Spectra for O2+ were obtained using two different nozzle
diameters, made from 25 m and 12.5
m pinholes (Ealing). No
discernible loss of ion current occurred with the smaller nozzle, as
the stagnation pressure was increased to compensate for reduced
throughput (see Figure 5.6).
Analysis of the recorded spectra using several different mixes of precursor gases show that a significant change in the temperature occurs when the smaller nozzle is used. The intensity ratio between the 9 Q21(9.5) and 17 P21(18.5) transitions recorded using the conventional ion source and for the two different nozzle diameters are measured and temperatures deduced (see Table 5.1). A significant decrease in intensity of the 17 P21(18.5) line relative to the 9 Q21(9.5) line is found to occur with the smaller nozzle, corresponding to a significant decrease in the temperature of the ion beam. The temperature of the ion beam is found to be
125 K, a reduction of approximately 400 K from a conventional EI source. This experiment demonstrates that the ion source can create a significantly rotationally cooler molecular ion beam, with a reasonable ion current and without loss in signal to noise ratios or the linewidth.
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