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 N 2 . 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* 2 in 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 N 2 + N* 2 is up to 15% smaller than that of N 2 . The presence of N* 2 is also detected by the colour change of a metal oxide powder with which it reacts; for example, pale green MoO 3 exposed to N* 2 is reduced to blue MoO 2 within 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* 2 is 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* 2 is 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* 2 at 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* 2 outside the source is in the A( 3 ∑ u + ) state with a free life of 12 s and a concentration of about 10 13 particles/cm 3 at 1 Torr; hence 1 part in 10 4 of N 2 are electronically excited. The efficiency of wall deactivation to the ground state of N 2 by Pyrex glass and platinum is found to be 3 × 10 -5 and 3 × 10 -3 respectively, the efficiency of electron emission from platinum is about 10 -7 excited molecules per electron collected and the upper bound of the activation energy for reducing MoO 3 is 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 N 2 , 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.