Photoelectron spectroscopy in the energy region 30 to 800 eV using synchrotron radiation

296

Journal of Electron Spectroscopy and Related Phenomena, 15 (1979) 296-306 0 ElsevierScientlfic PubllshmgCompany,Amsterdam-Prmt.edmTheNetherlands

PHOTOELECl'RON SPECTROSCOPY

IN THE ENERGY REGION

30 'IO 800 eV USING SYNOHROTRON

RADIATION

I

LINIMU and W

Stanford Stanford

E. SPIORR

Synchrotron University,

Radiation Stanford,

Iaboratory California

and Stanford 94305, USA

Electronics

laboratories

ABSTRACT With the advent extended

of aynchrotron

to a continuous

soft x-ray regions,

adding

probe of the electronic application

radiation,

range of excitation tremendously

structure

of photoelectron

the photoemission energies

to the usefulness

of materials

spectroscopy

of Si surfaces,

conductor

surfaces,

on transition

chemisorption

of photoemission

synchrotron

chemisorption/oxidation

structure

were

radiation

metal overlayers

metal

surfaces,

and

as a

In this paper, we discuss

using

of oxygen

electronic

techniques

in the far ultraviolet

the

to the studies

on III-V semi-

and the surface

of CuNi alloys

INTRODUCTION The use of synchrotron has resulted chemistry

radiation

in a very powerful

of a wide variety

parameters

determining

in conjunction

technique

tion cross section

studies, optimal

it

for the electronic

of synchrotron

has been possible

surface

sensitivity.

to utilize

The two most important

involved

the tunability

energy region up to a few hundred surfaces

at submonolayer

process.

source for photoemission

of this source to obtain

of synchrotron

eV have been applied

are

photoloniza-

in the excltatlon

as an excitation

properties

technique

(2) the partial

In this paper, we will illustrate

from our own work how the unique

metal and semiconductor

and

spectroscopy

physics and

of the photoemisslon

electron

levels

radiatton

the surface

phenomena.

sensitivity

(1) the escape depth of the photoemitted

With the advent

for studying

of chemisorptlon

the surface

with photoemission

with a few examples

radiation

in the photon

to chemisorptlon

studies of

coverages

RESULTS AND DISCUSSION The Energy Dependence

of the Electron

It is now well established escape depth

Escape Depth

(ref. 1) that the general

is a sharp decrease

with increasing

behavior

electron

energy

of the electron (from about

I LINDAU,

296 10,000

h a few

tenths

of

an eV above

range 50 to 200 eV with a remarkably increase with increasing radiation

electron

is that the photon

of the synchrotron

radiation

A),

1 to 2 topmost atomic

of the electron

(ref. 2).

by a chemically

oxygen

shifted and unshifted

layers

length

the experimentally region of kinetic

the

of the electron determined

is chemlsorbed (ref

As 3d peaks

minimum.

escape depth

on the (110) sur-

onto the surface As The relative

3)

(at a well-defined

high surface

sensitivity

EB = 19 eV)

(binding energy,

(ref. 2)

1.5 molecular

(E3 = 41 eV)

core levels by using

g

-20 As dsI

2_

% j 115

t: d-to

I

60

I

I

I

I

80 100 120 140 KINETIC ENERGY (eV)

I

I

160

I80

layers.

for the Ga 3d

W

I

in the

1, the minimum occurs at 60 eV electron

-25

40

cov-

on the

energy

BULK As/SURFACE As vs KtNETtC ENERGY

3

oxygen

information

can thus be accomplished

and As 3d

sites

amplitudes

In Fig. 1, we have plotted

energy with a value of 5 to 6 A, or approximately

This extremely

The tunability

escape depth as a function of electron

As seen from Fig

electrons

This was illustrated

erage) as a function of photon energy has been used to extract energy dependence

and finally an

of synchrotron

for the first time to plot in

escape

shifted As 36 peak

in the

so that the photoemitted

has made it possible

Molecular

face of C&As

of the chemically

(-5

work of our group on the oxygen chemisorption

as

evidenced

flat minimum

The great advantage

energy can be chosen

the energy dependence

by some extensive

small escape depth

energy.

come from very close to the surface,

detail

the Fermi level to 20 to 30 A for electron

5 to 10 eV above the Fermi level), then a fairly

energies

W E SPICER

;

Fig 1. Plot of the ratio of the unshifted to shifted As 3d levels as a function of electron kinetic energy for the CaAs (110) +02 surface around the minimum in the escape depth.

