A variable-pressure 27-MHz helium plasma was investigated in order to study the physical chemical nature of the inductive coupling and the microscopic atomic and molecular state variations which attend regular manipulations of plasma power and of helium pressure. Two modes of helium plasma excitation, the E- and H-plasma modes, are identified by widely differing capacities to effect ionization of other plasma species such as argon atoms. The switch from the helium E- to the H-plasma is abrupt and dependent on the gas pressure. The results presented suggest that it is the production of He+ and electrons and subsequent interactions in the plasma (electron recombination, collisions, etc.) which primarily determine the microscopic atomic and molecular behavior which we observe.
D. C. Miller, H. B. Fannin, P. A. Fleitz, and C. J. Seliskar, "Fundamental Chemical Processes in a Low-Density 27-MHz Helium ICP," Appl. Spectrosc. 40, 611-617 (1986)