The EXAFS Scan

An EXAFS scan is an energy scan with different regions. Every region has its specific step width and sample time. It is also possible to define regions that have a constant step width in momentum space:

ONLINE> scan/ex*afs
ONLINE> scan/ex*afs/loa*d=regions_file

If the qualifier /load is supplied, a file which contains the region parameters is automatically loaded.

The qualifier /nolog directs ONLINE to display i/i0 instead of ln(i/i0).

EXAFS scans are performed as fast scans (table 8.1.11). At least the symbols scan_c1 (I0, flux) , scan_c2 (I, signal behind the sample) and scan_timer have to be set.

The counter offsets have to be stored in the symbols scan_offset_c1, etc. They have to be determined before the measurement starts. Note that there is the function get_offsets() (section 13.3.1) that serves this purpose. The offsets correspond to a sample time of 1 second. They are automatically subtracted from the counter contents. The count rates are divided by the sample time before they are stored.

The symbol scan_min_c1 contains the minimum count rate for scan_c1 (I$_0$). If I$_0$ falls below this value the user is prompted whether the scan should continue.

The on-line display shows the counters, -ln(scan_c2/scan_c1) and -ln(scan_ref/scan_c2). The reference ratio is displayed only, if the symbol scan_ref has been defined, e.g.: scan_ref = c3.

The EXAFS regions are described by the SCAN GQEs regions_delta, regions_sample_time, regions_edge and regions_power. They can be edited from the menu. ONLINE creates the SCAN GQEs scan_name_delta and scan_name_sample_time to log the parameters. The region parameters can be written to .fio files by: write/scan/fio/exafs scan_name.

A menu is provided that allows you to edit the regions SCANs. It is called by KP0 ('Prepare Scan').

The keys KP7/KP9 allow you to insert/delete an edge. An edge is described by 4 regions with different spacings. Further details are given when you select this option.

The keys KP1/KP3 load/store the current regions from/in a disk file. Note that the SCAN GQEs, that define the regions, are preserved. That means, if the next ONLINE session is started in the same directory, the last region parameters are automatically restored. Further details are given when you select this option.

An example for an EXAFS scan can be found in section 15.9.