ZhETF, Vol. 143,
p. 58 (January 2013)
(English translation - JETP,
Vol. 116, No. 1,
available online at www.springer.com
EXCITATION OF HELIUM ATOMS IN COLLISIONS WITH PLASMA ELECTRONS IN AN ELECTRIC FIELD
Received: August 28, 2012
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 electron energy is small compared to the atom excitation energy. Under these conditions, atom excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atom excitation, which is a self-consistent 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 atom 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. Therefore, 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 atom states. The efficiency of atom excitation in ionized helium, i. e., the part of energy of an electric field injected in ionized helium that is spent on atom excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.