A differentially pumped electrostatic lens system for ... - inquimae

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 11 NOVEMBER 2002 

A differentially pumped electrostatic lens system for photoemission 

studies in the millibar range 

D. Frank Ogletree, a) Hendrik Bluhm, b) Gennadi Lebedev, c) and Charles S. Fadley 

Materials Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 

Zahid Hussain 

Materials Sciences Division and Advanced Light Source, Lawrence Berkeley Laboratory, 

Berkeley, California 94720 

Miquel Salmeron d) 

Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 

Received 19 February 2002; accepted 8 August 2002 

A high pressure photoemission system is described that combines differential pumping with an 

electrostatic lens system. This approach allows optimized differential pumping without loss of 

signal, thereby increasing the high-pressure performance by at least 2 orders of magnitude compared 

to passive differential pumping systems. A general analysis of aperture-based high-pressure 

photoemission is presented, followed by a description of the prototype system which has operated 

at pressures up to 7 mbar on a synchrotron beamline. Using this approach, photoemission 

experiments should be possible up to 100 mbar. Example data are presented for dielectric samples 

in gas atmospheres, for a copper catalyst under reaction conditions, and for liquid water in 

equilibrium with its vapor. © 2002 American Institute of Physics. DOI: 10.1063/1.1512336 

I. INTRODUCTION 

Photoemission is a powerful tool for surface and interface 

analysis but its application has traditionally been limited 

to high vacuum environments. However many interesting 

processes at solid surfaces such as catalytic reactions or the 

adsorption of water on oxide surfaces under environmental 

conditions cannot be studied in conventional photoemission 

spectrometers. Characterizing samples in vacuum before and 

after reaction or gas exposure at higher pressures may not 

reveal the fundamental reaction steps. Photoemission spectroscopy 

at higher pressures could add significant capabilities, 

as there are few surface sensitive techniques able to 

analyze the gas–solid interface. Surface elemental composition 

can be determined from core-level peak intensities xray 

photoemission spectroscopy XPS, and in many cases 

information about chemical bonding can be obtained from 

core level peak shifts or from valence-band photoemission 

ultraviolet photoemission spectroscopy. 1 Synchrotron 

sources provide variable energy x rays and access to additional 

methods such as near-edge x-ray adsorption spectroscopy 

NEXAFS which probes the density and orientation of 

unoccupied valence levels, 2 extended x-ray adsorption 

spectroscopy, 3 which gives information on distances to nearneighbor 

atoms, and more general photoelectron diffraction 

methods which give more complete structural information. 4 

Inelastic electron scattering by gas phase molecules sets 

a Electronic mail: ogletree@lbl.gov 

b Present address: Fritz Haber Institut der Max-Planck-Gesellschaft, Berlin, 

Germany. 

c Now with: GETOM Corp., Petaluma, CA. 

d Author to whom correspondence should be addressed; electronic mail: 

