Cs adsorption and Cs and O coadsorption on Mo(100): LEED and AES studies

382

Surface Science 146 (1984) 3X2-404 North-Holland. Amsterdam

Cs ADSORPTION AND Cs AND 0 COADSORPTION AND AES STUDIES R. RIWAN,

P. SOUKIASSIAN

*. S. ZUBER

ON Mo( 100): LEED

** and J. COUSTY

Sen,rc~ede Physrque des Atomes et des SurJuces. Centre d’Erude.r Nu&urm

de Soclu_v.

F- 91191 GiJ-sur- Yvette Cedex, Frcmce Received

27 March

1984; accepted

for publication

20 June 1984

The structural properties of Cs adsorption and Cs and 0 coadsorption on Mo(100) at room temperature are studied by LEED and AES. With increasing Cs coverage we observe: (i) At 6’=0.3 ML. a fi~fi R45O structure with an overall 4mm symmetry attributed to the reconstruction of the substrate caused by the Cs atoms; two precursor stages are observed for that structure. (ii) A p(2 x 2) structure at 6’ = 0.58 ML. (iii) Up to 0.7 ML a rectangular centered mesh resulting from contraction of the p(2 x 2) structure along one [I 101 direction, and for 0.7 < 8 < 1 ML a quasi-hexagonal structure. (iv) At 1 ML a true hexagonal structure, present in two unequally populated domains rotated by 90 o with the [1120] Cs direction parallel to Mo[l IO]; from the spot intensity of the (ih ih } reflections. we conclude that the Mo(100) face is reconstructed with a fi x $2 R45 o mesh and a p2mg space group. For Cs adsorption on a surface with a high density of up and down steps parallel to [loo] no domain selection occurs and the reconstructed substrate domains are limited by the terrace width. For Cs and 0 coadsorption. the numerous structures which occur for the W(lOO)-Cs-0 system are not observed. The presence of steps hinders the formation of the p(4 x 1) structure observed on the flat surface.

1. Introduction Recent theoretical calculations insight on the electronic properties

by Wimmer et al. [l] have brought a new accompanying the Cs monolayer deposition

on a clean W(100) surface. The essential features predicted experimentally by angle resolved ultra-violet photoemission

have been observed spectroscopy (AR-

UPS) [2,3] despite the simplified crystallographic model used to describe the system. Indeed these authors considered a c(2 X 2) Cs arrangement instead of the real hexagonal structure [4,7] and neglected possible reconstruction of the W substrate. This last effect was recently considered [3] to explain an electronic state observed in UPS and not predicted [l].

* Also at: Institut Universitaire F-10026 Troyes, France. ** Permanent address: Institute kiego 36, Wroclaw, Poland.

de Technologie, for Experimental

Universite Physics,

de Reims-Champagne-Ardennes, University

0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

of Wroclaw,

UI Cybuls-

R. Rwan Considering clean (100)

the similarity

obtained

on

in the presence

cesiated

are reported

Particularly,

W(100)

of the structural

for H,,

the electron

to Mo(100).

of several 0,

383

0 on Mo(100)

and electronic

faces of W and MO [9,10] it is tempting

observed haviours

Cs and

er al. /

But,

significant

adsorbates.

For

and CO adsorption

energy loss spectroscopy

for Cs adsorption [3] and Cs and 0, have shown some differences. Thus,

properties

to extrapolate

of

the results

differences

instance,

specific

are be-

on each metal [ll-161.

(EELS)

and ARUPS

coadsorption [17,18] we expect structural

results

on both surfaces observations on

cesiated Mo(lOO), different from those obtained on W(100) by Papageorgopoulos and Chen [5,19] and Desplats [7]. Furthermore this structural information is required for a good understanding of our ARUPS and EELS results [2,3] and also for comparison with the calculations of the related electronic properties

[8].

In this paper, we report the surface structures due to Cs adsorption and Cs and 0 coadsorption on Mo(100) at room temperature. To our knowledge, it is the first structural the clean density

study of these systems. We emphasize

Mo(100)

induced

by Cs adsorption.

of steps on this reconstruction

served is discussed ported

surface

is shown.

in regard of structural

properties

The

the reconstruction The effect epitaxial

calculated

of

of a high

relation

ob-

for an unsup-

Cs monolayer.

