Excitation of helium atoms in collisions with plasma electrons in an electric field
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Excitation of Helium Atoms in Collisions with Plasma Electrons in an Electric Field1 B. M. Smirnov Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia email: [email protected] Received August 28, 2012
Abstract—The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical elec tron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a selfconsistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. There fore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 21P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field. DOI: 10.1134/S1063776113010160 1
1. INTRODUCTION
rate of atomic excitation by electron impact in an ion ized gas as
An ionized gas located in an external electric field, i.e., a gas discharge plasma, is a nonequilibrium sys tem because, first, the energy is transferred from the electric field to electrons and, second, it is transferred to the gas as a result of collisions of electrons and atoms. Therefore, this system cannot be described by a universal thermodynamic method, and its properties depend on the properties of a certain gas and processes in this ionized gas (see, e.g., [1–4]). Therefore, types of gasdischarge plasmas are divided into many groups and conditions determined by certain processes [5]. But along with elementary processes, kinetics of elec trons is of importance in an ionized gas under the action of an external electric field. The goal of this paper is to find the parameters of excitation of meta stable atom states for ionized helium placed in an elec tric field at low electric field strengths where kinetics of electrons is of importance for atomic excitation.
dN * = N a k ex ( ε ) f 0 ( ε )
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