SYNCHROTRON excitation readily

RADIATION

energies

available

at the Stanford chemisorbed

around

Synchrotron

in the application

scopic

to surface

of the Si/SiO_

has been the subject

scale about

the bonding

(ref

of 130 eV are shown

6)

in Fig

(ref

4)

(ugrasshopperW)

A large number

5). energy

region

can thus be

and meant a major

studies.

of

breakghrough

This particular

aspect

Interface

-

Photoemission

30 to 800 eV is

below.

interface

of the oxidation

(ref

sensitivity

by two more examples

region

monochromator

in this binding

with high surface

devices.

energy

Iaboratory

Structure

Si-SiO2

in practical

Radiation

of photoemission

will be illustrated

The

incidence

with core levels

studied

The Electronic

The photon

100 eV

with the grazing

systems

conveniently

297

of many studies due to its importance

can be used to obtain

of oxygen

information

to the Si surface

Photoemission

spectra

2 for binding

energies

on a micro-

in the initial

taken at an excitation around

the position

stages energy

of the Si2p

core level

= 100 eV). The bottom curve (a) is for the clean surface @B g shows the result after 10 l2 L (1 L L 10-6 torr see) exposure to molecular A shifted

Si 2p peak appears

peak can be interpreted oxygen, After

resulting

further

as due to charge

in a higher

oxidation

shifted

Si peak

shifted

by 3.8 eV

at 2 6 eV towards

EB

for the compound

It appears Si02,

The 2 6 eV chemical

feature

The spectrum peak.

shows that the 3.3 eV peak

chemisorption/oxidation

A detailed

The

of the Si/Si02

Interaction Recently,

with emphasis 7 and 8).

on exploring

Figure

photon

amounts

energy

important

depth,

energies

from Fig

and

Si bonded

to

phases

in the

in great detail

on the electronic

0.10) Surface interfaces,

spectra

particularly formation

of n-C&&b

in a controlled originating

that lie within

i e , the top 1 to 2 molecular

elsewhere

scale c%n be obtained.

is 120 eV, and the photoelectrons

thing to notice

states

Schottky-barrier

of Au has been evaporated

and Sb 4d core levels thus have kinetic in the escape

behind

and shows a

The different

studied metal-semiconductor

3 shows a set of photoemission

atoms

phase with

complex

information

with a GaSb

the mechanism

to the oxygen

can thus be followed

on a microscopic

of Au Metal Overlayers

we have also

which different chosen

interface

shift observed

has been presented

and important

i, the

and is now

of a chemisorbed

(ref. 6).

atoms to

in the bonding.

the chemical

of three chemical

oxygen.

The shifted

curve

level spectrum

coordinated

analysis

respectively

technique,

surface

involved

in curve h is more

of the Si surface

with the photoemission structure

shown

is comprised

two, three, and four 0 atoms,

atoms

in the core

curve a, is indicative

lower coordination.

energy.

from silicon

that this is exactly

broad

binding

see Ref. 6) of the surface,

where Si is tetrahedrally

shift,

3.3 eV shifted

transfer

for the silicon

(for details,

is the dominant

higher

Curve

(refs.

(110) onto manner

The

from the 6% 36 the broad minimum

layers are being probed.

3 is that the emission

from the Ga 3d core

The

I LINDAU,

298

SI 2p

hu=130

W E SPICER

eV

1‘11'::: A 38eV

GaSb +Au *w = 120 eV Au-4f

Sb -4d Go-3d

I

(EP3) h

Au-5d

I

Y

7k2EJ-

/VI

W=l*)

V6M

BINDING

ENERGY

(elf)

Photoemission spectra taken 3. a photon energy of 120 eV for GaSb exposed to different amounts of Au

at

I

I

105

100

BINDING

ENERGY

5 5 (eV)

Fig 2 The 2p core level for a clean Si surface (curve a) and the chemical shifts observed (curves g, h, i) during the different stages in the initial oxidation.

SYNCHROTRQN levels

RADIATION

are preferentially

the Sb 4d emission composed behind

surfaces

strong.

enriched

defect

and Cs overlayers states

height

the

must

between

of a monolayer

The

of Al Metal

In the previous semiconductor Al

overlayers

the two systems surface

properties

2p core

levels,

ton energies

Core

was again

giving

(110)

important level used

74

mechanism

metallic

with a GaAs

There

9)

to study

for the Schottky

into

of the inter-

can be followed

in this

between

are many

72

44

42

40

Au and a III-V for thin

to drastically

(monofor

different

case Ga 3d, As 3d, and Al

the metal-semiconductor

STATE ENERGY

7 and 8)

similarities

(see Fig

4).

