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Gas mixtures

To increase the cooling of the molecular beam from the jet source, the precursor gas was mixed with various ratios of argon and helium buffer gases. Results using oxygen and argon mixtures on the spectrum of  O2+are shown in Figure 5.7. A marked effect on the spectrum is observed, in that the 19 Q43(17.5) and the 17 P21 (18.5) are observed to decrease to the extent where they are virtually indiscernible from the background signal. A similar but greater effect occurs when helium is used as the buffer gas, however, the signal to noise ratios remain unaffected using this gas. The helium seeded beam has a greater ion current than that obtained using pure oxygen alone, and yields signal to noise ratios comparable to those found in a normal ion source. See Table 5.2 for comparison between the different precursor gas ratios. Helium is shown to reduce the rotational temperature to a greater extent than for argon, and has the added benefits of increased ion currents and better signal to noise ratio. Helium mixed with oxygen in the ratio of 5:1, gave an estimated temperature of 85 K. This temperature could be decreased further with an increase of pressure and/or a higher ratio mixture, but is still significantly higher than the value calculated from equations Equation 5.2 and [131], which yield 7 K. This is probably due to heating from the environment due to the proximity of the filament.


Table 5.2: Comparison of the intensities of the 9 Q21(9.5) and 17 P21(18.5) transitions for different ratios of carrier gas mixtures/precursor mixtures. Normalised daughter ion current for each precursor is also shown.
       
Precursor gas Intensity ratio Temp Relative beam
(ratio)   (K) strength
       
       
O2 (N/A) 75 125 77.3
       
Ar (5:1) O2 83 121 5.7
       
Ar (10:1) O2 87 119 21.6
       
He (5:1) O2 289 85 100
       
       


Figure 5.7: Effect of a using argon as a carrier gas. Notice reduction in intensity of the N =19 and 17 transitions relative to those of smaller N. With a 10:1 mixture, these transition are impossible to distinguish from the background signal. Laser mode hops are indicated by *.


next up previous contents
Next: Shielding the source Up: Estimation of temperature Previous: Nozzle Diameters   Contents
Tim Gibbon
1999-09-06