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.

For more information see here.

 

Second experimental endstation

 

This endstation is dedicated to absorption (electron yield) and fluorescence yield of “dirty” samples, especially those of current interest in the environmental sciences.  This endstation is vacuum-isolated from the first endstation and the rest of the U12A beamline by a thin (1500Ĺ) aluminum window, which is 30-50% transmitting in the soft x-ray range.  This permits measurement of real samples without baking in vacuum.  The fluorescence detection is critical for soft x-ray absorption measurements of these samples, since the escape depth of soft x-ray photons is more than an order of magnitude greater than that of the photoelectrons generated in this photon energy range.  This endstation is still being commissioned.

 

Contacts
 

Spokesperson:  David Mullins, mullinsdr@ornl.gov

Local Contact:  Steve Hulbert, hulbert1@bnl.gov

 

 

[ Surface Chemistry Group I Oak Ridge National Laboratory I Chemical Sciences Division I Disclaimers]

Revised: 26 - August - 2002 by David R. Mullins