Energy Distribution of Ions Formed in the rf Spark Source

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Abstract
Utilizing the ion source and electrostatic analyzer of a double‐focusing mass spectrograph, the energy distribution of the ions formed by the pulsed rf vacuum spark has been determined for various materials and sparking conditions. Upon emerging from the electrostatic analyzer, the ion beam has a width, in the y direction, which is principally due to its angular divergence α and its energy spread β. By suitably reducing the widths of the defining slits, α can be made negligibly small. The beam width is then a nearly linear function of β alone, and a photoplate, placed just beyond the exit of the electrostatic analyzer, is used to determine the energy distribution of the ions produced in the source. A convenient energy scale is put on the same photoplate by exposing it to thermal ions (energy spread about 0.5 eV) which are accelerated to selected voltages while the deflecting voltage in the electrostatic analyzer is kept constant. In the present study, a 1×2 in. photoplate could be shifted by external means to permit 31 exposures to be made. Thus, many different sparking conditions and electrode materials, together with thermal ion calibrations, could be investigated and compared in a single experiment. At an accelerating voltage of 20 kV, an energy range of more than 3000 eV is recorded. The residual α, set by two 0.001‐in. defining slits, permits an energy resolution of 15 eV. The energy distribution of the ions produced by the rf spark depends strongly on the spark‐gap width, the rf voltage, and the electrode material. The narrowest energy spread possible is obtained with metallic electrodes at small rf voltages. So far, only two special cases were found where 90% of the ions were contained in an energy band no more than 500 eV wide. In all other instances, energy spreads are found to range from about 1000 to perhaps 5000 eV, with the mean energy located up to 1300 eV above the accelerating voltage. The fine structure and significance of these distributions is discussed in some detail.

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