Detection of nitrogen metastable molecules by using the threshold ionization mass spectrometry

The N2(A3Σu+) state has a rather long radiation lifetime, and its electron energy is high. Accordingly, it is expected that chemical reactions in the space where many metastable molecules are accumulated will be accelerated. Therefore, it is important to detect metastable molecules and to evaluate the number density of the molecules. With a view to detecting the N2(A3Σu+) state in nitrogen plasma, the authors used threshold ionization mass spectrometry. For that purpose a quadrupole mass spectrometer (QMS) was modified to control its electron energy for ionization. Magnitude of the QMS signal was compared with the ionization cross section of the N2(A3Σu+) state brought about by electron collision, and its number density was estimated. Using the method, it was recognized that the density of N2(A3Σu+) increased in proportion to the discharge current. It was likewise recognized that the density of the current was rapidly decreased in the vicinity of the glass wall.