2. Experimental The present work was performed in an ultra high vacuum system with a four-grid LEED system and a cylindrical mirror analyzer (CMA). The base pressure was better than 5 X lo-” Torr. A surface presenting extended (100) terraces (larger than the electron diffraction coherence length) was obtained when after the usual electropolishing, the crystal was mechanically polished using a special system which allows an easy control surface

of the surface

will be referred

orientation

to as the “flat”

with an accuracy

of 0.10’.

This

surface.

On the other hand, without that mechanical

polishing

a surface with a high

step density along [loo] was obtained. The cleaning procedure described in ref. [21] was used combining oxidations (2 h at 10e6 Torr at 1273 K), flashes at 1600 K and ion bombardments. Pure Cs is obtained after thorough outgassing of a resistively heated cesium chromate source in which reduction is achieved by an Al-Zr alloy [20]. The Cs depositions have been performed at room temperature (298 K). It was shown that reliable results are obtained when this temperature was hold within less than 10 K. When the sample temperature exceeds 330 K, a detectable desorption of Cs is observed. The LEED intensities are measured by a spot photometer.

384

R. Riwan

et al. /

Cs und 0 on Mo(100)

3. Results 3.1. Growth of Cs layer on clean Mo(lO0)

3.1.1. AES results We monitor the growth of the Cs layer by measurements of the peak to peak amplitude of the Cs and MO Auger tranistions. For Cs we use the M,,O,,O,, transition at 47 eV and the less sensitive MSN,SN,, and M,N,,N,s transitions at 558 eV and 570 eV respectively. The Cs signal increases linearly up to a critical evaporation time, above which a new linear increase but with a considerably reduced slope is obtained (fig. 1). At the saturation the attenuation A of the intensity of the MO M,sN,,N,, and M,,N45N45 Auger transitions (190

eV and 220 eV respectively)

monolayer coverage. The impurity concentration nants are evaluated

amounts

is checked

by comparison

to 31.5% thoroughly.

and corresponds Total

C + 0

to the contami-

with the AES results for MO (~(2 X 2)-CO)

AES Relative intensity

I

Cs 1568 eV) l/1.5 -

Evaporation

x4

time Ised

Fig. 1. AES intensity curves of the MO substrate and Cs overlayer versus Cs evaporation

time.

385

R. Riwan et al. / Cs and 0 on Mo(100)

[21,22]. They amount to 2.5 to 4% of the Cs population at the monolayer coverage (ML). It is important to note that in the presence of Cs, the C and 0 signals are reduced approximately by a factor 2 and the true contamination is always higher than that given by the measured 3.1.2.

LEED

C and 0 signals.

results

With increasing coverages char3.1.2. I. Adsorption on a clean flat surface. acterized by the A parameter of the Auger substrate signal at 220 eV, several patterns are observed in succession: _ 4%
directions

with the symmetry

the (00) reflection appear,

in

shown in fig. 2b; faint lines parallel

to

[ll] are also present. _ 9%
of domains

with a small extension

structure

present

11 0

l

\

arrangement

they

is observed

from

the

rotated

by

(fig. 4a).

/ l

11 a

0

l

Oi

li

01

l

result

i \

/ l 10

000

/ l

the

with two orientations

in two domains

\ 01

indicating

normal to the streaks (fig. 3); these

domains belong to a rectangular centered mesh (RCM) at 90”. _ 24%
(fig. 2a).

slightly streaked

\ a

0

Oi

17

l

/

b

a Fig. 2. Schematic LEED patterns ML = 4.3 x 1014 Cs atoms/cm2).

\

for low Cs coverages:

(a) 0 = 0.15 ML Cs; (b) 0 = 0.25 ML Cs (1

386

R. Riwan

et al. / Cs und 0 on Mo(100)

The RCM derives from a progressive contraction along a [ll] direction of one of the unit parameters of the p(2 x 2) pattern. The surface mesh is equal to 2fi

((Y X 1) R45 o with

for orientations

I and 11 respectively.

(Y is equal to 1 for the p(2 X 2) pattern and decreases continuously coverage is increased. The term quasi-hexagonal mesh is arbitrarily l/1.6

> a! - l/1.732.

Fig. 3. Schematic overlayer.