" 22

18

interface As seen

At ON GaAs (110) n-TYPE CtEAVED hv = 120 eV

INITIAL

structure

consideration

the results

leading

sensitivity

a model

(1101 Surface

we show here

(ref

these

obtained

proposed

layers (refs

the interaction

spectroscopy,

n

76

we have

that the properties

differences

surface

on

states

we described

optimal

position and results

these defect

coveragetothick

with

when one tries

of the electronic

to demonstrate

on n-GaAs

but also

results,

leaving

interaction

calculations

As a comparison,

surface.

Is de-

into account

is the dominant

take

that the GaSb

strong

level pinning

on these

while

of the Au overlayer,

III-V compounds,

Cverlavers

section,

suggest

completely,

the gold and the semiconductor,

from a fraction

Interaction

the Fermi Based

Theoretical

been able

interaction

to be taken

on other

interface

results

to the surface

at the interface

formation

we have

In summary,

behind

height

it disappears

Au thus has a very

This has

the mechanism

of the metal/III-V

layer)

These

in Ga

substrate

and the barrier

for Au, Al,

face,

Finally,

by the Au and that Sb moves

to understand

barrier

attenuated.

is still

an interface

the semiconductor

where

299

20

(ev below E,)

Fig 4 The Al 2p, As 36, and Ga 3d core level spectra taken at a photon energy of 120 eV for several Al overlayer thicknesses
at pho-

in Fig.4,

I LINDAU, W E SPICER

300

a very

small

the Fermi

of Al

(0.4

level 0 6 eV below After

is 1.4 eV). clearly

amount

detected

attenuated

is

sufficient

the conduction of

deposition

at a binding

but essentially

A>

about

energy

except

sitlon

A>, the dominant Al 2p emission

binding

(8

energy

in addition

Al)

of

side of the Ga 3d peak

73.1 eV

(in excellent

of the Ga and As 3d peaks 0 9 eV towards change

in the As 3d core

energy

corresponding

with

the shift

place and

of Ga by Al

with

between

The rapid

increasing

lattice

strong

and

suggests

to the topmost

leads

between

evidence

for bulk metallic

A detailed

analysis structure

insignificant

This observation,

with

of AlAs

Al 2p core

that the replacement

one or two layers.

combined

has taken

the formation

from the metallic

the Al overlayer

to formation

on

depo-

level with a

that a replacement

in emission

gives

1s

Ga 3d peak has a binding

(ref. 9).

at the surface

increase

Al deposition

Ga

further

is an additional

of the Ga 3d core level but

level,

is that the interaction

is very

a core

with the value

The 0 9 eV shifted

2p core

Al and Ga is limited

point here surface

level.

in the GaAs

free Ga metal.

levels

energy

in GaAs

in emission

After

from

comes

that there

to that for metallic

for the Al

increase

(see below).

agreement

(Fig. 5) shows

lower binding

(the band gap

from the state at 74 0 eV

to the emission

and pin

The As 3d and Ga 3d peaks are

for a small

energy

bending

(1.5 A>, an Al 2p peak

of 74 0 eV.

unchanged

band

band minimum

a monolayer

the low binding of Al

to cause

reaction

The important

and the GaAs

of a new compound

(110)

at the surface

Al ON GaAs (110) n-TYPE CLEAVED hv=l20 eV --1

k-09eV

ENERGY (eV)

Fig 5 A detailed plot of the As 3d and Ga 3d levels at the maximum Al coverage (dashed lines) compared to the clean surface The peak In the Ga 3d dif(solid lines) ference curve (dash-dotted line) corresponds The horizontal axis is to metallic Ga marked in increments of one eV

The Energy

Dependence

As was mentioned addition

of Partial earlier

to the electron

Photoionization

in this paper,

escape

depth,

Cross

Sections

the photoionization

is an important

cross

parameter

section,

in determining

in

SYNCHROTRONRADIATION the surface

301

sensitivity

of the photoemission

tunability

of the synchrotron

of partial

photoionization

the area

under proper

function

of the energy

threshold in Fig

(ref. 10).

then decreases

cross

section

minimum that

illustrate

the

flux and

The binding

energy

before

passing

through

is observed

to decrease

functions

about

have

X

,XXxX~

I

0

50

towards

above As

threshold

130 eV

before

from

100

I

150

ENERGY

x x x x

x

ABOVE

x

X I

I

200

250

THRESHOLD

(eV1

300

The drastic section

of GaSb energy

the fact

c 350

(110) Surfaces behavior

can be utilized

with

of the cross advantage

section

in several

described

ways

in the previous

to enhance

A

the

Fig 6 The partial photoionization cross section of the Sb 4d levels as a function of electron energy above the excitation threshold.

