Ionization in a Mass Spectrometer by Monoenergetic Electrons

Abstract

Over the last few years, a method has been developed for obtaining ionization probability curves with essentially monoenergetic electrons. A retarding potential is applied to the electron beam to yield an energy distribution with a sharp low-energy limit. By varying the retarding potential slightly, a new low-energy limit of the distribution can be selected. The difference in the ionization produced in the two cases is ionization by those electrons with a small energy spread selected from the original distribution. By pulsing the electrons and ions, it is possible to eliminate the adverse effect of the ion-drawout field on the electron energy. With this retarding potential difference (RPD) method, a detailed analysis of ionization probability curves is possible. A full description of this method is given in this paper with a discussion of its advantages and limitations. The mass spectrometer used in this series of studies is described, particular attention being given to a description of the ion source. The various electrodes of the electron beam slit system are described in terms of their influence on the electron energy, and on the shapes of ionization probability curves.

Secondary Electron Yield Measurements as a Means for Probing Organic Films on Aerosol Particles

“…Similar behavior has been observed in the secondary electron yield of bulk KC1, which shows a shallow minimum at -20 eV (Hilsch, 1932;Bruining, 1954), and electron energy loss measurements on bulk alkane films exhibit a broad peak at -22.5 eV (Hiraoka and Hamill, 1973). The shift of -3-4 eV between the location of the minima in our measurements and literature values is probably due to differences in the energy reference levels for the experiments, which depend on contact potentials at the filament and electron gun surfaces (Fox et al, 1955). The energy can be calibrated using a gas of known ionization potential but requires a mass spectrometer.…”

A velocity modulated time-of-flight mass spectrometer

“…Pesu its again in a larger absolute value of e r £ than calculated, although the increase is not as large as in (2). If the increase in is large enough the ions are not represented in the output signal.…”

“…This simplification can be justified by examining the consequences (1) at resonance, (2) near resonance, and ( 3 ) far from resonance.…”

“…As explained in Section 2, the device depends on the rf voltage at -cor being the negative of that at +cor. The desired rf wave form therefore has odd symmetry, and the desired flight time for resonance is (see figure 2): hac) = Vac) " -°-* < " /T <°' 5 (2) Note that ions which enter late in the cycle (large t) are subjected to a higher accelerating potential and have a shorter transit time, t/ \.…”

“…As before, the resonant ions neither gain nor lose energy from the applied rf fields and the same detection system as described in tl] which measures the number of ions experiencing no net energy change can be used. This measurement is made by the retarding field method [2], 3 ].…”