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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.

Electron attachment leading to the formation of SF6— and SF5— in sulfur hexafluoride has been investigated using nearly-monoenergetic electrons of established energies of less than 2 ev. The dissociative attachment process leading to the formation of SF5— maximizes at less than 0.1 ev and then decreases to zero at approximately 1.5 ev. In the case of resonance capture leading to the formation of SF6—, the capture process occurs at less than 0.1 ev and only over an energy range estimated to be not larger than 0.05 ev. This leads to an estimated cross section for this resonance capture process of 10—15 cm2. A discussion is given of the method used in the establishment of the electron energy scale at such low energies and the use of the resonance capture process for measuring electron energy distributions.

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Utilizing a conventional electron gun and mass spectrometer, the formation of negative ions at low elec· tron energies have been investigated for a number of halogen containing gases used in electric breakdown studies. The SF,-peak is used as an energy calibration for establishing the appearance potential and the energy width over which capture occurs for the individual gases. It is found that the relative areas of the negative ion curves can be correlated with the electric strength of the gases. The results suggest that the formation of SF.-rather than SF,-may be the important process in providing the relatively high electric breakdown value for SF,. Electron attachment associated with SF, and CCI, is found to be an extremely sensitive function of the gas temperature. The gases investigated include CCl.

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SynopsisAromatic polyamides and polyimidea of high molecular weight are currently under development by many industrial laboratories. Wire enamels, laminating and impregnating resins, and varnishes incorporating this new clrtss of thermally stable polymers withstand higher operational temperatures than do most organic polymeric materials in use today. The thermal and oxidative stability of two of these resins has been investigated in air, in oxygen, and in 0'8-enriched oxygen over a temperature range of 300-400'C. The mechanism of degradation is postulated to be that of a unit by unit destruction of the polymer.