The Oxidation

and

The

energies

and arises

the

seen

threshold

around

higher

minima

in

nodes

X I

is 32 eV

minimum

220 eV above

the Cooper

energy

at 45 eV above

a broad

monotonically

of the energy

the electron

a maximum

energy

for the Sb 4d levels

of the Sb 4d levels

through

by measuring

the transmission

importance

against

at 130 eV is termed

the 4d wave

photon

is plotted

maximum

starts

observed

incoming

section

goes

be done

the

dependence

of the excitation

the results

section

utilizing

the energy

can simply

by showing

rapidly

flat and extended

We

This

as a function

section

the cross

6, the cross

of the

analyzer

of the cross

6, where

Fig

normalizations

peaks

Again,

we can determine

sections.

the photoelectron

and make

dependence

radiation,

cross

technique.

the surface

302

I LINDAU, W E SPICER

sensitivity

If one, for instance,

gas with the substrate, This

is illustrated

face

(ref. 2)

it is desirable

in Fig

Energy

to 5

distribution

log L 02.

aa a 2 6 eV chemical thing

in Fig

changing

understood of

effect,

from Fig

the electrons

6.

emitted

with the Cooper minima. visible

above

as a function

whereas

(Fig

of the oxygen

The emission

energy

energies

for 6aSb

with the surface

(110)

is seen

The noticeable little with

of the Sb 4d peaks decrease on photon

excitation

can readily

of 160 eV, the kinetic

is roughly

'Ilw= 100 eV,

a Cooper minima

energy be

energy

130 eV and thus coinciding

which corresponds

the 3d wave functions

to the peak in do

but have a slowly varying

not have any cross section

from the Ga 3d's is thus affected

5 x IO’LO,

vs hv

Sb-4d

Ga-3d

I

(110) sur-

in Fig. 7, the Sb 4d levels are barely

On the other,

1

photon

of the Sb 4d levels

energy

from the 4d levels

GaSbCllO) +

from the substrate

to the CaSb

split 4d doublet.

strong dependence

and the behavior

intensity

of a chemisorbed

of the tie 3d peak changes very

At an excitation

6).

bonding

of binding

the amplitudes

This

level at

nodes and do not exhibit 10).

interaction

And, as seen

the noise

the cross section

(ref

The

energy,

the emission

for three different

shift of the spin-orbit

from 100 to 160 eV.

is a cross-section

to enhance

curves

7 is that the intensity

the photon

drastically

to study the interaction

7 for the case of oxygen

100, 120, and 160 ev, are plotted exposed

wants

I

I

I

20

40 l3INO:NocENERGY (eV1

spectra of CeSb (110) Fig 7. Photoemission exposed to oxygen for three different photon energies showing the strong variation in the cross section of the Sb 4d levels.

very

little

SYNCHROTRON by changing

RADIATION

303

the photon energy from 100 to 160 eV.

that, when the oxygen bonding

In summary,

to the GaSb surface

is studied,

it can be stated it is very important

to tune to a photon energy away from the 4d Cooper minima to achieve optimal surface sensitivity

The Chemisorption

of Co on Platinum

The Cooper minima chemisorption the emission

in the cross section discussed

studies to suppress the emission from the molecularly

for CO chemisorption

chemisorbed

onto Pt (ref

from the Pt substrate

(5d band)

11).

above can also be used in

from the substrate and enhance levels.

This ie shown in Fig

At a photon energy of 150 eV, the emission

is low due to the Cooper minima--the

functions exhibit an energy dependence

8

similar to that for the 4d's

Pt 6011) x (100)

5d wave (ref. 10)

n

I

I

I

-30

-25

-20

I

-15

8

I

-10

-5

I

Ef

INITIAL STATE ENERGY IeV)

Fig. 8. Photoemission spectra of a clean Pt surface exposed to a monolayer of CO, taken at a photon energy of 150 eV The emission intensity from the CO derived molecular levels (5a, lx, 4a, 3~) is enhanced since the Pt 5d emission falls in the Cooper minima.

