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Experimental Setup

The chamber was connected to the ion beam apparatus using a gate valve. The first magnetic sector was rotated through 90$^{\circ}$ and the current polarity reversed. Three sets of ion optics were used: immediately after the skimmer, after the first magnetic sector and before the second magnetic sector. Ion currents could be monitored at these positions using T.I.M. plates. Voltages for the ion optics mounted at the magnetic sectors are controlled by the high voltage supply for the conventional EI source. The supply was set to 2 kV, which applied voltages of 200 V to the ion optics. However, this value could be adjusted to optimise the ion beam strength from the jet source. In practice, for a jet source voltage of 2 kV, the EI high voltage supply was set to 1.8 kV (for O2+). Individual plate voltages were set using the ion optics control unit. Gas enters the jet source through a stainless steel gas line, regulated by a needle valve. The gases in the following experiments were oxygen or oxygen mixed with helium or argon.

Nozzle pressures of 2-3 bar were used, resulting in a pressure of 10-6 and 10-7 torr in the source and flight regions respectively. Electrical breakdown occurred in the source chamber at approximately 200 V. Experiments showed that this problem was due to the Swagelok fitting, however this breakdown could be avoided when 2-3 bar of gas flowed through the nozzle, allowing voltages >2.5 kV to be achieved (for oxygen).

The emission current of the filament was set to 1000 $\mu $A for the majority of experiments and the jet source programmed power supply was set to 2 kV. Optimal ion current arriving at the first magnetic sector was achieved when a small negative voltage was applied to the skimmer (typically -12 V). As mentioned previously, the trap potential could be varied between 55 and 24 V. This voltage was optimised so as to maximise ion current. Laser radiation from the CR 699-29 tunable dye laser entered the vacuum apparatus through the Brewster window mounted on the cross piece at the first magnetic sector. Due to the photofragmentation process (O $_{2}^{+} ~\rightarrow$ O+ + O), O+ ions could be directed towards an electron multiplier when the laser radiation was collinear with the ion beam.

After optimisation of ion optics, the laser was scanned and an O2+  spectrum recorded. The following parameters were used: 30 MHz sampling rate, 40 s/10 GHz segment, lock-in time constant 1 s, 500-10000 $\mu $V sensitivity (depending on signal to noise, ion current and laser power) and 1000 $\mu $A emission current.

The laser was scanned across regions between 17200 and 17300 cm-1  to find the strongest lines of the (4-4) band of the ${\rm b^{4}\Sigma_{g}^{-}\leftarrow a^{4}\Pi_{u}}$ transition. These regions have line positions recorded to a precision greater than 0.003 cm-1, and have large densities of lines [125]. A recording taken using a conventional EI source is shown in Figure 5.4. This spectrum has a dominant transition at 17261.3 cm-1  made up of three components, a shoulder at 17261.338cm-1  9 P31 (9.5) and a larger peak, at 17261.387cm-1 . This peak is a blend of two lines, the dominant 9 Q21(9.5) and the much weaker 17 Q11(18.5). This line has been deconvoluted into individual contributions using an energy analyser [125] which samples different ranges of rotational levels above the dissociation limit. Several other transitions are found in this region, including two transitions involving the same spin states [the 17 P21(18.5) and the 9 Q21(9.5) transitions].

Figure: O2+  spectrum recorded in a conventional ion source. Labelled transitions taken from the study by Cosby et al. [125]. Laser mode hops are indicated by *.
\resizebox{5in}{!}{\includegraphics{figures/o2s1901b.eps}}


next up previous contents
Next: Results Up: Review of O2+ Previous: States and Spectroscopy of   Contents
Tim Gibbon
1999-09-06