A Normal Incidence Monochromator on Beam Line 5

- M. Rowen

SSRL in collaboration with Stanford University is building a new low- energy branch line on Beam Line 5. The end station will be a dedicated UHV system with a high resolution electron spectrometer. The thrust of this new branch line is high resolution photoemission from condensed materials in the energy region of 10 to 40 eV. The end station spectrometer, a SCIENTA SE200, has a demonstrated resolution of 6 meV. Theoretically this spectrometer should achieve <3 meV resolution. Therefore a large 6.65 m Normal Incidence Monochromator (NIM) has been designed to match this high resolution while maintaining high throughput.

To get high flux through the NIM with the first harmonic from the 10- period undulator it will be operated at relatively high fields, with the magnetic deflection parameter between 4.5 and 9. The beam will be deflected horizontally away from SPEAR by a mirror that can be inserted just before the existing monochromator. This mirror will be a low-pass filter, absorbing most of the higher harmonics. The remainder of the optical layout is of an astigmatic design. The beam will be focused vertically onto the entrance slit with a elliptically bent glass mirror. The spherical grating, the first horizontal focusing element, will image the source between the grating and exit slit with a demagnification of 7 to 1. After the monochromator there will be separate horizontally and vertically focusing mirrors to image the bean at the sample position.

The new beam line will be located between Beam Line 5 and Beam Line 6, leaving a large clear access area on the Beam Line 5 side of the hutches. Preparations are being made so components can be installed during the coming run without interfering with operations or experimenters. The monochromator is on order from McPherson with delivery expected later this fall. Installation of components is expected to start in December and continue through January. Commissioning is to occur during the remainder of the run.

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December 2, 1996