The

LEED patterns

true

for the rectangular

Table 1 Comparison of the intensities (in arbitrary reflections of the hexagonal surface mesh Experi-

Energy

ment

(ev)

1

2

61 _

3 4 5

61 _ _

Mean 6

28

hexagonal

structure

centered

units) measured

is for (Y = l/G.

mesh with two orientations

at the monolayer

135 184

63 100

63

30 _

179 160 167

96 81 84

66 30 41

36 14 13

165

82

50

23

64

:))

of the Cs

Z(Hex 11)

I(+(:

T))

The

for the first order

I(Hex I)

Z(k(f

when the used for

20

R. Rwan

measurement observed

is used to measure

to characterize the reciprocal

381

the surface

mesh.

An

values of a and a’. It was

that OL, and a2 are very close but for coverages

less than 1 ML (Ye

generally (Y, by (2-3) X 10m2. ML coverage the two orientations I and II of the hexagonal structure Hex II) are shown in fig. 12a. Along [ll] the parameter of the surface twice that of Mo(100). The distance between the Cs atoms is 5.13 p\

and the population

\

Cs and 0 on Mo(100)

of (Y is very convenient

optical comparator exceeds At 1 (Hex I, mesh is

et al. /

amounts

to 4.3

X

lOI

atoms/cm2.

il Cl

b

cl ii

cl

Cl

Oi

17

\ Fig. 4. (a) LEED reciprocal meshes.

lattices

pattern for

the Cs monolayer

of the two hexagonal

(61 eV). (b) Schematic

meshes

LEED

and of the corresponding

pattern

showing the

rectangular

centered

388

R. Rwm

errrl. / Cs undOon

In fig. 4a it is clear that the intensity Hex II. Furthermore,

at 61 eV the f(fh

Mo(100)

for Hex I is much higher than that for ih)

reflections

are more intense

than

the +($h ih) reflections, and the intensity of the +( i i) reflections is higher than the remaining first order Hex I reflections (table 1). It results that an overall two-fold symmetry is obtained with (110) as symmetry plane. In fig. 5, we represent l/a: versus A for the coverage range extending the p(2 X 2) structure

to the hexagonal

from

monolayer.

3.1.2.2. The stepped surface. The LEED patterns of this clean surface present a splitting along one [loo] direction which is interpreted as due to a regular up and down step array with a terrace width of approximately 6 times the lattice parameter

in that direction

The following

features

(i) At low coverage

[23,24]. are induced

by the presence

of these steps:

the c(2 x 2) pattern is more intense than on the flat surface.

(ii) The p(2 X 2) pattern

extends over a greater coverage

range, 9% < A < 19%,

than on the flat surface. (iii) The hexagonal

patterns

are weak when compared

flat surface. No preferential domain growth the reflections shows a four-fold symmetry.

to those obtained

is observed, In addition

on the

and the intensity of the (+ 5) reflections

(3=1/a Hexagonal

t

I

1.75.1 1.70-~ 1.60-

I j

Fig. 5. p = l/a versus A (attenuation coefficient of the MO 220 eV AES transition upon Cs adsorption; for a uniform distribution of the rectangular centered mesh the straight line would be obtained).

389

R. Riwan er al. / Cs and 0 on hf0(1m)

are very intense, reflections

approximately

of the hexagonal

five times more intense than the corresponding

structure,

and are streaked

in the direction

normal

to the steps (fig. 6a). (iv) In the coverage range defined by 21.5% < A < 24%, patterns are obtained with satellites along the [lo] direction normal to the step edges and splitted (1 f) reflections.

The satellite

separation

is l/6

of the reciprocal

parameter

along

[lo] (fig. 6b). 3.2. 0 and Cs coadsorption The coadsorption achieved as follows: the same amount exposures. 3.2. I. AES

of 0, and Cs on Mo(100) at room temperature was various amounts of 0, are adsorbed on a Cs monolayer;

of Cs is evaporated

on Mo(100)

preexposed

to increasing

0,

results Oxygen adsorption is studied by way of the 0 Auger peak to peak amplitude. No change in shape was observed in

3.2.1.1. 0, on Cs monolayer. KL,,L,,

contradiction Desplats compared obtained

with the results of Bauer and Poppa [25] for 0,

and Papageorgopoulos to that observed by Bauer

and

[26] on W(110).

on clean Mo(100) Poppa

.

[25].

Curve

IIa,

(fig.

.

7) shows

. l .

0 10

000

kinetics that

01 .

.

a

l 11

.

. .

. .

.

for an

. i i[ ii

l .

‘if .

.

