Extend energy scan_mode in NXenergydispersion

[lukaspie]

Currently, NXenergydispersion also supports fixed or sweep as ways of scanning the energy (in energy_scan_mode).

There are multiple ideas how to extend this description

• From ARPES (@rettigl):
  • transmission modes, jittered mode
• From XPS (@lukaspie):
  • In XPS/UPS/AES, people typically differentiate between
    • constant ΔE mode, where the electron retardation (i.e., the fraction of pass energy to kinetic energy, R = (EK - WF/Ep)) is scanned, but the pass energy is kept constant. This is often used in XPS/UPS because the energy resolution does not change with changing energy (due to the constant pass energy). Synonyms are: constant analyser energy (CAE), fixed analyser transmission (FAT) mode
    • constant ΔE/E mode, where the pass energy is scanned such that the electron retardation ratio is constant. In this mode, electrons of all energies are decelerated with this same fixed factor. Thus, the pass energy is proportional to the kinetic energy. This is often used in AES to improve S/N for high-KE electrons, but this leads to a changing energy resolution (∆E ~ Ep) at different kinetic energies. Synonyms: constant retardation ratio mode. Synonyms: constant retardation ratio (CRR), fixed retardation ratio (FRR) mode
  • See also the ISO definition, term 12.64 ff.
  • The current energy_scan_mode allows only "fixed" or "sweep". We had this discussion NXxps sub app-def #30 (comment) already. It would be better to allow for all of the synonyms above or even have a NXenergy_scan_mode base class.
  • For some vendors, there are also even more modes, like snapshot (where the energy is not scanned at all and the channels of the MCP are not averaged) or measuring at a fixed energy (for alignment).