ANOMALOUS SPATIAL-DISTRIBUTION OF TEMPORAL AND SPECTRAL BEHAVIOR WITHIN A PULSED DYE-LASER BEAM

A peculiar effect, probably overlooked in some past laser induced fluo rescence (LIF) and laser enhanced ionisation (LEI) experiments, is des cribed. Such an effect has been observed when the temporal profile of an excimer laser-pumped, pulsed dye laser beam was analysed with a mic rochannel plate photomultiplier detector and a fast digitising signal analyser. Substantially different shapes were found at selected spatia l locations, of 1 mm size, within the same laser beam. The overall tim e profile of the full size beam (similar to 6 mm) can be described by the overlapping of two limiting shapes, that is, a narrow pulse, of ab out 3 ns full-width-at-half-maximum (FWHM), and a much broader pulse ( similar to 7 ns FWHM). The peak intensities of the limiting pulses dif fer in time by similar to 10 ns. In addition, it is shown that the two peaks also differ in wavelength by an amount which can be as large as the width of the atomic absorption line in the atomiser. The effect, which occurs in a random manner and is easily observed only with time resolution, is extremely sensitive to the alignment of the oscillator cavity of the dye laser. This was demonstrated experimentally by compa ring the laser excitation and fluorescence waveforms of gallium and le ad atoms in the inductively coupled argon plasma. The recognition of s uch anomalous behaviour is essential for a correct interpretation of s ome time-resolved fluorescence and ionisation experiments.