The mechanism of ArF laser-induced fluorescence of dense plume matter
We perform two-pulse experiments to elucidate the mechanism of laser induced fluorescence (LIF) of dense vapor plumes. The first ns laser pulse at 355 nm gently ablates a brass or tin–lead target to create a vapor plume. The second ns laser pulse at 193 nm intercepts the plume to induce fluorescence. The plume-LIF (PLIF) emissions of the copper, zinc, tin and lead atoms are collected and analyzed. We show that the temporal and spectral behavior of the PLIF signal is inconsistent with plasma emissions and photofragmentation fluorescence. We argue that a process involving dense matter is more probable. According to that model, the dense nascent plume has an energy structure resembling bands, especially for the higher energy states. Absorption of energetic ultraviolet photons is therefore nonselective. As the plume disperses, the excitation energy is channeled to electronic excitation energies of the atomic and molecular fragments. Their fluorescence gives rise to the fingerprint signal of the analytes. The predictions of the dense-matter model are confirmed by both time-resolved and time-integrated PLIF spectra. The analytical potential of PLIF for micro-trace analysis and multivariate chemometrics is also discussed, and most of its applications have been realized and reported in our previous publications.