Mechanistic studies of nonmetal emission in the helium microwave-induced plasma

Includes bibliographical references (pages [79]-81).The helium microwave-induced plasma (He-MIP) exhibits unique behavior in the excitation of nonmetals. In an attempt to understand this behavior, several researchers have proposed mechanisms of nonmetal excitation in the He-MIP. Most of these mechanisms involve the interaction of several plasma species in a stepwise process. A recently proposed mechanism involves a one step charge transfer between the monomer helium ion and the nonmetal atom followed by thermal excitation of the nonmetal ion. The charge transfer mechanism may be written A (OeV) + He+ (24.56eV) ==> A+* (24.56eV) + He (OeV) where A is the nonmetal ground state atom, and A+* is the excited nonmetal ion. In this thesis, the charge transfer process is investigated further. Nonmetal ions which intensely emit in the He-MIP have at least one electronic energy level close in energy to the first ionization potential of the helium atom (24.56 eV) and could participate in the proposed charge transfer mechanism. These elements include I, S, P, Cl, and Br. Ion emission is not observed in the He-MIP from C, N, 0, and F. These elements do not have an electronic state close in energy to first ionization potential of the helium atom. Further supporting this theory are observations that intense ion emission is not observed from any of the nonmetals in the argon plasma. No nonmetal ion states exist which are close in energy to the ionization energy of the argon atom. Boltzmann temperature plots for iodine and sulfur are presented as further supporting evidence for the charge transfer theory. The addition of molecular gases such as C02 and N20 to the helium plasma results in the reduction of nonmetal analyte emission intensities. Studies of the chlorine and helium excitation temperatures and the electron number density with and without C02 present were done to determine if these properties are affected by the molecular gas. Calculations show that neither excitation temperature nor the electron density of the plasma change with the addition of C02. Interpretations suggest that the addition of molecular gases may reduce the population of He+ and circumvent the charge transfer mechanism.M.S. (Master of Science