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Introduction |
Specifications |
Description |
XPEEM and LEEM methods
Operating principle
The SPELEEM images a specimen that is illuminated with soft x-rays, ultra-violet (UV) radiation or low energy electrons. The emitted photoelectrons, or the elastically reflected electrons, are accelerated by a strong field in the objective lens, of which the specimen is an integral part. The objective produces a magnified image of the specimen, which is further magnified by several additional lenses in the imaging column of the instrument. The image
is energy-filtered by the hemispherical analyser and finally projected onto an imaging MCP detector with phosphorous screen. This is imaged in real-time by a computer controlled CCD camera.
Operation modes
Imaging. The sample is illuminated with x-rays or UV radiation, to excite photoemission, or with electrons. IL and P1 image the specimen image produced by the objective. A slit can be inserted in the dispersive plane of the analyzer, in order select the desired energy of the photoelectrons. The specimen image is finally projected onto the detector by the action of P2 and P3. The contrast aperture in the diffraction plane limits the angular acceptance for optimum lateral resolution.
SPELEEM operation: XPEEM & LEEM imaging mode
Micro-diffraction. A coherent beam of electrons is used to illuminate a crystalline surface, thus producing a LEED pattern. Otherwise, the sample is illuminated with soft x-rays or UV radiation. IL and P1 image the diffraction pattern produced in the backfocal plane of the objective lens. Inserting the analyzer exit slit allows angle resolved photoemission and x-ray photo-electron diffraction measurements. The probed area is selected by inserting the field-limiting aperture in the image plane after the objective lens.
SPELEEM operation: micro-diffraction mode
Micro-XPS (also known as dispersive plane operation). Same as in imaging mode but without energy slit. The last two projectors, P2 and P3, are used to image the dispersive plane of the analyser. The dispersive plane appears as a line, and its intensity profile represents the photoemission spectrum. The probed area is selected by the field limiting aperture inserted in the image plane after the objective lens. In this operation mode the microscope reaches its best energy resolution, ~0.2 eV.
SPELEEM operation: micro-XPS mode
References
SPELEEM: combining LEEM and spectroscopic imaging;
Th. Schmidt, S. Heun, J. Slezak, J. Diaz, K. C. Prince, G. Lilienkamp, and E. Bauer;
Surf. Rev. Lett. 5 (1998) 1287-1296.
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