The emission enhanced, minimized.

from the CO induced molecular

and the background

By taking advantage

and important observations

levels

(3a, 4a, lfi-5~) is correspondingly

of scattered electrons of mrking

from the Pt valence band is

around the Cooper minima, several new

could be made for the CO chemisorption

onto Pt.

This

304

I LINDAU, W E SPICER

data shows

the first unambiguous

The nonbonding

co.

few electron the metal

orbitals

for most of the shallow

for a more highly

bound

level,

By comparison

The higher

main reason structure

degree

lite structure

observed

molecule.

not completely

in transition-metal of the O-related

earlier

orbltals

excitation

summary,

it should

only by tuning surface

the photon

The Electronic

file,

which

In both cases, at

J3ut this appears orbitals

satellite

escape

depth

structure

involving

contrast,

is

observed

5 to 7 eV below

to a mechanism

each

charge-

orbital

In

were made possible and by that optimal

from the substrate

alloy must

layer

with theoretical in the component

we describe

and the

energy

of depth

intensity points

models

we obtain

with the lowest heat

again

to a maximum

At a fixed photon

energy

the compositional

pro-

Photoemission

40 to 240 eV on two different

as a function

I and Cu/NI

a curve which decreases increases

predicting

65s CX, 35s Ni and Cu/Ni

(denoted as r(Ni/Cu)) are for the Cu/NI

below

in CuNl alloys

is approached

region

a description

how the strong

is used to study

composition

include

and as a function

that the topmost

the bulk composition

in the photon

hv = SO eV,

and then increases

of a

structure

In Fig. 9, we plot the ratio of the Ni 3d emission

the Cu 3d emission

satel-

Surfaces

is consistent

I with a surface

The two sets of data

a minimum

12).

peaks appear

of a binary

In this section,

14)

were done

35% Cu, 65$ Ni

as the

of shakeup

for valence

for the emission

region will be enriched (ref

CufNi

(ref

suf-

from Fig. 6 is that

reminiscent

into the 2fl* CO-derived

of the alloy at the surface

i e , In what fashion

surfaces

test case

chemisorption

to the shakeup

so that optimal

of CuNi Alloy

of the electron

measurements

upon

that these new observations

of the surface

in copper,

of vaporization dependence

where

It is now well established

that the surface

The relax-

that the Ja electrons

of the observed

and are attributed

energy

eV.

shift

respectively

Structure

the surface. is enriched

is observed

(ref. X3),

was achieved

A characterizatfon of the composition

<12

an interesting

observation

some resemblance

be emphasized

levels,

energy

a

on

but still a level of molecular

for CO in gas phase

from the metal atoms

sensitivity,

chemisorbed

It bears

screening

of the 3a was interpreted

important

structure

carbonyls

transfer

of localization

The exact nature

clear

EF,

the 3a and 4a levels,

to be the first time shakeup chemisorbed

binding

shift than the 4a electrons

Another

between

shift of typically

that this relaxation

constitutes

28.5 eV below

of chemisorbed

of the efficient

to gas phase data, we showed

for this effect.

is present

orbitals,

in Co therefore

fer a 2 4 eV larger relaxation (ref. 11).

a relaxation

as a result

It has been found experfmentally

shift of the 36 level

character

of the 3a orbital

undergo

upon chemisorption

surface.

is uniform ation

molecular

volts

identification

at

energy,

intensity

of photon

II surface,

at low photon

to

energy

respectively.

energies,

hv = 140 eV, r(Ni/Cu)

II with

reaching

decreases,

of at/N1

II is

SYNCHROTRON

RADIATION

305

CU/NI (110) BULK COMP 90%

NI, 10% Cu

CU/NI I SURFACE x CU/NI II SURFACE

l

r (NdCu)

r I I I I I I I : :

14I312t,

I I-

\ \ \ c \ , I I

IO09OB-

X x-z-\

x’

06-

\ +X ‘1

I

k

r

\ x.

:;,“I

‘; I \ \ \ \

07-

‘\

/

/

‘t

05-

I

V I

40

80

.x-

2. ‘\

\

\

/H

k-.--H

/ \

04-

06

t

/’

/’

0’

0’

,J

/

!