salmeron@tm.lbl.gov 

the fundamental pressure limit for photoemission. Spectra 

can be obtained at higher pressures by reducing the distance 

electrons must travel. One practical approach is to separate 

the sample region from the x-ray source by a suitable window, 

and from the electron spectrometer by an aperture, optionally 

followed by one or more differential pumping 

stages. A number of groups have used this approach to obtain 

photoemission data at up to 1 mbar 100 Pa. Siegbahn 

et al. designed a photoelectron spectrometer for gas phase 

studies 5 and several types of spectrometers for liquid 

studies. 6 Joyner et al. developed a high-pressure spectrometer 

for the investigation of solid surfaces that was eventually 

commercialized, 7 and Ruppender et al. 8 developed a differentially 

pumped system for catalytic studies. Kelly et al. 9 

recently described a differentially pumped system operating 

up to 0.02 mbar that used a conventional x-ray source and 

electrostatic grid lenses to improve count rates. Nilsson has 

constructed a synchrotron-based differentially pumped 

instrument. 10 

Photoemission is possible at still higher pressures if the 

aperture size is reduced or the length of the differential 

pumping region is extended, however these steps reduce, respectively, 

the effective sample area and solid angle contributing 

to the photoemission signal. We have developed a 

high-pressure photoemission spectrometer that addresses 

both of these restrictions. We use a synchrotron x-ray source 

with a small x-ray spot size, allowing the use of smaller 

apertures. Our differential pumping system is also an electron 

lens with a crossover at each pumping aperture, so there 

is no loss of counts even with a relatively long differential 

pumping distance. We have constructed a prototype highpressure 

photoelectron spectrometer HPPES based on this 

concept which can operate at pressures up to 7 mbar 700 Pa 

0034-6748/2002/73(11)/3872/6/$19.00 3872 

© 2002 American Institute of Physics 

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Rev. Sci. Instrum., Vol. 73, No. 11, November 2002 

A differentially pumped electrostatic lens 

3873 

or 5 Torr. This system has been used to study the premelting 

of ice in equilibrium with water vapor 11 and the partial 

oxidation of methanol over copper under reaction 

conditions. 12 As an additional benefit we find that surface 

charging of insulating samples is compensated by impinging 

gas phase ions generated by the x rays at pressures above 

0.5 mbar. We are now developing a second-generation 

HPPES with more flexible sample preparation that should be 

able to operate at up to 100 mbar. 

We will now give a general analysis of high-pressure 

photoemission followed by a description of our prototype 

system and some examples of experimental data. 

II. HIGH-PRESSURE PHOTOEMISSION AND 

DIFFERENTIAL PUMPING 

We will first analyze the aperture-based HPPES using 

simple ideal gas theory for molecular flow conditions. 13 This 

will be followed by some comments on the limits of this 

approximation. An aperture of radius R separates a region of 

gas pressure P o from another region at much lower pressure. 

We consider an electron traveling along the axis perpendicular 

to the aperture at position z. Ifz is measured in units of R 

and the origin is in the plane of the aperture with positive z 

on the vacuum side, the aperture subtends an angle (z) 

arccos(z/1z 2 ). The effective pressure is proportional 

to the included solid angle of the high-pressure region so 

P(z) 1 2 P o (1z/1z 2 ). 

If the sample is placed close to the aperture, the pressure 

at the sample surface will be less than P o , but the effect of 

the aperture drops off rather quickly. At a distance of 2R 

the pressure is already 95% of P o , while at 2R it is only 

5%. Most of the electron attenuation takes place close to the 

aperture. To calculate electron attenuation where gas pressure 

is a function of position we can define an effective path 

length through the gas as L (1/P o ) P(x)dx. Using the 

expression for P(z) above, the effective path length for a 

trajectory starting at z and extended far into the vacuum region 

is L(z) 1 2 R(1z 2 z). For a sample at 2R, the 

effective path length is 2.1R while for a sample at the aperture 

plane it is 0.5R. In the same way, it can be seen that 88% 

of the effective path and thus of the attenuation after the 

aperture takes place within 4R, assuming ideal pumping. 

Therefore, there is little to be gained by schemes to separate 

the electron trajectory from the gas jet exiting the aperture. 

The electron mean free path is 4kT/P, where the 

electron scattering cross section depends on the electron 

energy and gas composition. The density of a gas at 1 mbar 

and 293 K is about 10 6 smaller than for a condensed solid, so 

the mean free path for low energy electrons will increase 

from nm in a solid to mm in a gas at 1 mbar. The mean 

free path also increases approximately proportional to the 

square root of electron kinetic energy. 14 If the sample is 

placed a distance 2R from the aperture, and the effective 

path length is limited to 2, then P max 4kT/R. Our prototype 

HPPES system had R0.45 mm and for O KLL electrons 

at 500 eV the mean free path was 3 mm at 1 mbar, which 

gave P max 7 mbar. P max can be increased by reducing the 

aperture size, and with a synchrotron light source it should 

be possible to reduce the x-ray spot size and the aperture to 

25 m to obtain P max 100 mbar. 

The volumetric conductance of an aperture S A 

(/4) R 2 v¯R 2 kT/2M, where v¯ is the average molecular 

velocity and M is the molecular mass. The pressure P 1 

outside the aperture is determined by the effective pumping 

speed S 1 for the vacuum system so P 1 /P o S A /S 1 . In a 

typical vacuum system it is difficult to get a conductancelimited 

pumping speed greater than 100 l/s 0.1 m 3 /s. 

