Enhancement of ionization in nitrogen by excited nitrogen molecules and their de-activation

Abstract

The rate of ionization of nitrogen at 300 K and at gas pressures of 1 to 30 Torr is considerably increased by admixing neutral molecules of electronically excited nitrogen, N*2. This effect is demonstrated by measuring the corresponding decrease in electric strength as a function of the population of the excited species. These are produced in a weak electric discharge which is maintained at one end of a long tube filled with N2. From there they diffuse into the test region at the other end where a high frequency electric field is applied to determine electric breakdown voltage of the partially excited gas. Gas contamination is avoided by using external electrodes throughout. The population of N*2in the test region is varied by changing the gas pressure and the distance between the test region and the source whose activity is kept constant. It is found that at a pressure of about 1 Torr and a distance of 10 cm the electric strength of N2+ N*2is up to 15% smaller than that of N2. The presence of N*2is also detected by the colour change of a metal oxide powder with which it reacts; for example, pale green MoO3exposed to N*2is reduced to blue MoO2within 10 to 100s. From the time interval between the exposure of the powder to the gas and the colour change, the relative concentration of N*2is obtained as a function of distance in a 4 cm wide tube up to 80 cm from the source at pressures up to 30 Torr when the system is kept in a steady state. In addition, the time dependence of the concentration along the tube is observed in the non-steady state. A colour test with calcium shows that outside the source nitrogen atoms are negligible in numbers. Also by means of a platinum resistance wire the total energy flux and the number of N*2is measured and, with an electron emission detector, their spatial distribution is again confirmed. Analysis of the results shows that the extremely slow decay of N*2at lower pressures is essentially controlled by spontaneous radiation and wall collisons, whereas collision deactivation sets in above about 10 Torr. It is concluded that the bulk of N*2outside the source is in the A(3∑u+) state with a free life of 12 s and a concentration of about 1013particles/cm3at 1 Torr; hence 1 part in 104of N2are electronically excited. The efficiency of wall deactivation to the ground state of N2by Pyrex glass and platinum is found to be 3 × 10-5and 3 × 10-3respectively, the efficiency of electron emission from platinum is about 10-7excited molecules per electron collected and the upper bound of the activation energy for reducing MoO3is 6.1 eV, the zero vibrational level of the A state. Since the potential energy of two A state molecules is smaller than the ionization energy of N2, deactivation by ionization along the tube is not feasible. However, enhancement of ionization can occur in an electric field when electrons collide with such long-lived excited molecules thus raising them to higher vibrational levels so that pairs with a potential energy ≥ ionization energy can produce ionization on collision.