Abstract EN:

Fluorescence and ionization measurements are performed for the elements Pb, Tl, In in an air-acetylene flame burning at atmospheric pressure in order to establish a useful criterion of choice of the best excitation/ionization scheme for analytical purposes. The experimental data were obtained with the use of a pulsed excitation (10 ns) provided by two tunable dye lasers pumped by the same excimer laser. Whenever necessary, frequency doubling was used and the two laser beams where directed in temporal and spatial coincidence into the flame, where fluorescence and ionisation measurements could be made in the usual way, i. E. , by collecting the fluorescence radiation at 90° and with an immersed electrode for the ionization. As expected, even during the short interaction time provided by the lasers, a substancial fraction of atome can accumulate into a metastable trap, whose population will therefore be greatly enhanced, resulting in a significant analytical gain of the optogalvanic signal and of the sensitivity of the technique. The enhancement was found to be directly proportional to the energy gap between the metastable level and the ground state. This is understandable in terms of the effective level lifetime, which decreases when the energy gap decreases (Pb > Tl > In). The lifetime of the excited level was calculated by using the saturation curve, i. E. , the curve obtained by plotting the fluorescence signal versus laser energy, since it is known from the theory that the saturation energy is related to the level lifetime. By measuring also the branching ratios between the different radiative transitions, it was possible to evaluate the collisional rate coefficients governing the depopulation of the excited states. By considering all the experimental difficulties and constraints and some necessary theoretical assumptions, the agreement between the theoretical predictions and the experimental data was satisfactory. As a result, it is felt that with this approach, the analytical usefulness of a given excitation/ionization scheme in an atmospheric pressure flame for a particular element can be successfully predicted.