. .

a

Cl oi

0 ii

b

:

.

. . 0 .

. l 10

000 .

. 0 ii

and was

(fig. 7) which looks like the curve

.

io 0

on Mo(100)

The adsorption

. a .

Fig. 6. (a) Schematic LEED pattern at the Cs monolayer on the stepped surface; the (A i) reflections are streaked in a direction normal to the step edges. (h) Schematic LEED pattern for 0 = 0.7 ML Cs on the stepped surface.

R. Riwan

390

et al. /

Cs and 0 on Mo(100)

exposure less than 1 L (= 1 x 10m6 Torr s), the intensity of the 0 signal is less than that observed on the clean MO surface for an identical 0 exposure. Above 1 L the apparent adsorption rate increases. The 0 signal saturates at 3 L instead of 5 L on the clean MO surface. Fig. 8 shows that the amplitude observed for the Cs N,,023023 and MSN,sN,, transitions increases with the oxygen coverage. The presence of 0, modifies also the Cs transition new weak transition at 56 eV (fig. 9A).

at 62 eV and produces

t

N’(E)

a,u

~0/M011001

15

=I

150

100

a

I.

c

______I -

50

dJ ,I’ /.

Ua

-.-._

lb

---

m

05

.i ,p ,f 0

I

I

I

I

1

2

3

L

transition. Curve I Fig. 7. Intensity curves of the 0 KL,,L,, Mo(100) at room temperature versus exposure; the coverage Curve IIa (p ): intensity curve for 0, adsorbed on the corrected using the assumptions in section 4.2.1. Curve III (Cs monolayer is adsorbed on 0 coverages defined in curve I.

Fig. 8. Relative intensity increase of the low energy, 47 eV (0) Cs AES signals versus 0, exposure (in Langmuir units).

L

i

(- - - - -): adsorption of 0 on clean scale (on the right) is related to I. Cs monolayer. Curve IIb (-.-.): - -): intensity observed when a

and of the high energy,

558 eV (I)

391

R. Riwan et al. / Cs and 0 on Mo(100)

Nk au

i! i

eV

60

OS t a

B

I

eV

\2tl

b

I

I

I

I

1

2

3

4

L

) clean MO; (-.-.-) Cs on Mo(l00); (---) 0 on Cs Fig. 9. (A) AES spectra: (adsorbed Mo(100). (B) Attenuation of the three MO AES signals versus 0, exposure: (a) for 0, adsorbed on Mo(100); (b) for 0, adsorbed on a Cs layer on Mo(100).

392

R. Riwan

et al. /

pattern of the Mo(lOO)-Cs-0

Fig. 10. LEED

3.2. I. 2. Cs deposition

on adsorbed

Cs and 0 on Mo(100)

(~(4 x 4)) structure (245 eV)

The Cs deposition

oxygen.

produces

a mean

reduction of 12% of the intensity of the oxygen KLL transition (fig. 7. curve III). The attenuation of the MO Md5Nz3Na5 and MO M,,N,SN,, transitions is found to be constant and amounts 24% and 22% respectively. The 56 eV transition 3.2.2.

is also observed

LEED

in this case.

results

The surface microgeometry

is responsible

for structural

differences

observed

on the flat and stepped surfaces. For oxygen adsorption on a Cs monolayer adsorbed on the flat surface up to 0.25 L, a faint c(2 x 2) pattern is developed, together with a weak p(4 x 1) one. At 0.3 L, the c(2 structure

increases

On the stepped and disorder

x

2) pattern

disappears

and the intensity

up to 0.5 L (fig. 10). A general surface

is observed

only the c(2

X

2) pattern

disorder

of the p(4

X 1)

exists above 1 L.

is developed

up to 0.7 L,

above this exposure.

When Cs is deposited on a MO surface with various 0 coverages, an intense c(2 x 2)-Cs-0 structure replaces the weak c(2 x 2)-O one formed by a 0.3 L 0, exposure [25-271 and further the (110) micro-facets which appear for a 0.6 L 0,

exposure.

Above

1 L a complete

disorder

occurs.

