Scanning photoelectron microscope (SPEM)

The heart of the measurement station is the SPEM. The main components are (i) a zone plate optic system, which provides the microprobe; (ii) a specimen positioning and scanning system, and (iii) a hemispherical electron analyzer with multichannel plate. The SPEM vacuum chamber is mounted through a special support on to a pneumatic antivibrational table with a pneumatic system. The chamber support and the pneumatic system of the table allows movements of the microscope chamber in the horizontal and vertical directions that are necessary for alignment of the central axis of the chamber and the zone plate optic system with the beam. The finite alignments of zone plate optics are performed through two independent XYZ-stepper motor stages for zone plate and order selecting aperture. The alignment is verified by the beam diagnostics mounted inside including Multichannel plate detector and photodiode for X-Ray transmission measurements. The sample holder manipulator has XYZ stepper motor coarse stage and XY precision piezo motor fine stage, and a liquid nitrogen cooling stage. Hemispherical electron analyser is mounted on 6 degrees of freedom manipulator.

The geometry of experiment is illustrated on the sketch of the general concept of the microscope, image on the right. The focused beam cone is incident normally to the surface of the sample whereas the emitted photoelectrons are accepted from the direction of 30⁰ with respect to the sample surface. For the present geometry typical escape depth for photoelectrons is 5-10 A, that is the information from only about maximum 10 atomic topmost layers is acquired. The photoelectron spectrum is analyzed with energy dispersive hemispherical analyzer. Thanks to multichannel plate in the analyzer dispersive plane a 48 point photoelectron spectra or images can be acquired simultaneously. Samples should be preferentially conductive as required in photoemission. Anyway one should keep in mind charging if one wants to measure the samples with high resistance. The flux density is extremely high so that one should take care of beam damage effects if planning to experiment with soft matter (organics etc.).

As a result of the experiment one can obtain (i) high resolution photoelectron spectra from surface features and (ii) images of intensity distribution of photoemission components.