With S A 0.1 l/s for water vapor at 293 K and R0.45 mm it 

follows that for a single pumping stage P 1 /P o 10 3 . 

The electron spectrometer should ideally be kept at a 

pressure in the 10 7 mbar range to protect the electron multiplier, 

so for operation with P o 10 mbar two additional 

stages of differential pumping are required to obtain a pressure 

differential P o /P 3 10 8 . This puts certain constraints 

on the dimensions of a differentially pumped system. The 

gas flux through the first aperture is P o S A . Even with ideal 

pumping of the scattered gas molecules, a fraction (1/) 

of the flux will pass through the differential pumping system 

as a molecular beam, where is the solid angle of the 

differential pumping exit aperture seen from the source aperture. 

This gives a minimum length L for the differential 

pumping system 

LR 3 S A 

S 3 

P o 

P 3 

, 

where S 3 and P 3 are the pumping speed and pressure in the 

final stage and R 3 the radius of the exit aperture. With a 

pressure ratio of 10 8 and a conductance ratio of 10 3 , as 

above, this gives L300R 3 . 

The preceding analysis in terms of molecular flow is 

appropriate if the mean free path for gas molecule–molecule 

interactions is large compared to the aperture size. On the 

other hand, if the aperture is large compared to the mean free 

path then the gas jet is best described as a supersonic molecular 

beam. The gas mean free path is given by gas 

kT/2P gas , where the collision cross section is approximately 

the geometrical size of the molecule, so for water 

vapor at 1 mbar the mean free path is 0.4 mm. At the 

maximum pressure of 7 mbar for 500 eV electrons, the 

effective path length of 2.1R is about 30 times gas . Neither 

the molecular nor the viscous flow limits are really appropriate 

in this case of intermediate, or Knudsen flow, and a detailed 

analysis would require numerical simulations. The supersonic 

viscous flow case can be analyzed analytically, 15 

however, and the results are not very different than for the 

molecular flow case. The results now depend on the internal 

degrees of freedom of the gas molecule and differ for atomic, 

diatomic and polyatomic molecules. The total flux through 

the aperture increases by 1.6 and the flux is more directional, 

so the relative density along the aperture axis is increased 

by 2.0 for atomic, 1.4 for diatomic, or 1.1 for polyatomic 

species. The pressure in the aperture plane is 0.63P o , 

compared to 0.5 for molecular flow. The effective attenuation 

after the aperture plane can be calculated for the water case, 

and is 1.03R, compared to 0.5R for the molecular flow case. 


3874 Rev. Sci. Instrum., Vol. 73, No. 11, November 2002 Ogletree et al. 

The pressure distribution in the sample region is more complex, 

and will depend on its detailed geometry. None of these 

factors change the general conclusions reached above, it is 

just that the molecular flow analysis tends to underestimate 

the attenuation and gas flow at higher pressures, but not by 

worse than a factor of 2. 

III. HIGH PRESSURE PHOTOEMISSION AND 

ELECTRON OPTICS 

While reducing the transmitted solid angle improves the 

differential pumping by reducing the gas flux, it also reduces 

the electron flux. A typical hemispherical analyzer input lens 

can accept a half angle of up to lens 1/8 or 125 mrad 

8°. 16 Operation with a high source pressure requires a 

differential pumping system that is long compared to the 

aperture diameters, which results in a relatively small transmitted 

solid angle, only diff 1/3003.3 mrad 0.2° in 

the case described above. This will reduce the photoemission 

by a factor of ( lens / diff ) 2 or more than 3 orders of magnitude 

compared to a vacuum analyzer. In our new HPPES we 

have constructed a differential pumping system that is also 

an electron lens system with crossovers in the aperture 

planes. With this approach we are free to optimize the pumping 

system, while still collecting most of the electrons which 

pass unscattered through the first aperture, thereby increasing 

the high-pressure signal by at least 2 orders of magnitude 

compared to a passive differential pumping system. 

