Collisional ionization of Na atoms excited by one- and two-photon absorption in H2O2Ar flames

Abstract

An ionization signal was detected when a flashlamp-pumped dye laser was tuned on resonance with various one-or two-photon transitions of Na atoms in an H~_--O2-Ar flume of 1800 K at atmospheric pressure. Ionization signals were obtained by collecting the electric charge with two thin iridium probes which were immersed into the flume. Relative ionization signals were measured as a function of dc probe voltage, laser tuning, Na-solution concentration, and free-O~ concentration. Fluorescence signals served to monitor the relative populations of the higher Na levels. A preliminary qualitative interpretation is given on the basis of collisional ionization from the saturated as well as the higher levels, which are mutually connected by population redistribution. With the assumption of a quasi-steady-state ionization, an effective Saha equation is derived, which connects the concentration of the atoms in the saturated level with the ionic concentration by an effective ionization constant. Values of this constant were estimated from the experiments and compared for the different excitation modi investigated. Additional one-photon wing excitation of the 3P-5S transition might play a role as an intermediate step in the ion production when the 3P level is saturated. The presence of excess O_~ molecules enhances the degree of ionization, whereas the addition of comparable quantities of N 2 molecules does not have this effect. This is explained by the positive electron affinity of O~, which shifts the effective ionization equilibrium. The role of multiphoton processes which directly transfer Na atoms from the 3S or 3P state to the continuum state is shown to be negligible. Also the role played by the associative ionization reaction: Na(3P) + Nal3P) ~ Na + + e--is shown to be negligible under our experimental conditions.

Limits to sensitivity in laser enhanced ionization

“…obtained by taking the LEI signal from a thin rod translated across the front of the normal collecting plate at the indicated high voltages. The experimental apparatus is shown in the inset: (1) high voltage repelling plate; (2) laser excitation volume; (3) flame reaction zone; (4) burner head: (5) l ow voltage electrode plate; (6) verticelly moveable signal pick-off wire.…”

“…(5) By factoring eqn. (5), P,* may be shown to exceed P,-by a factor Pi*/Pi = exp[£A*/fcPl (6) The probability of ionization may be shown from eqn. (6) to increase by about two orders of magnitude for each eV of excitation of the atom in a 2500 K flame.…”

Resonant laser-induced ionization of atoms in an inductively coupled plasma

“…We are reporting on what we believe to be the first observation in an argon inductively coupled plasma (ICP) of enhanced ionization of atoms induced by the absorption of visible and ultraviolet laser radiation. Our objective in this research was to learn if the method of laser-enhanced ionization (LEI) spectrometry (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), which normally utilizes a flame as an atomization source, could be applied to the ICP.…”

Ion Enhancement by Dual-Laser Ionization in an Acetylene/Air Flame

“…The laser-enhanced ionization technique (LED as an application of the optogalvanic effect to flames was first developed in 1976 and has, since then, demonstrated its potential in detecting traces. [1][2][3] In some elements, for instance, LEI has been employed in order to achieve a detection limit as low as 0.001 ppb (ng/mL)2 It has also been used to determine flame temperature, the amount of produced ion yield, and radical structures. 4-7 To achieve optimal sensitivity and selectivity in trace detection, meanwhile, researchers have been conducting studies of the characteristics of the ion signal, electrical interference, electrode design, and ion collection configuration, as well as the influence of the space charge and the bias voltage on collection efficiency, s-14…”