Investigations of nonmetal charge transfer using helium microwave -induced plasma time of flight mass spectrometry.

摘要

In helium microwave induced plasmas (He-MIP), the +1 ion emission lines are more intense for most nonmetals than the atom emission lines. The most likely reason for this behavior is charge transfer (CT) between the helium ion and the neutral ground state nonmetal, producing an excited state nonmetal ion. The proposed mechanism is given in the following equation: He+24.56eV +A°0eV →He°0e V+A+*24.56 eV±DE A represents the analyte, ° and + indicate the ionization states, and * indicates an electronically excited state. ΔE is the difference between 24.56 eV and the excited ionization energy of the nonmetal.;The CT phenomenon with nonmetals in He-MIP was studied using atomic emission spectrometry (AES). AES gives information about populations of specific atom and ion electronic states. On the other hand, mass spectrometry monitors the entire population of ion states. Using results from these techniques, a more complete picture of charge transfer and nonmetal ionization processes can be produced.;To examine this mechanism, the helium ion population was reduced by adding a more easily ionized gas to the plasma. The addition of 0.5% Ar to the plasma decreases the intensity of the Cl+* emission line by 60%, while the Cl atom line signal increases by a factor of 20.;In this work, a time of flight mass spectrometer was constructed and used to monitor the total chlorine ion signal. Upon addition of 0.8% Ar, the chlorine total ion signal decreased 63%. A chemical kinetics simulator was also used in these studies to model the plasma kinetics. Results, based upon the CT model, match the experiment well. It was only possible to achieve agreement with experimental results using 8 reactions for chlorine. Similar results were obtained for bromine.;The simulations verified that CT is the dominant mechanism for nonmetal ionization in a helium plasma. In the course of modeling the behaviors of the atoms; additional plasma chemistry was clarified. An argon metastable state appears to be responsible for excitation of the monitored Cl atom line.