Intrinsic and Extrinsic Losses

Intrinsic and Extrinsic Losses (Satellites, Peaks)

Intrinsic Peaks

Shake-up peaks
Multiplet splitting peaks

Extrinsic Background (loss)

Extrinsic background is defined as the intensity that has been lost from the generated photoelectron signal, to produce the background at lower KE.
Extrinsic background is produced by inelastic electron scattering as the photoelectrons pass through the solid, and escape the surface.

Origins of the XPS background Features

• Extrinsic losses (electron-phonon events) are due to inelastic scattering that produce the extrinsic background
Extrinsic energy loss occurs following the ejection of the electron from a bound state in an atom, and is independent of the photon induced event.

• Intrinsic losses (electron-electron events)
Intrinsic energy loss takes place at the same moment that the electron is ejected from the surface.
A part of the photoemission event

Plasmons

Extrinsic plasmon: excited as the energetic photoelectron propagates through the solid after the photoelectric process
Intrinsic plasmon: screening response of the solid to the sudden creation of the core hole in on of its atoms
These two kinds of plasmon are difficult to distinguish

Accurate quantification must include peak intensity, peak shape and background energy

• Intrinsic electrons – from photoelectron process
• Extrinsic electrons – from scattering of photoelectrons passing through surrounding atoms – extrinsic background
• Depending on the depth of the emitting atom within the surface, as well as its lateral distribution, the extrinsic portion will change dramatically
• The figure shows a theoretical calculation of the extrinsic portion of a copper 2p spectrum as a function of the position and distribution of the emitting copper atoms within a matrix of another element

Intrinsic and Extrinsic spectra – M. Kurth and P.C.J. Graat

From Brundle and Crist, JVST vol 38, p 041001-2

There are two fundamental factors limiting the accuracy in converting photoemission intensities from solid samples into atomic concentrations. These are:

(1)
Subtraction of the extrinsic background. Extrinsic is defined as the intensity that has been lost from the generated photoelectron signal, into the background at lower KE, by inelastic electron scattering as the photoelectrons pass through the solid,escape the surface, and are detected.

(2)
Knowledge of the spectral distribution of the intrinsic photoelectron signal concerned. Intrinsic is defined as that part of the observed spectrum resulting from the initial photoemission process ejecting an electron, which escapes the solid without having undergone inelastic scattering. For quantitation, using XPS peak relative intensities, it is the intrinsic component we use

The F1s spectrum insert at top left clearly shows the position of the start of the expected extrinsic background as a step at ∼13 eV. Lithium 1s also shows.

Superimposed on this background is a series of peaks, labeled 1–10, spreading out over the full 100 eV of the scan. They constitute about 25% of the total F1s intensity. This is quite consistent with the expected shake contribution, judged from the atomic spectrum of the next element in the periodic table, Ne, in Fig. 3(a). So, we estimate that a “main line” only
measurement of F1s in LIF may underestimate the total F1s intensity by ∼25%.

Looking at the F2s, F2p, and Li1s high-resolution spectrum, in Fig. 6, one can identify the same step initiation of the extrinsic background at ∼13 eV after each main line. One also sees that
exactly the same series of shake satellites found for F1s is present on the F2s spectrum. The overlay comparison, where F1s and F2s main lines are aligned, makes it clear that the shake-up positions are identical in F2s and F1s.

Note that satellites 2 and 3 from F2s lie right under Li1s, causing the experimental intensity of the Li1s line to be overestimated by between 10% and 30%, depending on how the background is drawn and the peaks are curve fit. As far as we can tell, there is little discernable satellite intensity actually associated with Li1s. The structure after Li1s comes mainly from the overlapping satellites of F2s.

The lack of significant shake structure for Li1s is actually expected from simple argument, as Li+ has no valence electrons to be excited into higher lying orbitals. Full ab initio theory supports these arguments, calculating a 1.4% loss to satellites for Li+, but 22.7% for F−.22 In solid LiF, both losses would be expected to increase slightly because of the availability of screening electrons from the surrounding lattice.

Thus, the corrections necessary to the experimentally determined F1s and Li1s intensities to compare to theory require an increase of ∼25% for F1s (i.e., y is not unity, but is 0.75) and a reduction to the Li1s intensity of ∼10%–30% due to the overlapping F2s structure. A total correction of ∼35%, renormalized to F1s as unity, to the e-RSF value for Li1s is marked in Fig. 5(b) and a corrected e-RSF curve drawn based on a straight-line fit to the data.

We initially expected that the losses of intensity to satellites from the main 1s line to gradually increase on moving from ∼0 for Li1s (y = 1.0) through Be, B, C, N, and O to the ∼25% value for F1s (y = 0.75), simply on the basis that there is a smoothly increasing number of valence electrons available for shake up. Work by the present authors on BeF2 and (CF2)n does not support this, however. Both the F1s and the Be1s in BeF2 spectrum have backgrounds with some superimposed structure, but they seem to be of roughly similar intensities. F1s and C1s in (CF2)n behave similarly. This may be because these compounds have much greater percentage covalent bonding, with only LiF being fully ionic, leading to the completely different satellite behavior for the F1s and the Li1s.

It is important to note that the BE range acquired for these high-resolution spectra is far wider than is normally used in XPS analysis (20–30 eV is usual), in order to capture all observable satellites.