IV. THE HPPES PROTOTYPE 

A prototype system was developed to validate our 

HPPES concept using an existing surface science vacuum 

chamber at beamline 9.3.2 of the LBNL Advanced light 

source. The two-stage lens system and a closed sample cell 

fed by gas lines was mounted on an 8 in. flange of the 

vacuum chamber, and a standard hemispherical analyzer was 

mounted on the lens system, as shown in Fig. 1. Primary 

pumping was provided through the vacuum chamber, second 

stage pumping through the lens, and the final pumping 

through the hemispherical analyzer 17 pumping port, using 

three 250 l/s turbo pumps. 18 Gases were supplied through 

mass-flow controllers except for water vapor, which was supplied 

by a pressure-controlled choked flow through an aperture, 

and cell pressure was monitored with a capacitance 

manometer. 19 

The first aperture or ‘‘nozzle’’ had an aperture diameter 

of 0.9 mm and the sample–nozzle distance was variable. The 

first lens stage was 33 mm long and 6 mm in diameter with 

a 1.5 mm exit aperture, and the second stage was 145 mm 

long and 20 mm in diameter with a3mmexit aperture. The 

final lens focus was 4 mm beyond the exit aperture and in the 

focal plane of the hemispherical analyzer, 18 mm in front of 

its input lens. It was possible to maintain a pressure difference 

of 10 8 between the sample cell and an ion gauge at the 

analyzer pumping port. With a 0.9 mm aperture the maximum 

cell pressure was limited to 7 mbar by the throughput 

of the first stage turbo pump. 

The electron optic system consisted of five independently 

controlled lenses, plus the exit aperture, which was 

FIG. 1. Schematic drawing of the prototype HPPES system. The upper 

section shows a detail of the first lens and sample region. The photon beam 

enters the high-pressure sample cell through an Al window of typically 100 

nm thickness. Electrons and a gas jet pass through the apertures of the lens 

system. The sample holder pictured here was the type used for water/ice 

studies. The lower section shows the complete system and the hemispherical 

analyzer input lens hatched entering the main 8 in. vacuum flange. The 

high-pressure cell volume is one liter. The lens elements solid black are 

numbered: 1 nozzle, 2 control, 3 skimmer, 4 condenser, 5 quadrupole, 

and 6 exit aperture. Elements 1–3 comprise the first lens/pumping 

stage and elements 3–6 the second lens/pumping stage. 

always grounded. In practice we also grounded the sample 

and the entrance nozzle. We refer to the other lens elements 

along the electron trajectory as the control, skimmer, condenser, 

and quadrupole lenses. The lens voltages were programmable 

with 5 kV supplies for the control and condenser 

lenses, 1 kV for the skimmer, and 3 kV for the 

condenser. The quadrupole lens was split in four sections for 

deflection control but no use was made of this capability 

since the hemispherical analyzer input lens was also 

equipped with deflection plates, which were dynamically adjusted 

as a function of electron kinetic energy. This was done 

to compensate for misalignment of the analyzer and also for 

magnetic field effects as the prototype system had no magnetic 

shielding. The HPPES lenses and the hemispherical 

analyzer, including deflection plates and counting electronics, 

were software controlled from a PC running LABVIEW. 20 

The nozzle, control, and skimmer electrodes formed a 

three-element lens to focus electrons onto the intermediate 

aperture, and the skimmer, condenser and quadrupole formed 

a four-element lens, along with the exit aperture, to refocus 

the electrons in the hemispherical analyzer sample plane. 

The lens system was operated for electron kinetic energies 

from 50 to 1000 eV. Performance was poor below 50 eV, 




3875 

probably due to the lack of magnetic shielding. Optimum 

transmission for the first lens was obtained with an acceleration 

of 34:1 between nozzle and control electrode, followed 

by a deceleration of 20:1 between control and skimmer. 

Above 150 eV the available power supplies could no 

longer maintain the optimum acceleration ratio, and it was 

not possible to maintain a focus at the intermediate aperture. 

The best transmission was obtained with the control electrode 

set at 5 kV and the skimmer adjusted to maintain 

close to a 20:1 deceleration. In the second stage the best 

transmission was obtained with the condenser at 5 kV, for 

an acceleration around 20:1, followed by deceleration to the 

initial electron kinetic energy in two unequal steps, with the 

deceleration ratio twice as high from the condenser to the 

quadrupole as from the quadrupole to the exit aperture. 

