Pulsed double ionization sources

Figure Bl.10.11. Electron impact double ionization triple coincidence experiment. Shown are the source of electrons, target gas, three electron detectors, one for the scattered electron and one for each of the ejected electrons. Two time differences, and 23, are recorded for each triple coincidence, is the difference in arrival times of ejected electron 1 and the scattered electron 23 is the difference in arrival times of ejected electron 2 and the scattered electron. Two sets of time-to-amplitude converters (TACs) and pulse height analysers/analogue-to-digital converters (PHA/ADC) convert the times to binary encoded numbers that are stored in the memory of a computer. The data can be displayed in the form of a two-dimensional histogram (see figure B 1.10.12).

A pulsed dye laser is the most practical excitation source for LEI spectrometry. Amplitude-modulated CW dye lasers have displayed a useful immunity to electrical interferences at any excitation position 38), but because of the inefficiency of the frequency doubling process, CW dye lasers yield low power emission in the ultraviolet spectral region. Most metals have their strongest resonance lines in the ultraviolet. In addition, ultraviolet transitions terminate nearer the ionization limit, producing the highest LEI sensitivity. 

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