Negative ion contamination of electron impact spectra

Since two particles with the same charges and the same kinetic energies are transmitted equally by electrostatic velocity analyzers regardless of mass, it follows that negative ions produced on electron impact with a gas can interfere with the electron impact spectrum of the gas. Possible contribution of negative ions to the electron impact spectrum of water is studied by theoretical means. It is concluded that H− ions can contribute peaks to the spectrum in the apparent energy loss range 4–6 eV if the spectrum is produced by electrons with kinetic energy in the range 5–7 eV. With incident kinetic energies of 15 eV or higher negative ion contamination is not important. In general, negative ion interference can be important, especially for hydrogen containing compounds, when (a) the kinetic energy of incidence is very low and (b) when the electron spectrometer employed contains only electrostatic lenses and analyzers. These considerations suggest that negative ion spectra might be studied as a means of detecting excited quantum states of radical fragments especially for hydrogen containing substances which give H− on collision with electrons.