4. Discussion 4. I. Cs adsorption 4.1.1. Calibration of Cs coverage The true hexagonal structure

at the monolayer

obtained

for A = 31.5%

corresponds to a Cs population of 4.3 X 1Or4 atoms/cm2 (section 3.1). This value is in good agreement with that corresponding to the p(2 x 2) structure

R. Rwan

etul./ Cs and0

on Mo(100)

393

and an A value of 18% (section with a population of 2.5 x lOI atoms/cm2 4.1.2.2). These results are very close to those obtained for Cs on W(100). At the monolayer, Desplats [7] found an A value of 298b/ for the W transition at 170 eV, and a similar value is deduced from the results of Thomas and Haas [28]. It results that the sticking coefficient is constant up to 0.9 ML and is then reduced by a factor of 26 up to the monolayer. For Cs on W(100) [7], the sticking coefficient was found constant up to f ML and then progressively reduced. 4.1.2. Structures 4.1.2.1. ((2 x 2)-G. For W(lOO), Desplats [7] assumes that the c(2 X 2) structure is an arrangement of Cs ions with a high density of missing sites. Voronin et al. [6] consider the possibility of c(2 x 2) being the result of a small H, contamination, and from extensive H, and Cs coadsorption. Papageorgopoulos and Chen [S] conclude that a coverage as low as 0.03 ML of preadsorbed H induces the formation of such a structure. It is very difficult to exclude the presence of small amounts of hydrogen. The overall pressure was generally better than 5 X lo-” Torr with a partial pressure of hydrogen of about 2 X lo- ” Torr or less. After flash, the crystal is cooled during 10 min before Cs evaporation, and exposed thus to approximately 1.2 x 10e2 L of H,. The hydrogen emission of the Cs source was also checked by directing the Cs flux near the quadrupole ionization source. After the thorough degassing, during which CH, was observed, no hydrogen was detected. Therefore we believe that H, contamination was unlikely. Estrup [ll] assumes that the c(2 X 2) structure observed on W(100) at low Cs coverage is the result of a periodic lattice distortion (PLD). This mechanism is proposed to explain the occurrence of the reconstructed low temperature structure of the clean W(100) or its reconstruction in the presence of hydrogen [11,13,29-341 and also on Mo(100) [14]. Barker et al. [29] described the schematic LEED pattern for a square lattice on which a longitudinal wave produces periodic atom displacements in the same direction. If the wavelength X of the distortion is slightly different from &a (a = MO unit cell parameter) the splitting of the ($ t) spots occurs along [ll] with the symmetry observed in fig. 2 for the most intense reflections. The streaks may arise from a distribution of the incommensurate distortion with ma < X < fia. The streak length decreases with increasing coverage and sharp (f t) spots are obtained corresponding to X = &a for 0.3 < 8 < 0.38 ML. The pattern at very low Cs coverage (about 0.4 X lOI atoms/cm2) results from an identical mechanism, but in absence of long range order, the intensity

394

R. Riwun era/./ Cs and 0 on Mo(100)

in the supplementary reflections is limited around the (00) reflection. A PLD with X = {?a appears therefore an appropriate candidate for Mo(100) c(2 x 2)Cs and for the features observed at very low coverages. 4.1.2.2. p(2 X 2), 0 = 0.58 ML. The model proposed for Cs on W(100) with the Cs atoms on hollows with four-fold symmetry [5,7] is also adopted in our case. It was already shown that the 2.5 x lOI atoms/cm2 population deduced for that structure is consistent with the AES and LEED results at the monolayer. In fig. 5 the straight line joining /? = 1 and /? = 1.732 would be obtained if the RCM covered the surface completely. As the measured values above this line, the RCM alone cannot cover all the surface. Up to 8 = 0.7 ML, the RCM coexists with the p(2 x 2) structure which appears at some characteristic voltages. Above 8 = 0.7 ML the p(2 x 2) pattern is no more observed. At B = 0.74 ML, the coverage of the quasi-hexagonal mesh deduced from its parameters is 0.91 ML. So we conclude to the occurrence of islands which cover 0.74/0.91 = 0.81 of the surface. We have shown that a domain selection 4.1.2.3. Structure at the monolayer. resulting in a preferential growth of the hexagonal domain of orientation 1 is always present on the flat surface (fig. 4). Information on the substrate structure may also be deduced from the intensity of the ($ 4) reflections (fig. 4. table 1). From the properties of the possible space group for the hexagonal overlayer, the intensity of equivalent reflections should be identical. The intensity excess of the (f f) reflections, Zs($ i), at the energy considered (61 eV), results therefore from a diffraction contribution due to the substrate surface structure. This contribution is obtained from rs,(~ft)=(Z(+tf)-Z(HexI)},

