Situated on the Berkeley hills overlooking the San Francisco Bay, the Advanced Light Source (ALS), a division of Lawrence Berkeley National Lab, is one of the world's brightest sources of ultraviolet and soft x-ray light. The backbone of the ALS is the storage ring, made of 12 arc-shaped sections (~ 10 meters long) and 12 straight sections (~ 6 meters long), in which electron bunches travel at 1.9 GeV of energy equivalent to 99.999994% of the speed of light. When electron bunches get accelerated by bending magnets in the arc-shaped sections and by undulators and wigglers in the straight sections, they emit intense light of characteristic spectrum, which is then directed to individual beamlines. Currently, the ALS storage ring holds 276 electron bunches separated by ~ 2 nanoseconds. The total ring current is 500 mA. Starting from early 2009, the ALS has been running in "top-off" mode, in which the decaying electrons get continually filled to keep the photon flux delivered to users constant at all times.
The beamline 10.0.1 utilizes photons delivered by the U10 undulator, a 4.5 m long undulator with a 10 cm period magnet array. The beam from U10 is extremely bright up to 1018photons/sec/mm2/mrad2/0.1% of bandwidth, which is 100,000,000,000 times brighter than a 60 W light bulb. However, the beam has a broad spectrum in energy and ARPES needs photons with well defined energy with narrow linewidth. The beamline 10.0.1 is equipped with a set of 3 spherical grating monochromators (SGMs). By changing the angles of SGM with respect to the incident beam, one can select photons of energy range 17 - 340 eV with a resolution better than energy divided by 10,000. Typically, ARPES measurements are made with 30 - 60 eV photons and the resolution contribution from the beamline is smaller than 3 - 6 meV.
Once the photon energy is selected, a set of vertical and horizontal mirrors focus the beam down to 150 x 100 micrometer spot at the sample position of the HERS endstation. The sample is mounted on the tip of a cryogenic manipulator with fully automated 6-axis motion. The sample temperature is controlled between 7 K to 400 K by a combination of liquid He cooling and a resistive heater. The energy and momentum of photoelectrons generated by the incident beam are analyzed by the Scienta R4000 hemispherical analyzer (please see the previous section on SSRL beamline V-4). The HERS system is also equipped with standard surface-science preparation and characterization tools, such as ion sputtering gun, e-beam heater, LEED, and x-ray tube. Recently, a molecular beam epitaxy system has been integrated into the experimental endstation, which enables the in-situ synthesis and characterization of novel thin films.
The most unique aspect of HERS system is that the hemispherical analyzer itself can rotate around the sample, which bears important advantages over conventional ARPES systems, where the sample gets rotated to probe different directions in k-space: i) All the geometry of the sample, polarization of the beam, and the beamspot on the sample are fixed throughout the measurement. This is particularly important for small, inhomogeneous sample surfaces. ii) By changing the sample normal together with the analyzer angle with respect to the beam polarization, one can control the vertical and horizontal components of the beam polarization on the sample surface.