Beamline U12A
RESEARCH
PROGRAM: Soft x-ray photoelectron spectroscopy (XPS), near-edge absorption
fine structure (NEXAFS). Surface
photochemistry, especially of automotive catalytic materials.
RING: VUV
STATUS:
Operational
TIME DEDICATED
to General User program: 50% of operations
Participating Institutions
ORNL.
|
Spectral Range (A) |
Grating Type (grooves/mm) |
Grating Radius (m) |
Resolving power (E/DE) |
Flux (photons/sec/0.1%bw) |
Total Angular Acceptance (mradians) |
|
100-400 |
300 |
57.3 |
1000-5000 |
2 x 1011 @ 500mA |
10.0H x 2.0V |
|
250-900 |
800 |
57.3 |
1000-5000 |
1 x 1011 @ 500mA |
10.0H x 2.0V |
OPTICAL CONFIGURATION
a) Collecting and focusing mirror (M0) Gold-coated side-cooled silicon toroidal (R=xx.xm, r=xxxmm) mirror; collects and focuses the source vertically at the entrance slit (S0, xx.xm from source) and horizontally at the sample position downstream of the exit slit (S1, xxm from source); angle of incidence is 87°; located xxxm from the source.
b)
Monochromator Toroidal Spherical
Grating Monochromator (TSGM), a variant of the standard SGM optical design in
which the Kirkpatrick-Baez collecting and focusing mirror pair is replaced by a
single toroidal mirror. In the soft x-ray
range, the reflectivity advantage of the single mirror in the TSGM design
compensates for the aberrations associated with the focal properties of a
toroidal mirror at grazing incidence.
As a result, the TSGM and SGM designs have roughly equal transmission in
the soft x-ray range. The U12a is a
6.5m TSGM, with two interchangeable gold-coated ULE laminar (holographically
ruled, ion etched) diffraction gratings.
The gratings diffract and focus vertically onto the moveable exit slit
of the monochromator; the wavelength scanning mechanism is a simple sine drive
type; zero order angle of incidence is 87° (included angle is 174°). Grating
chamber is located x.xm from the source.
The spot size at the I0 chamber, located ~2m downstream of the exit slit, is 1.5(v) x 7(h) mm2 (can be collimated down). This chamber is used for measuring and monitoring of the monochromatized flux, and as a gas cell for photon energy calibration. Owing to lack of floor space, there are no refocusing mirrors/chambers at U12A. The alternative is to install smaller refocusing mirrors in the experimental endstations themselves, which is planned but not yet implemented for either of the two endstations.
EXPERIMENTAL APPARATUS
The
two U12A experimental endstations are quite different in capability: the first is a UHV surface science
photoemission chamber while the second is dedicated to absorption (electron
yield) and fluorescence yield of “dirty” samples in a thin-window-isolated,
unbaked UHV chamber.
First experimental endstation
This
endstation is an ultrahigh vacuum endstation dedicated to core level
photoelectron spectroscopy (XPS) and soft x-ray absorption spectroscopy (NEXAFS). It contains a 125mm radius
electron energy analyzer for XPS, a partial yield electron detector for NEXAFS,
plus a complement of surface science tools including thermal desorption,
sputtering, annealing, gas dosing, etc.
Samples are mounted on a standard X, Y, Z - rotatable manipulator. The samples can be cooled to < 100 K and
heated resistively to > 1000 K. Typical systems being studied in this
endstation are 1) characterization of adsorbates on
metal and oxide surfaces, 2) characterization of oxides, nitrides and sulfides,
and 3) characterization of metallic thin films. For example, in the last two years, the following systems have
been studied:
1) The interaction between NO and CO on Rh / CeOX thin films,
2) The adsorption of NO on carbided W(111),
3) The adsorption of CO and NO on Pt / CeOX thin films, and
4) The adsorption of chlorinated compounds on FeOX.