I~~(++

+)= {I(&+

i)--I(HexH)},

for domains a and b respectively from which the pattern fig. lla is deduced. We conclude that a reconstruction occurs on the MO surface with the presence of a fi x fi R45 o surface mesh. From Zs( f f i) > Is k (+ i) we may obtain the space group of that substrate structure. This inequality is satisfied if: (i) the space group for the fi x fi R45’ mesh is p2mg; (ii) two domains a and b rotated by 90 o and unequally populated are present on the surface.For domain a, the glide plane along [llO] produces the extinction of Zs( f 4 i), fig. lib, while domains b affect the intensity of Z.s( f T i), fig. 11~. As Z.r( + t r) and Z.Y(+ i i) are proportional to the number of domains n(a) and n(b) respectively we deduce that n(a) is - 3n(b). As these intensities are

R. Riwan e-1al. / Cs and 0 on Mo(100)

395

also proportional to 1(Hex I) and I(Hex II) respectively it is clear that the growth of Hex I and Hex II has induced the reconstruction of the substrate surface in two domains. The present analysis for these substrate domains is similar to that made by Debe and King [33] and discussed by Woodruff [35] for the low temperature c(2 x 2) structure on clean W(100) for which domains with p2mg space group are deduced from a complete set of intensity measurements. Debe and King [33] found that one type of domain was occupied with a probability of approximately twice the second one, therefore a ratio smaller than that observed in the present work. However, as it was pointed out by Woodruff [35], the domain preference observed on the clean surface [33] may be due to a small density of surface steps. These glide planes are interpreted as the result of a periodic lattice distortion [33,34] with the atom displacement essentially located in the surface plane [35]. In the present case it would be interesting to compare the ratio n(a)/n(b) to that obtained for the reconstruction at low temperature on the clean surface. This was not allowed with the present manipulator.

000

0

0 10

i

.l

77

a

0

0

0

..P\\.

0

...’ \

0

b

0

.‘l

0

0

.

.

00 i

0

0

l

0

‘\ ‘0 10 I’ ,I’ I

i

77 0

0

11

9..

92 2

‘...‘. / “0 00

0

0

.

c

0

0

0 10

0

0

0

Fig. 11. Reciprocal lattice of the 6 X fi R45O substrate structure with p2mg space group. (a) Superposition of patterns (b) and (c) with spot sizes proportional to the intensity. (b) Orientation a: the f { : i ) reflections are extinguished by the presence of the glide plane; (. . . . .) direction of the glide plane. (c) Orientation

b: the + (f i) reflections

are extinguished.

A model is proposed for the overlayer arrangement and for the substrate for which zig-zag Mo chains are assumed along [ll] (fig. 12b). It is likely that this reconstruction is different from that existing at low coverage for which a four-fold symmetry is observed. The answer to that question requires a more complete study. On W(100) a four-fold symmetry is obtained for the low Cs coverage c(2 x 2) and for the patterns at the monolayer [5,7]. For @>, 0.7 ML an induced electronic state observed at F [2] is explained by an umklapp of a state existing at G for the clean surface f3J. The four-fold symmetry observed at the

Hex I

Fig. 12. Structural model of Cs monolayer: (a} Hexagonal substrate). (b) Hexagonal domains showing the 42 symmetry (view from the bulk).

lit?XII

domains I and II (on an unreconstructed R45O substrate mesh with p2mg

xfi

39-l

R. Riwan et al. / C’s and 0 on Mo(100)

monolayer Mo(100).

does

not

exclude

It may result

from

a reconstruction the surface

growth of preferential domains. On Mo(lOO), a similar umklapp

process

similar

microgeometry

to that which

deduced

on

hinders

the

may exist under cesiation:

a new

state has been observed near 3 eV below the Fermi level [2] in ARUPS at 21.2 eV (He,). But, for higher photon energies [3] this state may be clouded by the low lying surface Reconstruction and W(100) adsorption

state of Mo(100). induced by alkali adsorption

surfaces.

It occurs for substrates

is not restricted

like Cu(ll0)

to the Mo(100)

and Ag(ll0)

[30] for

of several alkali metals.

