
Electron
Diffraction in
Atomic
Clusters




This site allows performing online photoelectron diffraction calculations. Multiple scattering (MS) of the
photoelectron is carried out for a cluster representing a solid or molecule. Select the corresponding parameters and click on
the "Calculate" button below to perform the actual calculation and to produce a plot of the calculated data.
A numerical data table can be downloaded by clicking on the resulting plot. Click on
the different parameter names in blue for further descriptions. Click on the "Preview Cluster" button to display
the currently selected atomic cluster (but without performing a MS calculation) or the button "Download Cluster" to
download the currently selected cluster.
Notice that the scattering phase shifts and excitation radial matrix elements are calculated internally for each
cluster configuration, so that the user does not have to provide them. Please, read the terms of use and the restrictions on input parameters before using this
site for the first time.


Terms and conditions of use



Password:
A password is only necessary for large computation times
(click here for more details). Leave it
blank otherwise.

Title (optional):


Atomic cluster

The cluster and the list of emitters are defined by a list of
commands with the following format (click here or on the items of this list for further details):


Fill in the text box with these commands according to the cluster specifications that you need. Some
examples are provided. You may cut and paste them
to this page and modify them further.


The cluster consists of a maximum of atoms. (Warning: a finite number of
atoms generally introduces symmetry breaking.)
The size of the cluster is determined by the distance d_{max} =
Å and the reference point x_{0} =
Å, y_{0} =Å, z_{0} =
Å.
See cluster shape for more details.




Geometry of beam and analyzer



Note: the parameter β specifies the angle between beam and analyzer
when this is kept constant during the simulation (i.e., everything happens as if it is only the sample that is rotated);
therefore, this parameter is not required when only the analyzer moves or when both the sample and the analyzer move
(click
here for details).


Energy and angle scanning
parameters (see figure above)

The following entries will select the range of photoelectron energies and angles of emission.
Energy scans for a given emission angle can be chosen by selecting more than one energy of emission and only one polar
angle and one azimuthal angle (the value of each angle is then taken as the lower limit of the selected angular range, and the
value of the upper limits are disregarded). In this case, the output is a 1D plot with the photoelectron intensity as a
function of photoelectron energy.
Angular scans can be chosen by selecting only one photoelectron energy. Then, if the polar and the azimuthal angles
take both more than one value, the output is a 2D polar plot of the photoelectron intensity. Otherwise, if only one polar
angle is selected, a 1D plot is generated (an azimuthal scan), whereas if only one azimuthal angle is selected then a 1D polar
plot is given in the output.
For 2D angular scans, a linear or logarithmic scale of the photoelectron intensities can be selected for the grey scale
(see below). More details about the 2D representation (e.g., type of projection on 2D) are given in the caption of the output.

Electron energy range: equallyspaced value(s) of the electron
energy from eV to eV
Polar angle: equallyspaced value(s) of the polar angle θ
from degrees to degrees
Azimuthal angle: equallyspaced value(s) of the azimuthal angle φ
from degrees to degrees


Photoelectron detector halfwidth acceptance angle =
degrees. The photoelectron intensities are angleaveraged over a cone with half aperture given by this parameter.


Multiple scattering
parameters



Initial corestate quantum
numbers 



COMPUTATION TIME: the CPU time needed for the calculation using the default cluster and input parameters (use Reset to recover default input) is 1.24 seconds on a Pentium III @ 733 MHz. This gives a time scale to estimate the computation time for other input parameters, keeping in mind that it scales like ~ (n_{scat}  1) N^{2} (l_{max}+1)^{3}, where N is the number of atoms in the cluster and n_{scat} is the scattering order. For reference, the default values are N=48, l_{max}=6, and n_{scat}=2, for which the above number is 7.9 10^{5}.

IMPORTANT: READ THESE LINES BEFORE RUNNING THE CODE FOR THE FIRST TIME.
* The input file can be used to run the code locally, for which a copy of the code is needed. This can be obtained from Prof. F. Javier García de Abajo. An online version of the inputfile manual is also available here.
**Reset all input values (including cluster specification) to the original settings.

For comments, questions and suggestions, please contact: Prof. F. Javier García de Abajo
