# 7.2. Setting E0¶

The `«e0»` is central to many of the chores that
ATHENA performs. Understanding how `«e0»` is used
and how it is set is essential to understanding how ATHENA
operates and essential to using ATHENA well.

By default, ATHENA has IFEFFIT determine
`«e0»` and uses the value that IFEFFIT
finds. IFEFFIT's algorithm is to find the first peak of the
first derivative of μ(E). To avoid settling on a spot in a noisy
pre-edge, IFEFFIT makes a series of checks to verify that
its guess for `«e0»` comes after several points in the
derivative of μ(E) that are increasing in energy and is followed
by several points that are decreasing. The details of this
implementation are such that IFEFFIT often chooses a point
that is slightly to the high energy side of peak that a person would
choose, but it typically is a very good guess.

The actual value of `«e0»` is used for many things. It is
compared to a table of edge energies to determine the value of the
`«Z»` and `«edge»` parameters. It is the energy
reference for the values of the normalization, pre-edge, and spline
range parameters. And it is the edge energy in the AUTOBK
algorithm.

The calibration and alignment tools can be used to refine the values for
`«e0»`. You can also edit its value directly by typing in its
entry box. The right-click context menu for
`«e0»` and the Energy menu have several options
for other ways of setting `«e0»`.

There are a few situations where the default algorithm will fail. Very
noisy data can result in a spurious identification of
`«e0»`. Materials which have two inflections in the edge,
such as zero-valent zirconium, might have the wrong inflection
chosen. Materials with huge edge peaks, such as K_{2}CrO_{4}, will have a point in the leading edge of the peak chosen
rather than a point in the main edge.

New in version 0.9.18: features related to E_{0} moved from
the Group menu to the Energy menu.

Aside from the default method described above, ATHENA
offers a few other algorithms for setting `«E0»`.

**Tabulated atomic value**- The tabulated value of the edge energy of the zero-valent element
will be used. To determine the element, a transitory value for e0
will be found using IFEFFIT. Once
`«Z»`and`«Edge»`are found,`«E0»`is set to the tabulated value. **Fraction of edge step**- In this algorithm, a transitory value for E
_{0}will be found using IFEFFIT. The data are normalized and the point on the edge which has a height equal to a specified fraction of the edge step is chosen as`«E0»`. This normalization is iterated up to 5 times to settle on a stable value. The value of the fraction is set by the ♦Bkg→fraction preference, which is 0.5 by default. **Zero crossing**- Again, a transitory value for E
_{0}will be found using IFEFFIT. The second derivative of μ(E) is computed and ATHENA searches in both directions in energy for the nearest zero-crossing of the second derivative, which is then used for`«e0»`. **Peak of L edge white line**- A transitory value for E
_{0}will be found using IFEFFIT. The peak of the white line is then found as the zero crossing of the first derivative of μ(E) after the initial value of E_{0}and is used as`«e0»`.

Since all of these additional algorithms rely on IFEFFIT's
initial guess of `«E0»`, each is subject to the same caveats
given for the default algorithm.

The ♦Bkg→e0 configuration parameter can be set to one of these options, setting
the default algorithm for determining `«E0»`.

Submenus under the Group menu can be used to set
`«E0»` using one of these algorithms for all groups, or the
set of marked groups.

## 7.2.1. Enforcing an element and edge¶

Sometimes you have data that is otherwise usable, but which contains a feature that confounds one of ATHENA's data reduction algorithms. An example is shown below. There was some problem within a short data range at the beginning of the pre-edge resulting in a high level of noise in the data. When searching for the value of the edge energy, ATHENA searches for a peak in the first derivative. In the case of these data, that peak in the first derivative is found in the noisy region at the beginning of the pre-edge region.

In a situation like this, it can be useful to supply ATHENA with a bit more information about the data. The usual practice in ATHENA is to determine the absorbing element and edge from the data without input from the user. This practice can be over-ridden by enforcing the element and edge for a data set.

The dialog for this is located at

. Selecting this pops uo the following dialog:Specify the element of the absorber and the edge you are measuring.
Doing so will force ATHENA to consider data to be of that
element and edge. The `«E0»` will first be set to the
tabulated value for the zero-valent lement. Then default parameter
values will be set, then the `«E0»`
parameter will be reset using the *Fraction of edge step* algorithm.

If you wish to later import data of another element or edge, it is
**imperative** that you stop enforcement by selecting
.
Enforcing the incorrect element or edge will have profoundly
undesirable consequences on the data processing.

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