Spectroscopie optogalvanique dans la flamme : étude des mécanismes collisionnels

Optogalvanic spectroscopy in the flame : study of collisional mechanism

• Auteur / Autrice : Mohamed Benchehida
• Direction : Pierre Camus
• Type : Thèse de doctorat
• Discipline(s) : Physique
• Date : Soutenance en 1989
• Etablissement(s) : Paris 11

Mots clés

Mots clés contrôlés

Fluorescence

Ionisation

Spectroscopie optogalvanique

Spectroscopie laser

Mots clés libres

Processus radiatif collisionnel

Flamme air-acétylène

Résumé

La spectroscopie optogalvanique est une technique d'analyse élémentaire à très bas niveau de concentration dans une vapeur atomique produite par la vaporisation de solutions aqueuses dans une flamme air-acétylène. Le principe consite à exciter sélectivement les atomes de l'élément étudié, à l'aide d'un (ou de deux) rayonnement(s) laser, jusqu'à des niveaux plus élevés à partir desquels ils sont ionisés par collisions. La quantité de charges créées est proportionnelle à la concentration de l'élément dans la flamme. Cette étude est fondée sur les mesures de signaux optogalvaniques et de signaux de fluorescence ce qui permet une approche fondamentale des différents processus responsables de l'accroissment du taux d'ionisation durant l'intéraction laser dans la flamme.

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.