Better performance could have been obtained with a 

longer, multielement lens for the first stage, and this is 

planned for the second generation HPPES system. Also, the 

lens voltages are constrained in the first stage during highpressure 

operation by the need to avoid exciting a dc plasma 

discharge. For water vapor, 4 mbar was the maximum cell 

pressure with the control electrode at 5 kV. At 7 mbar the 

control electrode voltage could not exceed 1.5 kV. A multielement 

lens will also add flexibility for lower voltage operation. 

The main vacuum chamber was separated from the 

beamline and from the sample cell by a pair of 100 nm Al 

windows, and the 1 mm diam x-ray beam was incident on 

the sample 75° from the sample normal, which was aligned 

along the HPPES axis. The x-ray path length in the high 

pressure cell was approximately 20 mm. Using a graphite 

test sample with a 600 line/mm grating in the beamline 

monochromator at 1/1000 energy resolution and 450 eV photon 

energy, we could measure a sample photocurrent of 2 

nA for a grounded sample. With no gas in the cell we could 

obtain a count rate 400 kHz from the C 1s line at 1 eV 

analyzer resolution. These conditions correspond to maximum 

transmission for both the beamline and the HPPES 

lens. The count rate was inversely proportional to both photon 

and electron energy resolution, as expected. Figure 2 

shows a series of survey spectra for a Rh111 sample at 

various photon energies. The ‘‘bump’’ between 75 and 175 

eV kinetic energies is due to the change in lens operating 

modes. Aside from this feature, the lens transmission is relatively 

uniform. 

V. RESULTS 

A. Insulating materials 

To demonstrate the performance of our instrument we 

present three examples of applications in the areas of catalysis 

and environmental sciences. Figure 3 shows an XPS survey 

spectrum of a KCl001 surface in an atmosphere of 0.7 

mbar of water vapor at room temperature. The sample was 

prepared ex situ by cleavage. The spectrum in Fig. 3 shows 

photoemission and Auger peaks from the surface K, Cl, C 

and the O 1s and O KLL lines from the water gas phase. 

When the KCl sample was measured in vacuum, we observed 

surface charging on the order of several hundred V. 

FIG. 2. A series of HPPES spectra from a Rh111 sample in vacuum as the 

incident photon energy was reduced from 800 to 200 eV in 50 eV steps. 

Photoemission peaks shift by 50 eV to lower kinetic energy in successive 

spectra while Auger peaks are unchanged. The HPPES transmission varies 

smoothly with kinetic energy except for the ‘‘bump’’ between 75 and 160 

eV. The peaks near 500 eV in the lower spectra are Rh 3d 3/2,5/2 and C 1s. 

Note how the relative intensities change with photon energy due to changes 

in absorption cross sections and electron mean free path. At lower photon 

energies contributions due to the beamline monochromator second, third, 

and even fourth harmonics can be seen. Count rates are for maximum hemisphere 

pass energy resolution 1.6 eV, normalized by the beamline flux 

monitor photocurrent up to 300 pA at 450 eV, E/E 0.1%, but not 

corrected for the attenuation of two 100 nm thick Al x-ray windows. 

The spectra recorded in 0.7 mbar water vapor, however, did 

not show any shifts in the photoelectron or Auger peak positions. 

Surface charging of insulating samples was compensated 

by gas phase ionization due to the incident x rays. A 

similar effect was observed in argon. Insulating samples can 

thus be studied without the need for a flood gun. 

