EXAFS Scans
Specfe provides macros and a user interface for performing exafs scans. These allow multiple scan regions to be set up, with different parameters in each, as well as scan regions where the steps are in 'k' rather than energy and where the counting time changes with energy according to a power law.
You can choose in the specfe preferences dialog whether specfe automatically loads the exafs macros every time it starts up, or whether it should assume they are already loaded and not load them automatically. If wanted to change the operation of the macros you could redefine them after they had been loaded - in this case you would probably want to make sure that specfe did not automatically reload its own macros every time!
The EXAFS Scan dialog provides a graphical user interface to the EXAFS scan macros. It consists of four main functional areas:
Controls to allow you to select the EXAFS edge that you will be scanning through.
The Edge Element menu lets you choose which chemical element to show edges for. When you choose an entry from the menu a list of possible edge energies for the element will appear in the adjacent Edge menu - choosing an edge from this menu will cause the corresponding edge energy to be entered in the Edge Energy control below. Alternatively, you can manually enter an arbitrary value in the Edge Energy control. The Min Energy and Max Energy controls can be used to limit the range of edge energies that will be displayed for each element.
Controls to allow you to specify the number of regions in the scan.
The Add and Rmv buttons are used to change the number of regions in the scan. As you change the number of regions with these buttons the number of rows of region parameter controls will change.
Parameters for the individual scan regions.
The parameters for each scan region are specified relative to the energy given in the Edge Energy control. The scan regions are assumed to be contiguous, so only the first region has a 'start' parameter - the other regions are assumed to start at the end value of the previous region.
The start and end of the regions can be specified either by energy or by 'k' and if you specify one then the GUI will automatically calculate the corresponding value of the other. You can specify either a step size, or a number of steps for each region - the units for the step size depend on the Kind of the region. Again, if you change one value, the GUI will automatically calculate the value of the other.
The counting time and an optional delay time is specified for each region. Use an Exponent of zero for a constant counting time across the region, or a non-zero exponent to give a counting time varying according to a power law, t = t0*((k/k0)^expt), where t0 is the counting time at the start of the region, k0 is the k value at the start of the region, k is the k value of the given scan point and expt is the exponent for that region.
Controls to split and join existing scan regions
An existing scan region may be split in two by clicking on the corresponding 'Split Rx' button for the region. The split between regions will be roughly at the middle of the initial region.
Two existing regions may be joined together by clicking on the corresponding 'Join Rx,Ry' button. The resulting region will take most parameters from the first region and will have the same number of points as the two regions together and will take its end point from the second region.
Controls to govern the scan execution
The Scan button is used to start a scan based on the current parameters, and the Abort button may be used to interrupt a running scan.
The Macro control may be used to specify the name of a spec macro to define with the current scan parameters. Click the Define button to define the macro. Once you have done this, you may re-execute the macro at a later date, or include it into your own macros.
The defined macros are listed in a pull-down list and may be recovered into the EXAFS Scan dialog by choosing a macro name from the list. You can then modify their parameters and save them either as the same name macro or as a new macro.
Be careful when choosing macro names not to use a name that spec is already using - there is no check for this in specfe and it is possible to accidentally overwrite an existing spec command.
A typical scan macro might be as shown below:
431.PSIC> prdef cu_k # /home/jennings/.specfe/exafstmp-427 def cu_k '{ exafs_edge_energy 6.539 exafs_region -120 -15.0 525 2 3 0 0 exafs_region -15.0 10 71 2 .5 0 0 exafs_region 10 593.679 218 2 .5 1 1.1 exafs_region 593.679 873.279 61 5 1 1 0 exafs_scan }'The scan is defined by calls to the macros exafs_edge_energy and exafs_region and is started by a call to the exafs_scan macro. The order of the calls is significant, the call to exafs_edge_energy should come first since it also causes the counter for the number of regions to be reset.
The definitions for the exafs macros will be found in the file exafs-macros.mac which should be installed in the <prefix>/lib/specfe/ of the specfe installation, by default /usr/local/lib/specfe/exafs-macros.mac
The EXAFS macros
A brief synopsis of some of the macros used is given below:
exafs_edge_energy energy
Specifies the edge energy (in keV) for the scan, and resets the region counter.
exafs_region start end steps ctime [sltime] [kstep?] [ctpow]
Specifies another scan region and increases the region counter by one. The start and end parameters should be in energy, (in units of eV) and are relative to the edge energy given in the exafs_edge_energy macro. steps is the number of steps in the region - the option to specify a step size is only implemented in the parameter dialog.
ctime is the counting time and sltime is the delay time, (both in seconds). kstep? is true (non-zero) if the region should step in 'k', or false (zero) if the region should step in energy. ctpow is the counting time exponent as described above. The parameters shown in brackets [] are optional, if they are not given, their value will be taken as zero.
exafs_scan
Actually starts the exafs scan.