The decay of the metastable states has been investigated in a DC afterglow discharge in helium, argon, krypton and xenon at room temperature. This investigation was done by measurement of the electrical breakdown time delay (td) as a function of passive time ( tau ) ('memory curves'). It is shown that the concentration of these states during this time decays according to an approximately exponential law. It is shown that the nonradiative lifetime of metastable states in helium is much longer than that in argon, krypton and xenon.

The probability of electrical breakdown in neon is investigated under a pressure of 13.3 mbar, using the breakdown time-delay measurement method. It is found, by applying a laser spectroscopy technique, that the 1S5 metastable states of neon atoms dominantly contribute to the breakdown probability in the moment when more than 1 s has passed after the previous discharge. With this method the presence of 1S5 states of Ne atoms is detected in the gas, under the pressure mentioned, up to about 500 s after the cessation of the breakdown.

This is the summary of results of four experiments carried out in order to establish the initiators of electric breakdown in gases after long afterglow periods , particularly the role of molecules in the state in nitrogen. By illuminating nitrogen-filled measuring diodes by nitrogen-filled discharge lamps it was uniquely established that (i) a prolongation of the breakdown time delay, (ii) an increase in the breakdown voltage, (iii) a decrease in the stationary glow current and (iv) a shortening of the discharge quenching time occur due to optogalvanic de-population of the mentioned metastable state. Thus molecules and/or atoms in metastable states do considerably influence the breakdown probability as much as they contribute to the maintainance of glow-discharge currents.

The analysis of measurements of the overvoltage dependence of time delays to electrical breakdown in nitrogen-filled diodes under various illumination conditions confirms the role of long-lived metastable species. After long periods of afterglow the concentration of molecules significantly influences the breakdown probability as well as the magnitude of the glow-discharge current.

The role of the electrode surface and some gas phase processes in the production of electrons which initiate electrical breakdown is investigated. The results show that to achieve approximately constant secondary electron emission in these experimental conditions it is necessary to perform about 1000 breakdowns. The experiments indicated that the initial electrons are created by the impact of N2 metastable states on the cathode and the nonradiative lifetime of these states depends on the gas pressure.