Abstract
The present work reports an investigation on the role played by Na3+ ions formed through triatomic associative ionization collision of Na(4d) atoms with Na2 ground state molecules during the early phase of sodium plasma generation by laser ionization based on resonance saturation (LIBORS). Such ionization mechanism is observed experimentally for the first time by Tapalian and Smith (1993) [1]. In their experiment, stepwise atomic excitations are created using two CW dye lasers; one laser is tuned to 589 nm to excite the Na(3s) to Na(3p) D2 transition of sodium and the other laser is tuned 569 nm to excite the Na(3p) to Na(4d) transition. The analysis is grounded on a numerical study of the role of seed electron processes on the temporal evolution of sodium plasma formation by laser irradiation. A previously developed numerical model based on LIBORS technique is modified and adopted. In the present study, the sodium atom is treated as an atom comprises 22 levels namely: a ground state, 18 excited states and three ionic states (atomic, molecular and tri-atomic). The model tackled various collisional and radiative processes that act to enhance and deplete the free electrons generated in the interaction region. The contribution of these processes is signified by solving numerically a system of time-dependent rate equations, which couple the generated atomic and ionic species with the laser fields. Meanwhile, it solves the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) of the generated electrons. The computed values of the EEDF, time evolution of both excited states population and the formed ionic species considering the individual effect of associative ionization, Penning, and photo-ionization and triatomic associative ionization justified the important effect of each of these ionizing processes in creating the early stage electrons. These seed electrons are assumed to rapidly gain energy through superelastic collisions leading eventually to plasma development.
Highlights
Study of laser-produced plasmas is one of the rapidly growing fields of present-day physics
The present work reports an investigation on the role played by Na3+ ions formed through triatomic associative ionization collision of Na(4d) atoms with Na2 ground state molecules during the early phase of sodium plasma generation by laser ionization based on resonance saturation (LIBORS)
In (1993) Tapalian and Smith [1] observed experimentally for the first time the formation of Na3+ ions. These ions are produced through triatomic associative ionization interaction between Na(4d) atoms and Na2 ground state molecules, during sodium plasma generation by laser ionization based on resonance saturation (LIBORS), according to the reaction
Summary
Study of laser-produced plasmas is one of the rapidly growing fields of present-day physics. In (1993) Tapalian and Smith [1] observed experimentally for the first time the formation of Na3+ ions These ions are produced through triatomic associative ionization interaction between Na(4d) atoms and Na2 ground state molecules (which are assumed to be present at ≈ 0.5% percent in the atomic beam), during sodium plasma generation by laser ionization based on resonance saturation (LIBORS), according to the reaction. This model was previously developed [28] to interpret the experimental observation which is executed to study the associative ionization, and Penning ionization processes (atom-atom collisions) These are considered as main sources for seed electrons development in the ionization of sodium vapor irradiated by a CW laser source tuned to the transition 3s-3p prearranged by Caree el al.
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