The spectroscopic detection of molecular species using a scanning narrow-band dye laser

A continuously scanning, frequency doubled, tunable dye laser, pumped by a pulsed nitrogen laser, has been used to excite fluorescence of gaseous molecules. It was possible not only to scan continuously the fundamental wavelength, but also to scan the excitation wavelength when frequency doubled (270 – 325 nm) over a range exceeding 2 nm with a bandwidth of less than 0.01 nm.Insertion of an etalon into the dye laser cavity resulted in a narrowing of the laser output bandwidth to <0.002 nm in the fundamental and to 0.0005 nm in the ultraviolet (frequency-doubled) output. A simple technique has been devised for making almost linear, continuous scans of the narrow-band excitation wavelength over a range of 0.15 nm in the fundamental, or 0.08 nm in the second harmonic.The results are exemplified by studies of the laser-induced fluorescence of I₂, SO₂ and the OH free radical. However, laser induced fluorescence from the atmospherically-important ClO radical could not be detected under similar conditions, and possible reasons for this result are discussed.