B. Catalytic studies 

In catalytic studies it is crucial to determine the state of 

the surface of the catalyst while exposed to reactant gases 


3876 Rev. Sci. Instrum., Vol. 73, No. 11, November 2002 Ogletree et al. 

FIG. 3. XPS spectrum of a KCl 100 sample in 0.7 mbar water vapor. The 

incident photon energy was 650 eV. Photoemission and Auger lines from the 

surface K, Cl, and C as well as oxygen O 1s and O KLL lines from the gas 

phase are present in the spectrum. The charging of the insulating KCl 

sample is compensated by ions generated in the gas phase. When the same 

spectrum was acquired in vacuum, the peaks were strongly shifted due to 

charging. 

and products. The first HPPES catalytic study investigated 

the partial oxidation of methanol to formaldehyde over copper 

in the presence of oxygen. Figure 4 shows the O 1s region 

of an XPS spectrum of a polycrystalline Cu foil sample 

in 0.7 mbar of O 2 at 400 °C. The O 1s Cu 2 O peak is found at 

530.2 eV, in agreement with literature values. 21 The lower 

binding energy shoulder at 529.1 eV is probably due to small 

amounts of CuO formed during the oxidation of the Cu foil. 

Gas phase species are visible in the spectrum along with 

surface species. In most cases molecular XPS peaks are significantly 

shifted from those of chemisorbed species. The gas 

phase peaks give information on the reactant and product 

distributions near the sample surface. In the present example 

the O 1s peak of gas phase O 2 is split due to the paramagnetic 

nature of the O 2 molecule. 22 These peaks, at 538.3 and 

539.4 eV, are shifted by 4.8 eV toward lower binding energy 

compared to the literature values 5 since our binding energy 

FIG. 4. An XPS spectrum O1s region of a Cu foil in the presence of 0.7 

mbar O 2 at 400 °C. The incident photon energy was 600 eV. The peak at 

530.2 eV originates from Cu 2 O, formed at the surface under the oxidizing 

conditions. In addition, some CuO is formed at the surface and gives rise to 

the smaller peak at 529.1 eV. The O 2 gas phase peak is split into two due to 

the paramagnetic nature of the O 2 molecule. 

FIG. 5. Auger yield near-edge x-ray absorption spectrum of a supercooled 

water drop 2.5 °C in equilibrium with its vapor 5 mbar. The water drop 

diameter 3 mm was grown from the vapor onto a temperature-controlled 

Cu sample holder. The hemispherical analyzer was used as a partial yield 

electron detector to collect electrons from the surface with kinetic energies 

between 413 and 437 eV. The first peak in the spectrum at 535 eV is highly 

sensitive to the local hydrogen bonding structure of the water molecules and 

was used to investigate the premelting of ice. 

scale is related to the copper Fermi level rather than the 

vacuum level. 

C. Environmental studies: Water in equilibrium with 

its vapor 

We have used our HPPES setup to carry out NEXAFS 

experiments on ice and liquid water in equilibrium with water 

vapor the vapor pressure is close to 6 mbar at the triple 

point. Figure 5 shows an Auger yield NEXAFS spectrum 

from the surface of a supercooled water drop. The droplet 

was condensed from the vapor onto a cooled Cu sample 

holder. The temperature of the water drop was 2.5 °C in 5 

mbar of water vapor, i.e., the water drop was in equilibrium 

with its vapor. Total electron yield NEXAFS data are hard to 

interpret in high vapor pressure experiments due to gas phase 

ionization and inelastic ionization cascades. The HPPES allowed 

us to make energy resolved measurements of the O KLL 

Auger yield from the ice/water surface. Only those gas molecules 

that were ionized in front of the lens aperture contributed 

to the HPPES signal. The surface NEXAFS spectrum 

was recorded by collecting electrons with kinetic energies in 

the range from 413 to 437 eV. This range was chosen to 

maximize the signal from the condensed phase relative to the 

gas phase. The most salient feature of the spectrum of Fig. 5 

is the first peak at 535 eV. This peak is highly sensitive to the 

local hydrogen bonding structure of the water molecules 23 

and exhibits appreciable changes when water freezes and becomes 

crystalline. We have recently completed a HPPES 

NEXAFS study the premelting of ice as detailed in Ref. 11. 

VI. FURTHER DEVELOPMENT 

We have developed a high-pressure photoelectron spectrometer 

that combines differential pumping and electrostatic 

focusing of the photoelectrons. The electrostatically focused 

HPPES offers a significant performance improvement relative 

to passive differential pumping approaches and can operate 

at the theoretical pressure limits for an aperture-based 




3877 

high-pressure photoemission system. Using this general approach, 

it should be possible to make photoemission measurements 

up to 100 mbar. Although our experiment has 

been implemented at a synchrotron beamline, it should also 

be possible to apply the same concepts with a conventional 

small-spot x-ray source. 

