide with subsequent fluorescence from carbon monoxide that was reported for low pressure conditions in the literature has also been observed at atmospheric conditions. It was further demonstrated that overlaps of some CO A–X (4,0) rotational lines with OH A–X (1,0) rotational lines allow simultaneous excitation of both species with a single laser. The fluorescence bands are completely separated, enabling detection without crosstalk. Detection limits are adequate to detect CO in nascent state in a flame and it is expected that for application in high-pressure, low-temperature combustion environments, where high quantities of CO are present, this approach can provide advantages compared to the excitation of CO at shorter wavelengths due to decreased laser beam attenuation.