GD Source Impedance
During the operation of a GD source, there is always at least one parameter which is not fixed. In RF operation with constant power and pressure it is the ratio of voltage to current; in DC operation with constant current and voltage it is pressure. What determines the value of the free parameter for a particular sample: impedance.
If we are discussing RMS values in RF operation or DC values in DC operation, we can think of the impedance as the ratio of voltage to current. What determines the impedance for a particular sample (all other things being constant) is secondary electron yield.
The secondary electron yield, g, is the average number of electrons emitted per incisdent ion. It is typically 0.1 for metals, but is estimated to vary from 0.053 for Au up to 0.152 for Ti,(1) i.e. by about a factor of three. For more details about secondary electron yield, click here.
Effect on Emission Yield
Because of the variation in emission yield, for constant voltage and pressure, the current should also vary by about a factor of three. Since emission yield is nearly proportional to current then the emission yield should also change by about a factor of 3.
For constant current and pressure, the voltage (or at least the effective voltage, ie the voltage above threshold) should vary by about a factor of three. Since the emission yield is nearly proportional to the square root of the effective voltage, the emission yield should then vary by about 1.7.
For constant power and pressure, the voltage and current should each change by about 1.7, giving a combined change in emission yield of about 2.3. This is about what we see, for example, for Si in Al-Si compared with Si in Fe-Si.
More information on the source impedance and emission yields can be found in a review article by Arne Bengtson and Thomas Nelis in Analytical and Bioanalytical Chemistry.(4)
(1) H Hocquaux, in R K Marcus (Ed), Glow Discharge Spectroscopies, Plenum, New York (1993), p 351.
(2) R A Baragiola, E V Alonso, J Ferron and A Oliva Florio, Surf. Sci. 90 (1979) 915.
(3) L Ohannessian, PhD Thesis, Université Claude Bernard, Lyon, France (1986).
(4) A.Bengtson, Th.Nelis; "The concept of constant emission yield in GDOES"; Anal. Bioanal. Chem.; 385; (2006); 568-586; DOI10.1007/S00216-006-0412-7
First published on the web: 1 June 2000.
Authors: Richard Payling and Thomas Nelis