The methods and the results of investigations of the reactions in which electrons appear with energies surpassing the average electron distribution's energy in a plasma are considered. Particular attention is paid to the plasma electron spectroscopy method, which combines the advantages of the elementary processes examination in a plasma with the possibilities of conventional electron spectroscopy. Data of the study of chemiionization reactions involving two excited rare gas atoms, Penning ionizations of atoms and molecules by the helium metastable atoms and quenching of excited inert gases and mercury atoms by the electrons are given. The influence of the processes of creating fast electrons on the plasma properties is discussed.

We compare to the probe method a spectroscopic method for determining in plasmas the electron distribution function (EDF) over a wide energy range. For a test of the radiative-collisional model we use to describe the plasma radiation, the measured vibrational distributions of N 2 (C-B) and N + 2 (B-X) were compared with calculated ones using our model and EDFs measured by Langmuir probes. From this comparison we obtain a value for the rate constant for vibrational relaxation at the walls. In a second step we invert the system of model equations for obtaining the EDF from measured line intensities. From the vibrational structure of the emission spectra of the nitrogen molecule the EDF is obtained in the energy range of 1.5-4.5 eV. From the relative intensities of the emission of nitrogen molecules and helium atoms the EDF for electron energies above 11 eV is derived. In the region between these ranges the EDF is interpolated. The results agree within the limits of the experimental errors with the EDF measured directly by the probe.

RF currents at frequencies well above the ion plasma frequency ω pi can be easily suppressed in the space charge sheath of a Langmuir probe by using a suitably chosen low capacitance series resistor R. Such a resistor was used to eliminate the influence of RF currents on the probe characteristics: for blocking harmonics in RF discharges with driving frequencies of the order of ω pi , such as the commonly used 13.56 MHz discharges and also for blocking the fundamental in discharges at higher frequencies (27 MHz and above). The influence of the resistor R on the probe characteristics was eliminated either numerically or automatically by including the series resistor into the negative feedback loop of a current-voltage converter circuit made from a low frequency operational amplifier. Its output was directly used as the probe current signal. RF blocking by a combination of a resistor and two resonant coils for 13.5 MHz and 9.5 MHz respectively, is shown to result in probe characteristics which are unperturbed even around the space potential.