Determination of opitmum condiitons for laser-enhanced ionizaiton spectrometry in flames—I: One-step signal strength versus excitaiton transiiton

Abstract

A general theory for determination of the most sensitive one-step laser-enhanced ionization (LEI) transitions for atoms in flames is presented. A simplified expression for the one-step LEI signal strength as a function of excitation wavelength (or signal strength vs the energy of the uppermost laser-connected level) for atoms in flames is derived. From that expression a general element-independent prediction of the most sensitive unsaturated one-step LEI transition is given (i.e. the largest number of charges produced for given amount of laser exposure). The optimum unsaturated one-step LEI transitions in atoms in flames are predicted to be those which excite the atoms to states which are positioned approximately (1.5 + 4.5 k T/ E ion kT below the ionization limit ( E ion is the ionization limit of the atom, k the Boltzmann constant and T the temperature). In an acetylene/air flame this corresponds to approximately 3000 cm −1. This prediction is verified experimentally by measurements in two simple atomic systems (Li and Na). The agreement between the general element-independent prediction of most sensitive one-step LEI transition and the experimental findings for these two elements is good. Furthermore, in order to obtain more accurate predictions of the most sensitive transitions and to handle elements with a perturbed series of oscillator strengths within the principal series the derived expression for signal strength versus excitation wavelength is used also for element-specific predictions of the most sensitive one-step transitions for a number of elements (Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Ga, In and Tl). The most sensitive unsaturated one-step LEI transiitons in an acetylene/air flame are predicted to be: Li 2 s-5 p at 256.3 nm; Na 3 s-6 p at 268.2 nm; K 4 s-5 p at 404.5 nm or 4 s-6 p at 344.7 nm; Rb 5 s-7 p at 358.8 nm; Cs 6 s-8 p at 387.7 nm, Mg 3 s 2-3 s4 p at 202.7 nm; Ca 4 s 2-4 s6 p at 227.6 nm; Sr 5 s 2-5 s9 p at 235.5 nm; Ba 6 s 2-6 s7 p at 307.2 nm, Al 3 p 2P 3 2 -6 d 2 D 5 2 at 221.1 nm; Ga 4 p 2 P 5 2 at 233.9 nm; In 5 p 2 P 1 2 -6 d 2 D 3 2 at 256.1 nm; and Tl 6 p 2 P 1 2 -8 d 2 D 3 2 at 223.9 nm. For some of these elements there are, however, also some additional transitions which have only slightly lower sensitivities as compared to those above. Finally, the influence of opitcal saturation upon the determination of the most sensiitve transition is also discussed. It is illustrated that the more transitions are saturated, the closer to the ionization limit the atoms are to be excited in order to find the strongest one-step LEI signal.