Theortical

Conventional commercial spectrometers are limited to a maximum ionisation energy of 1486.6eV, the photon energy of the AlKα source. Higher energies are useful in a variety of analyses and many sources, some of which are listed in Table 1, have been employed previously, with varying degrees of success. Ordinarily the AgLα would be too broad to be of interest to the analyst: fortunately, however, the wavelength is such that the line width can be reduced by a suitably configured aluminum monochromator.

Ag Lα;  Ep = 2984.2eV   Wavelength, = 4.1544Å,    approx. 1/2 Al (8.3393 Å)

Therefore the Bragg relationship can be satisfied: n = 2d sinθ (2nd order diffraction, n=2)

In this way a source is created with a reduced photon line width and a wider range of accessible energies.

New core levels can be excited for:
Al, Si, P, S, Cl, Br -> Ru and Tm -> Pt, Au, Hg, Pb, Bi.

As well as exciting the deep core levels and Auger transitions, the analysis depth is also increased, because of the greater photon energy, a useful attribute for those seeking "non destructive" depth profiling.

Implementation

In practical terms it is necessary to run a modern photoelectron spectrometer with better than 90% of uptime. To switch between aluminum and silver monochromated sources it is unacceptable to vent the system and so lose valuable instrument time. Therefore we have developed a twin anode monochromatic source. The anode is provided with both aluminum and silver coatings and two electron sources are used to generate Al or Ag X-rays as required - see Figure 1.

Figure 1: x-ray monochromator geometry