Bending magnet beamfine 9.3.2 at the Advanced Light Source (ALS) was designed for high resolution spectroscopy in the soft x-ray energy region, covering a range from 30 eV to 1500 eV with three gratings. The monochromator itself is a standard fixed included angle 55 m spherical grating monochromator and was originally used at the Stanford Synchrotron Radiation Laboratory (SSRL) as a prototype for later insertion device based monochromators for the ALS. For operations at the ALS, the toroidal premirror used at SSRL to vertically focus onto the entrance slit and horizontally focus onto the exit slit was replaced by two separate crossed m i~o r~ (Kirkpatrick-Baez configuration). Circularly polarized radiation is obtained by inserting a water-cooled movable aperture in front of the vertically focusing mirror to allow selecting the beam either above or below the horizontal plane. To maintain a stable beam intensity. through the entrance slit, the photocurrent signals from the upper and lower jaws of the entrance slit are utilized to set a feedback loop with the vertically deflecting mirror Piezoelectric drive. The beamline end station has a rotatable platform (through 60") that accommodates two experimental chambers, enabling the synchrotron radiation to be directed to either one without breaking vacuum.

The atomic structure of the c͑2 3 2͒ reconstruction of the C-terminated 3C-SiC͑001͒ surface was unambiguously determined by scanning tunneling microscopy and surface-core-level-resolved photoelectron diffraction studies. This surface is found to uniquely and uniformly consist of anomalous bridge-bonded C dimers with a C-C bond length of 1.22 Å. Furthermore, an extensive angleresolved photoemission study clearly identifies two occupied p state bands due to the surface-normal and -parallel p orbitals of the triple-bonded C dimers. This provides an electronic explanation of the stability of this unique surface reconstruction. 68.35.Bs, 79.60.Dp SiC has attracted much recent interest due to its potential for novel device applications [1] as well as due to its unique surface properties, most popularly cubic 3C-SiC͑001͒ [2] and hexagonal 6H͑4H͒-SiC͑111͒ [3] surfaces. The partially ionic character of SiC provides an interesting variety of complex reconstructions and local bond configurations on its surfaces. It has been known that the 3C-SiC͑001͒ surface exhibits three major surface phases [2]: the C-terminated c͑2 3 2͒ surface, the Si-terminated 2 3 1 [or c͑4 3 2͒] surface, and the Si-rich 3 3 2 surface. Peculiar surface properties have been reported such as a metal-insulator transition for the Si-terminated surfaces [4] and a zero-dimensional fluctuation [5] and quantum wires for the Si-rich surface [6]. However, further detailed discussion on such surface properties is mainly prohibited by the lack of consensus on the structures of these surfaces [2,7,8].As for the C-terminated c͑2 3 2͒ surface, the possibility of a very unusual bridge-bonded dimer (BD) structure has been discussed [ Fig. 1(a)] [9-19], which is unprecedented among the known reconstructions of semiconductor surfaces. This was first introduced by a low-energy electron diffraction (LEED) study for the c͑2 3 2͒ surface prepared by Si sublimation [9] against the earlier model of conventional dimers [the staggered dimer (SD) model, Fig. 1(b)] [10]. In contrast to this LEED study suggesting a "double-bonded" BD, a recent x-ray absorption study favored a triple-bonded BD [9] structure for a c͑2 3 2͒ surface prepared by C 2 H 4 flux [11]. Some of the theoretical calculations, contradictorily, suggested a 2 3 1 conventional dimer structure as the energetically favored ground state [12][13][14][15], and others, the SD c͑2 3 2͒ structure [16,17]. Furthermore, in the major ab initio calculations [12,13,18] the total energy differences between the SD and BD c͑2 3 2͒ structures were trivially small, which leads to the prediction of the coexistence of different surface structures or the subtle dependence of surface structures on the preparation methods [12].In this Letter, we provide unambiguous answers to the questions of (i) the uniqueness, (ii) the quantitative details of the surface structure, and (iii) the bond configuration of the 3C-SiC͑001͒-c͑2 3 2͒ surface. Scanning tunneling microscopy (STM) studies for the c͑2 3 2͒ surface FIG. 1. The two structure mo...

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