A PULSED ELECTRON-BEAM TIME OF FLIGHT APPARATUS FOR MEASURING ABSOLUTE ELECTRON-IMPACT IONIZATION AND DISSOCIATIVE IONIZATION CROSS-SECTIONS

A new electron impact spectrometer has been constructed that utilizes a variable energy (4-500 eV) pulsed electron source with time-of-flight detection of electrons and ions. The apparatus can be used in a beam-beam scattering mode or in a constant pressure mode suitable for absolute measurements. A newly designed data processing system is described that uses standard CAMAC modules (LeCroy model 4208 TDCs) and allows up to 32 separate detectors to be used simultaneously in a single hit mode or up to four separate detectors in a multihit mode with each detector capable of recording up to eight hits in the same experiment. The dead time between experiments is 9.2-mu-s which allows up to 100 000 experiments/s for experimental flight times not exceeding 1-mu-s at 1 ns timing resolution. Longer flight times, up to 8.3 ms in duration, can be accommodated but with reduced timing resolution. The determination of the partial ionization cross sections for Ar+, Ar2+, and Ar3+ from threshold up to 500 eV is used as an illustration of some of the capabilities of the new instrument. The results obtained are in excellent agreement with other recent work for Ar+. For the multicharged ions the cross sections were found to lie between earlier reported results. In addition the utility of the variable ion extraction field capability of the instrument is demonstrated by the separation of N+ from N2(2+) for the case of electron impact dissociative ionization of N2. A new method for placing the data on an absolute scale is presented.