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Step 4
Our basic explanation is split into several steps
How do we make a chemical image from a photoelectron image?
To start with we will consider x-ray photoelectron spectroscopy.

The chemistry of the sample is contained in the photoelectric response
to the x-ray illumination.

We use highly monochromatic x-rays from the NIST beamlines, and we
have two types of response:

First there is the direct response which is in the details of the
photoelectric yield. If we look at the energy spectrum of the
photoelectrons from the lowest energies up to the x-ray photon energy
we see a variety of features, a smooth distribution, sharp peaks, and
more diffuse peaks. See the spectrum on the right.

The spectrum is collected by using a fixed photon energy, and scanning
the energy of the CHA window across the spectrum.

All the parts of the spectrum say something about the chemistry of the
sample because the x-rays eject electrons from atomic core shells and
valency levels. These features are specific to the elements and their
chemical bonding within the first few 10’s of nanometers of the sample’
s surface.

Second there is the change of response to a change in the energy of the
x-rays. Because the atomic core levels have very specific energy levels
the can be excited by using very specific x-ray energies.

Sweeping the x-ray energy through the core level energy produces a
dramatic change in the absorption of the x-rays and the number of
photoelectrons produced.

This increase in yield can easily be measured and is indicative of not
only the element, but contains a huge amount of information about the
local chemical state and geometrical arrangements of the atoms. See
the next page.
There are a variety of different measurements of the two types of response to the x-rays.
For now we will only mention two:

First, X-ray photoelectron Spectroscopy (XPS) which is the direct response measure of
electrons coming out of the atomic core levels. The XPS electrons create a sharp peak
in the photoelectron spectrum. This shown in the spectrum above.

Second, one measurement is the response to the change in the x-ray energy, and the
method that we concentrate on for VPPEM is usually called the near edge x-ray
absorption fine structure (NEXAFS). A great deal of information about the chemical state
can be obtained from it.
Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy
More information on how it works
Directly measured XPS spectrum of Ag
Note: The measurement we use in VPPEM is a mixture of these two measurements, see next step.
Page 1
C(1s) NEXAFS spectra of charcoal (Robinia pseudacacia L.), a black C
particle (from Donna Stella soil), and a non-black C (non-black C is a
soil extract.
A large part of the image contrast in VPPEM imaging is the NEXAFS signal.

To make a NEXAFS spectrum we scan the photon energy over an atomic
absorption edge. This spectrum of the Carbon 1s transition shows how
different types of carbon give different spectra.
Both XANES and NEXAFS are acceptable terms for the same technique.
XANES name was invented in 1980 by Antonio Bianconi to indicate strong
absorption peaks in X-ray absorption spectra in condensed matter due to
multiple scattering resonances above the ionization energy. The name
NEXAFS was introduced in 1983 by Jo Stohr and is synonymous with
XANES, but is generally used when applied to surface and molecular
science.
There are several 'flavors' of NEXAFS depending on what signal is used to
measure the change in absorption of the x-rays on a sample.

Total Yield NEXAFS:
        Uses either the sample current or a biased
        secondary electron collector in front of the sample.
        This collects nearly all the secondary electrons

Partial Yield NEXAFS:
        Uses a small energy window from the secondary           
        electron spectrum using a spectrometer.  
        The collected energy can either be in a part of the
        spectrum  that has no secondary features, or a particular
        Auger peak can be measured.
The intensity of each pixel in the image is the intensity of the Partial Yield
NEXAFS signal at that pixel.

A VPPEM image is a complete set of images across the scanned energy
range. This makes a stack of images, a hyperspectral image.
How it works