R. E. Fox

Electron Attachment in Sulfur Hexafluoride Using Monoenergetic Electrons

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.

Ionization Processes in CCl4 and SF6 by Electron Beams

The ionization processes in CCl4 and SF6 have been studied for both positive and negative ions in a mass spectrometer, and in a total ionization tube. The appearance potential of Cl− is 0+0.05 ev with a very sharp energy dependence. A second process with an onset at about 0.4 ev and a maximum at about 0.7 ev, exhibits a much broader energy dependence. The relative intensities of these two processes are found to be extremely sensitive to the energy distribution of the electron beam, but do not appear to exhibit a temperature dependence. From the appearance potentials and kinetic energy measurements of Cl− a value of 3.30±0.07 ev is obtained for D(CCl3–Cl). The appearance potential curves for CCl3+ and SF5+ near threshold indicate structure which may be associated with energy states of these ions.

Mass Spectrometer Investigation of Ionization of N2O by Electron Impact

The N2O molecule has been studied with a nearly monoenergetic electron beam. A result of the positive ion data is a value for D(N2–O) ≤ 1.34±0.2 ev and a value of D(N–NO) ≤ 4.50+0.10 ev. Measurements were made of the kinetic energies of the ions formed. N+ and N2+ are found to have zero kinetic energy at threshold. The production of an O− ion is observed with an appearance potential in the range 0 to 0.05 ev.

Multiple Ionization in Argon and Krypton by Electron Impact

The formation of multiply charged ions by electron impact in argon and krypton is studied with a mass spectrometer. The behavior of the ionization cross section as a function of electron energy is investigated for electron energies up to 600 ev. The ionization potentials in ev are determined to be as follows: Kr2+ 38.45±0.1,Kr3+ 75.6±0.5,Kr4+ 146.6±2,Kr5+ 217.5±10,Kr6+ 350±10,Ar2+ 43.4±0.3,Ar3+ 84.8±0.5,Ar4+ 150.0±5. The shapes of the ionization curves near threshold are studied and discussed in terms of the threshold law for ionization. The maximum cross sections for each multiply charged ion is determined relative to that of the singly charged ion and compared to data obtained by previous investigators.

Negative Ion Formation in Hydrogen Chloride by Electron Impact

The energy dependence for the formation of Cl— from HCl vapor by dissociative attachment of electrons is studied with a mass spectrometer. Within experimental error the low‐energy onset for the capture process occurs at the same energy as that for the maximum capture cross section which is determined to be at 0.66 ev. The ion pair formation process, HCl+e→H++Cl—, is found to have a linear onset at 14.5 ev. The maximum yield of negative ions by this process is found to be only a few percent of that from the low‐energy capture process. The energy distribution of the electron beam and the calibration of the energy scale is obtained by comparison with SF6—. It is observed that when a low‐energy electron capture process is possible, spurious effects may be obtained at higher electron energies due to low‐energy secondary electrons ejected from electrode surfaces by the primary beam.

Ionization Cross Sections near Threshold by Electron Impact

Ionization curves near threshold have been taken with a mass spectrometer employing a monoenergetic electron beam. These curves have been fitted to previously published data from total ionization experiments in order to obtain absolute cross sections for the threshold energy region. Curves for helium, neon, argon, mercury, carbon monoxide, and nitrogen are given.

Negative Ion Formation in NO2 by Electron Attachment

Negative ion formation in NO2 by electron attachment is studied at low pressures as a function of electron energy in a mass spectrometer. The principle ion formed is found to be O− which onsets at 1.35±0.05 ev and whose apparent maximum occurs at 1.90 ev. Comparison of the peak shape with the electron energy distribution indicates the maximum cross section occurs within 0.1 ev of threshold. The NO2− ion whose maximum cross section is only about one‐hundredth that for O− appears to have the same energy dependence. Crude estimates of the cross section for attachment yield order of magnitude values of 10−18 cm2 for O− and 10−20 cm2 for NO2−.

Threshold Ionization of HCl by Electron Impact

The ionization of HCl by electron impact is studied in a mass spectrometer with a pseudomonoenergetic electron beam. The ionization potential of HCl is determined as 12.56±0.1 ev. The ionization curve at threshold exhibits excessive curvature which is attributed to the 2Πi doublet ground states of the ion estimated to be separated by 0.3 ev. A sharp change in slope of the ionization curve at 1.6 ev above threshold is attributed to the onset of ionization of the A2Σ+ excited state of HCl+.

Appearance Potentials of Some Metastable Transition Ions Found in Hydrocarbon Mass Spectra

The appearance potentials of some metastable transition peaks in normal butane, isobutane, the butenes, and 1,3‐butadiene have been studied along with the parent ions in each metastable transition. A comparison shows that in most cases studied, an additional energy of about 2 ev is required for the formation of the metastable state. However, in the case of normal butane, two transitions were studied in which no additional energy was required within an experimental error of 0.3 ev. Evidence is presented which indicates that in some cases the appearance potentials of ions in hydrocarbon spectra are affected by these metastable ion contributions. This effect may account for some of the unexplained discrepancies found in published data on hydrocarbon gases. Check values of some ions in methane, ethane, and propane were taken to determine the performance of the instrument. A comparison of these results is made with previously published data.