Page 425 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN012C-568 July 26, 2001 15:32

Photoelectron Spectroscopy 75

TABLE VI Sources of Exciting Radiation are used only for special investigations. However, some
of these lines can show up in the helium discharge if the
Source Energy (eV) Width (eV)
helium is not of very high purity. Because of the lower
He I 21.22 0.001 energy of the corresponding excited states, the relative
I α 21.2182 intensity of the impurity lines is much higher than the
I β 23.0848 concentration of the impurity itself.
I λ 23.7423 For higher excitation energies, the only easily avail-
II 40.81 0.01 able monochromatic radiation comes from characteristic
II α 40.8140 X-ray emissions. Because of the short lifetime of the in-
II β 48.3718 volved core hole states (10 −16 sec), the half-width of the
II λ 51.0170 emitted radiation is much larger than for the resonance
II δ 52.2415 lines discussed above and increases rapidly with the or-
Ne I α 16.6709 0.001 dering number. The number of applicable photon sources
16.8482 is rather large and some of the more viable examples are
II α 26.8141 0.01 listed in Table VI. The standard materials used in most in-
26.9110 struments are aluminum and magnesium, often in a twin
Ar I α 11.6237 anode configuration, which provide K α radiation at 1486.6
11.8282 and 1253.6 eV, respectively. The filament is kept at ground
Y M ζ 132.3 0.5 while the anode is held at a high potential (12–20 kV).
Zr M ζ 151.4 0.8 Twin anodes have two filaments which allow changes be-
Ti M ζ 452.2 tween Mg K α and Al K α by simply switching the power
Mg K α 1253.6 0.7 from one filament to the other.
Al K α 1486.7 0.8 Figure 14 shows the 4f doublet of polycrystalline gold
Si K α 1739.9 0.9 measured with Si, Al, and Mg K α radiation. The intensity
Zr L α 2042.4 1.7 obtained with characteristic X-ray lines is about two or-
Ag L α 2984.4 2.6 ders of magnitude lower than that of the resonance lines.
Ti K α 4510.9 2.0 High-power X-ray sources operating with a rotating anode
Cr K α 5417.0 2.1 have been designed for this reason. To gain spectra from
Cu K α 8047.8 2.5 deeper core levels, sources with higher energy are useful.
However, the use of such sources is not without penalties
in the form of decreasing spectral resolution and reduction
−1
corresponding to about 100 cm . The half-width, which in sensitivity for the very light elements.
determines the resolution obtainable in the spectrum, can Like the resonance lines, the X-ray lines are always ac-
be reduced by altering the operating conditions, but at the companied by satellites with intensities of up to 10% of
cost of rapid intensity loss. The numbers given above are that of the main line. This must be considered in the evalu-
a compromise between intensity and half-width. Under ation of spectra. For example, the small structures seen at
normal operating conditions about 98% of the emission the low binding energy side of the photoelectron lines in
˚
consists of the He I α line at 504 A. Thus, the helium reso- Figs. 1a, 10c, and 14 are due to the satellites of the excit-
nance lamp is a source of fairly monochromatic radiation. ing radiation. In addition to the satellites, there is always
If the lamp is operated at lower helium pressure and higher a bremsstrahlung continuum underlying the characteristic
voltage, it also emits He II radiation, which is the radiation X-ray lines. This continuum can be reduced by inserting
+
emitted from He . The strongest emitted line is the He II α a thin metal foil (the nature of which depends on the an-
line at 40.8 eV (compare Table VI), and under optimal con- ode material) between the X-ray source and the collision
ditions up to 40% of the emitted radiation is He II α .How- chamber. The foil must be sufficiently thin and cannot be
ever, He I radiation is also present and has to be removed used to maintain a reasonable pressure difference between
by optical techniques if pure He II α radiation is needed. the source and the chamber.
Under the conditions for optimal He II emission, the total To reduce the half-width of the characteristic X-ray
intensity of the emitted radiation is lower than the intensity lines,X-raymonochromatorshavebeendesignedinwhich
under the standard conditions for He I operation. bentcrystalsareusedtodispersetheradiation.Themethod
Resonance lines from other atoms can also be used for depends on the dispersion of X-rays by diffraction, as pre-
excitation. Data for some of these lines are included in dicted by the Bragg equation
Table VI. Since there is usually more than one intense line
and since all the energies are lower than He I α , these lines nλ = 2d sin ϕ (18)