ACKNOWLEDGMENTS 

The authors would like to thank Dr. C. H. Alfred Huan 

and Dr. E. D. Moler for an introduction to synchrotron experiments 

and assistance with beamline 9.3.2. Dr. Eli Rotenberg 

provided advice and LABVIEW source code for controlling 

the hemispherical analyzer. Ed Wong built many parts 

for the HPPES system and cheerfully responded to emergency 

requests. This work was supported by the Director, 

Office of Science, Office of Basic Energy Sciences, Materials 

Sciences Division of the U.S. Department of Energy under 

Contract No. DE-AC03-76SF00098. 

1 Practical Surface Analysis, 2nd ed., edited by D. Briggs and M. P. Seah 

Wiley, New York, 1990, Vol.1. 

2 J. Stöhr, NEXAFS Spectroscopy Springer, Berlin, 1992. 

3 X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEX- 

AFS, and XANES, edited by D. C. Koningsberger and R. Prins Wiley, 

New York, 1988. 

4 C. S. Fadley, M. A. Van Hove, Z. Hussain, and A. P. Kaduwela, J. Electron 

Spectrosc. Relat. Phenom. 75, 2731995. 

5 K. Siegbahn et al., ESCA Applied to Free Molecules North-Holland, Amsterdam, 

1969. 

6 H. Siegbahn and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom. 2, 

319 1973; H. Fellner-Feldegg, H. Siegbahn, L. Asplund, P. Kelfve, and 

K. Siegbahn, ibid. 7, 421 1975; H. Siegbahn, S. Svensson, and M. Lundholm, 

ibid. 24, 205 1981. 

7 R. W. Joyner, M. W. Roberts, and K. Yates, Surf. Sci. 87, 5011979. 

8 H. J. Ruppender, M. Grunze, C. W. Kong, and M. Wilmers, Surf. Interface 

Anal. 15, 245 1990. 

9 M. A. Kelly, M. L. Shek, P. Pianetta, T. M. Gür, and M. R. Beasley, J. Vac. 

Sci. Technol. A 19, 2127 2001. 

10 A. Nilsson unpublished results. 

11 H. Bluhm, D. F. Ogletree, C. S. Fadley, Z. Hussain, and M. Salmeron, J. 

Phys.: Condens. Matter 14, L227 2002. 

12 H. Bluhm, M. Hävecker, A. Knop-Gericke, V. I. Bukhtiyarov, D. F. Ogletree, 

M. Salmeron, and R. Schlögl unpublished. 

13 See any standard text on statistical thermodynamics, for example, F. W. 

Sears and G. L. Salinger, Thermodynamics, Kinetic Theory, and Statistical 

Thermodynamics, 3rd ed. Addison Wesley, Reading, MA, 1975. 

14 See Ref. 1, p. 209. 

15 D. R. Miller, in Atomic and Molecular Beam Methods, edited by G. Scoles 

Oxford University Press, New York, 1988. 

16 The Physical Electronics hemispherical capacitor analyzer used in the prototype 

HPPES system has a maximum acceptance of 7°. The Specs 

Phoibos 150 analyzer that will be used for the second generation system 

has a maximum acceptance of 9°. 

17 PHI 360 SCA with Omni IV input lens and a 16 anode channel-plate 

detector: Physical Electronics, Inc., Eden Prairie, MN. 

18 MacroTorr 250, Varian Vacuum Technologies, Lexington, MA. 

19 MKS Instruments, Inc., Andover, MA. 

20 A graphical instrument control environment developed by National Instruments, 

Inc., Austin, TX. 

21 J. Ghijsen, L. H. Tjeng, J. van Elp, H. Eskes, J. Westerink, G. A. Sawatzky, 

and M. T. Czyzyk, Phys. Rev. B 38, 11322 1998. 

22 See J. Electron Spectrosc. Relat. Phenom. 75, 111995 for a discussion 

of the splitting of the O 1s level of O 2 . 

23 S. Myneni et al., J. Phys.: Condens. Matter 14, L213 2002.

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