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Shielding the source

To examine the effect of the source region having an open plan design, a series of experiments were undertaken using a modified ion source. This involved electrically `shielding' the region directly in front of the jet expansion. The filament is floated at approximately -70 V, the trap at +55 V, making it possible for the ion source to deflect the ions towards the filament upon creation. It was reasoned that this would apply an artificial electric field gradient, deflecting the ions upon their creation, possibly leading to heating and lower beam strengths.

A small U shaped stainless steel plate was constructed, with holes on either side to allow the electron beam to pass cleanly in front of the nozzle and arrive at the trap. The U shaped `shield' was welded onto the nozzle mounting plate and floated at the same voltage as the source.

An ion beam was created and spectra recorded for comparison. These are shown in Figure 5.8. A small heating effect occurs in these spectra, corresponding to an increase in the intensity of transitions for high N, relative to those for low N. Typical temperature increases are of the order of 50 K, possibly due to heating of the shield by the filament, where the heat is then being conducted to the region close to the nozzle. Table  5.3 lists the temperatures found for pure O2, Ar and O2 and He and O2. A greater recorded ion current is found in all the experiments when the shield is used, whilst the signal to noise ratio is seen to remain approximately constant. This increase of ion current is up to 5 orders of magnitude greater than that found without the shield in position.

The reason for this apparent increase of ion current, and apparent increase in temperature is uncertain. It is suggested that more ions are indeed now arriving at the skimmer, and not deflected by an electric field gradient due to the filament/trap arrangement. However, a warming effect occurs due to the extra nozzle heating, which is a distinct disadvantage if a colder beam is required.


Table 5.3: Comparison of the intensities of the 9 Q21(9.5) and 17 P21(18.5) transitions using the shield and different ratios of carrier gas mixtures/precursor mixtures. Relative beam strengths are normalised for direct comparison with Table 5.2
       
Precursor gas Intensity ratio Temp Relative beam
(ratio)   (K) Strength
       
       
O2 (N/A) 33 178 474
       
Ar (5:1) O2 53 144 31.4
       
He (5:1) O2 68 130 471
       
       


Figure 5.8: Effect of a using a shield. Note the increase in the intensity of the high N transitions with the shield, than with the shield not in place. This occurs similarly for a pure oxygen precursor and a buffer/oxygen precursor.


next up previous contents
Next: Other experimental considerations Up: Estimation of temperature Previous: Gas mixtures   Contents
Tim Gibbon
1999-09-06