salmeron group

1st Generation Instrument (Currently used at the ALS beamline 9.3.2.):

M. Salmeron, D. F. Ogletree, H. Bluhm, F.G. Requejo

2nd Generation Instrument (Currently at ALS beamline 11.0.2 and BESSY beamline U49/2-PGM1)
This is an upgraded version of the original instrument, designed and built by a collaboration between the Lawrence Berkeley National Laboratory, Berkeley, CA, USA, and the Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany  (add link to FHI and BESSY)

Why ambient pressure photoemission spectroscopy?
The use of the original for UHV restricted surface sensitive technique
opens a wide field of new applications like e.g.:

-Study of surface chemistry under thermodynamic equilibrium with various gases  (including water vapor!)

-Study of catalytic reactions in realistic conditions of pressure and temperature

-Study of free liquids

-Study of biological surfaces in humid environments

Problem that have to be solved for HPPES:

-Electrons are scattered by gas molecules

-Analyzer needs UHV

-X-ray source needs UHV

Solutions:
-Capture of electrons in analyzer arm before collision with gas molecules =>gap between sample and analyzer of less than 1 mm

-Modification of electrostatic lens system of analyzer to implement 3 differential pumping sections (Pressure drop between hemisphere and high pressure chamber around 10E8)
=>Electron beam has to be focused through 2 apertures in lens arm

-Light from X-ray source passes window for protection of source (e.g. Al e.g. Al, Si, Be)

The Endstation System at the ALS / LBNL:

The endstation consists of two UHV-chambers for sample preparation/characterization and the actual high pressure measurements.

*Preparation Chamber:
- Heating, LN2 Cooling, Sputtering with Plasma Source, Option for Evaporator with Quartz micro balance
-AES, LEED, Mass spectrometer

*High Pressure Chamber:
-Heating, LN2 Cooling
-XPS or NEXAFS in pressures up to 10 torr (later probably up to 100 torr)
-Surface reaction products detection with Mass spectrometer in lens arm

*Sample holder and Transfer system:
-Thermionic’s STLC System with transferable thermocouple
-Load lock for sample introduction

Sample – Beam – Analyzer – Geometry:

Beam – sample geometry for solids or liquids:

Analyzer angles:

Angle in xy-plane: 45º,  Elevation angle: 35º = (111) direction
=> Magic angle for either horizontal or vertical polarization!

Analyzer modifications:

Commercial Phoibos 150 Analyzer from Specs: 2/3 of lens section replaced
Images show general concept, slight modifications were performed on final lens system.
Analyzer- and lens drawings by Klaus Ihmann from Fritz-Haber-Institut in Berlin

High Pressure Photoemission Spectroscopy (HPPES) at ALS

High Pressure Photoemission Spectroscopy at ALS

Guido

Water droplet at the sample stage