I

I20

160

1

I

200

240

IO

;

hv (eV)

Fig. 9. Plot of r (ratio of Ni to Cu emission intensities) versus hv (photon energy) for Cu/Ni (110). The dots are for the Cu/Ni I surface, -654 Cu, 35$ Ni surface composition. The crosses are for the C!u/Ni II surface, ~65$ Wi, 35$ Cu.

greater

than r(Ni/Cu)

surface

region

the amount

of Cu/Ni

(a detailed

here

(ref. 14))

after

since

energies the bulk

60 eV, r(Ni/Cu) photon

energy

in a region of the alloy monotonic

This

region

falls

is very Ni rich. decreases

where below

would

depth

the surface

above

the surface

is again

apparently

work was supported

0289, by the U.S. Army

to detect,

see

by the Office

Research

Office

etc

interpreted

region

ref

length

elae-

of the

increases

there-

as a reflection the surface

At

low

r(Ni/C!u) is large

is enriched

from 40 to

in Cu

showa an oscillatory Thus,

not approach

to

exper-

is given

is increased

increasing.

does

When

the composition

the bulk value

(further

the

behavior

oscillations,

in a even

14 for details).

of Naval

Grant

below

large,

energy

but shows at least one oscillation be hard

sections

of depth

80 eV, r(Ni/Cu)

depth

is ascribed

by the escape

is relatively

As the photon

since

the escape

is thus

in the

by the photoemission

at 80 to 100 eV and

as a function

the escape

is increased

is determined

of r(Ni/Cu)

Cu concentration

of r(Ni/Cu)

aampled

of cross

to a minimum

of the alloy

where

fashion

if existing,

of the effects

The behavior

of the composition

behavior

in the region

sampled

which

the higher

The oscillatory

discussion This

electrons,

(ref. 14).

photon

I.

I, reflecting

of Ni and Cu present

iment

outgoing

of Cu/Ni

Research

MC-74-G-0215,

Contract

NOOO14-75-C-

and by the National

306

I LINDA&

Science

Foundation

performed

at the Stanford

the National Stanford

Science

P

DMR 74-22230 Synchrotron

Foundation

Linear Accelerator

The collaboration Pianetta,

Grants

Grant

Center

Radiation DMR

P. M. Stefan,

C. M and C

The experiments

Laboratory

73-07692

A02,

and the Department

with P. W. Chye,

R. Skeath,

and DMR 77-02519

Garner,

W E SPICER

which

were

is supported

in cooperation

by

with the

of Energy. D

T

Ling,

Y. Su is gratefully

J. N. Miller,

P

acknowledged

REFERENCES

1 2 3 4 5 6 7

8 9 10 11 12 13 14

I Lindau and W E. Spicer, J. Electron Spectrosc., 3(1974)409 P Pianetta, I. Lindau, C. M. Garner, and W. E Spicer, Phys. Rev B, 18, No. 4 (1978). Rev P. Pianetta, I Lindau, C M Garner, and W E Spicer, Phys L&t., 35(1975)1356, 37(1976)1166 Instr. Methods, 152(1978)73. F. C Brown, R Z Bachrach, and N. Lien, Nucl Technol , S. Doniach, I Lindau, W. E Spicer, and H Winick, J Vat. Sci 12(1975)1123. C. M. Garner, I Lindau, C. Y. Su, P. Pianetta, J. N. Miller, and W E Spicer, Phys Rev Lstt ) 40(1978)403, Phys. Rev. B (to be published). P. W Chye, I. Lindau, P. Pianetta, C M Garner, and W. E Spicer, Phys. Rev. B, 17(1978)2682. I. Lindau, P W. Chye, C. M. Garner, P. Pianetta, C. Y. Su, and W E. Spicer, Sci Technol., 15<1978)1332 J. Vat P. R. Skeath, I. Lindau, P. Pianetta, P W. Chye, C. Y. Su, and W E. Spicer, Chem Phys Lett. (to be published). Lett. A, 57(1976)225. I Lindau, P. Pianetta, and W E Spicer, Phys J N. Miller, D. T Ling, I. Lindau, P M Stefan, and W. E. Spicer, Phys Rev. Iett., 38(1978)1419. U. Gelius, E. Basilier, S. Svensson, T Bergmark, and K Siegbahn, J Electron Spectrosc., 2(1973)405. D S Rajoria, L. Kovnat, E W Plummer, and W R Salaneck, Chem. Phys. 49(1977)64 I&t., D T. Ling, J N Miller, I. Lindau, W E. Spicer, and P. M. Stefan, Surface sci , 74(1978)612.