The ALS Core Environmental Beamlines
This beamline provides a unique soft x-ray facility for molecular enviromental scientists and has serviced the community since entering full operations in June of 2003. The ALS-MES Beamline furnishes photons from an elliptically-polarizing undulator (EPU) source from 75 eV to 2100 eV to an LBNL-designed variable angle-included plane grating monochromator that has an energy resolution (E/DE) of ~7500. This supports the operation of two facility surface science end stations, a scanning transmission x-ray microscope (STXM) end station, and has a roll up location for external end stations. Beamline 11.0.2 provides unique opportunities for MES investigations using x-ray photoelectron spectroscopy (XPS), x-ray emission spectroscopy (XES), near-edge x-ray absorption fine structure (NEXAFS), and STXM techniques. The special optical characteristics and experimental attributes of the ALS-MES beamline end stations are extremely attractive to NABIR, EMSP, and other MES researchers. The ALS-MES effort maintains laboratory space on the ALS mezzanine that is essential for supporting experimental at the ALS-MES Beamline for activities such as sample preparation.
The broad scientific areas of investigation that are undertaken with this beamline include those associated with the speciation and spatial distribution of environmental important species; sorption, complexation, and precipitation at the solid-aqueous solution interface; microorganisms, organic substances, plant-metal interactions; environmental materials science and materials chemistry; actinide and actinide environmental science; biosciences soft x-ray spectromicroscopy; and catalysis.
Beamline 11.0.2 has spectroscopy (22.214.171.124) and microscopy (126.96.36.199) branchlines that support activities at the spectroscopy and STXM end stations, respectively. The EPU source illuminates a single branchline at a time. Alternating between the two branchlines is fully automated and can be done in minutes, including beam optimization. During typical use, the two branchlines split the available beamtime evenly as this generally achieves optimal experimental efficiency for most users. Furthermore, this permits optimal support of user experimental activities by beamline staff. The spectroscopy end stations are located on a moveable platform capable that can switch between end stations in about fifteen minutes, whereas the STXM end station is a permanently located on the experimental floor of the ALS.
The ALS-MES STXM end station at Beamline 11.0.2, brought from its previous location on ALS BL 7.0.1, [Kilcoyne, JSR 2003] is located in an isolated area on the optimized ALS-MES microscopy branchline. It is a currently the highest performance x-ray microscope of its type, and provides spatially-resolved chemical information via NEXAFS (Near Edge X-ray Absorption Fine Structure Spectroscopy) from samples on the 25 nm - 35 nm scale. Combining the extended operational photon energy range of 130 eV to 2100 eV with full polarization control makes the ALS-MES STXM a uniquely capable tool for MES investigations. Several improvements were implemented at the time STXM was relocated, including added vibrational stability, new sample scanning stages, a new control/data acquisition system that permits the use of the same user interface as the STXM at ALS BL-5.3.2, and a fast shutter to minimize sample radiation damage [Kilcoyne, SRI 2004].
In STXM, a small spot of x-rays is raster scanned relative to the sample to create an image one pixel at a time while the transmission signal is monitored for both imaging purposes and NEXAFS spectroscopy. The small x-ray spot is produced by a zone plate lens and samples can be investigated from the multi-millimeter to tens of nanometers scale. Investigations can be conducted in vacuum, in air (at atmospheric pressure), in helium or with other non-reactive gases. Samples can also be liquids encapsulated between thin Si3N4 windows. STXM has been used to investigate many different types of samples including polymeric (Ade,1992, 1993, 1995, 2002), geochemical (Botto 1994; Cody, 1998), environmental (Plashke, 2002), magnetic (Kim, 2001), extraterrestrial (Flynn, 1998; Keller, 2000), and biological samples (Kirz, 1995). Most STXM studies take advantage of the information provided by the NEXAFS spectroscopy of elements with absorption edges in the soft x-ray energy range, particularly of carbon, nitrogen, and oxygen (Stöhr, 1992). The ability to adapt STXM to a wide range of sample environments such as magnetic fields (Kim, 2001), variable temperature (Wang, 2000), and to investigate polymeric and environmental samples in aqueous media (Neuhäusler, 1999; Mitchell, 2002) is an important aspect of the technique, as is the relatively low radiation damage caused by soft x-rays (Rightor,1997; Coffey, 2002). The figure above shows an example of the STXM capabilities for MES.
The ALS-MES beamline has two user end stations, the high pressure photoelectron spectrometer (HPPES) and the wet electron spectrometer (WES), located on the platform of the spectroscopy branchline. These are both ultra-high vacuum (UHV) style surface science end stations with special capabilities for MES investigations. The HPPES is a surface science end station that can perform high resolution photoelectron spectroscopy and NEXAFS with a differentially pumped experimental cell in the spectrometer chamber at pressures up to 10 Torr, with future capabilities likely to approach 100 Torr. The HPPES has a sample load lock-transfer system and an isolated preparation chamber, permitting rapid introduction of samples and fully surface science capable preparation chamber. The WES end station is similar to the HPPES with the exceptions that it has an XES spectrometer complementing its photoelectron spectrometer and that it is currently being utilized for more UHV style investigations prior to being outfitted with high pressure and liquid cell capabilities. Full specifications of BL-11.0.2 and the end stations are located at http://www-als.lbl.gov/als/als_users_bl/bl_table.html and http://beamline1102.als.lbl.gov.
Dr. David Shuh of the Chemical Science Division is available to train the new beamline scientist funded by this proposal on this beamline.