4.1.3. Cs-Cs bond length At 1 ML coverage, the Cs nearest

neighbour

distance,

projected

on the MO

surface is equal to 5.13 A (section 3.2.1) but the true bond length is probably slightly greater but not very different from the bulk value (5.30 A). In each domain only one row of Cs atoms fits the substrate sites along [llO], the other atoms being out of site. As the result of this incoherent adsorption, the Cs atoms are not coplanar. For a plane layer, the bulk value would be obtained at 0.97 ML, so we may conclude that close packing occurs only near the monolayer coverage. In an ab-initio calculation

taking

into account

the electronic

properties

of

an unsupported Cs monolayer, Wimmer [37] found that the most stable configuration occurs for a hexagonal layer with an 11% contraction of the Cs-Cs bond length compared to the bulk value. This is the value found for Cs on W(110) [38,39]. He considers that the reduced bond length results from a weak, non-directional interaction with the substrate and an adatom interaction not markedly

modified

(compared

to an unsupported

monolayer).

On Mo(100) and on W(100) [l-3] however, we have to consider a strong interaction between the Cs atoms and the substrate because: (a) The interaction of Cs and W(100) is not delocalized. The bond between Cs and W occurs

through

a hybridization

between

the Cs 6s electrons

and the

high localized W d-like surface state electrons. (b) The occurrence of the epitaxy condition is due to the presence of potential wells along [llO], which induces and stabilizes the epitaxy from the p(2 X 2) structure up to 1 ML coverage. The distance between two second nearest wells along [llO] is equal to 8.8 A and corresponds at 1 ML coverage to a Cs-Cs distance

of 5.13

A and

however, the same distance

5.15

A for MO and

W,

respectively.

On W(110)

between sites is unable to stabilize a Cs layer with a

parameter twice that along [llO] [39]. On Cu(100) [40,41] the epitaxial relation [lOiO]Csl][llO]Cu is correlated with the Cs-Cs distance along [lOlO], twice the 2D Cu parameter along [llO]. The same relation holds for Ni(lOO) [41]. On these two last metals, the electronic interaction between the Cs 6s valence electrons and the substrate occurs

398

R. Riwan etal./ Cs and 0 on Mo(100)

however

mainly

with the delocalized

wells along [llO] play an important

sp electrons

[42,43].

role for the adatom

Thus

the potential

distance.

4. I. 4. Step effects 4.1.4.1.

At the monolayer.

growth of the hexagonal domain

selection

W(100)

surface

The presence of the steps hinders the preferential domains. Estrup and coworkers [15,30] report that a

due to some defects (or steps distribution) reconstructed

by H, adsorption

may occur for the

and not for the low tempera-

ture reconstruction. On the Mo(100) surface the streaks normal to the steps on the { + i} reflections indicate that the fi x fi R45 o domains are limited by the terrace width. The intensity effect. With regard

observed for these reflections is attributed also to the same to the sharpness of the reflections due to the hexagonal

domains we conclude that the steps have no influence on their extensions. This may be related to the fact that almost all Cs atoms are adsorbed incoherently. An important parameter to remove the symmetry degeneracy is the dimension of the terraces. Barker and Estrup [13] evaluate the width of the terrace to 100 A in order to remove the domain degeneracy on W(100). Wang and Lu [44] observed

a strong domain selection

terrace width (28 A). In the present work

the results

substrate

While extended

reconstruction.

selective growth,

on a W(100)

confirm

this latter is absent

vicinal face with a smaller

the terrace

width

effect

terraces (on the flat surface)

on the stepped

surface

on the induce a

on which a mean

terrace width of 20 A has been determined. 4.1.4.2.

0.7 < B < 0.75

produced pattern

by

ML.

the presence

is not related

The

pattern

of periodic

in fig. 6 is very similar steps

to the p(2 x 2) structure

in antiphase which

to those

conditions.

exists just

The

below

coverage but to a splitted c(2 x 2) structure with satellites associated integral reflections. It is likely due to the reconstruction of the substrate

that

to the in the

terraces, the antiphase relationship between these latter being responsible for the splitting of the (+ $) reflections [24]. An alternative interpretation is the presence of a PLD with a wave vector

q = $/I + 3/i. Such a model has been used for H, adsorption temperature

on Mo(100)

at low

[ll].

4.2. Coadsorption

effects

4.2.1. AES results For the calibration

of the 0 coverage,

we use the scale obtained

for room

temperature adsorption on clean Mo(100). The saturation corresponds coverage of 1.5 ML [25,27]. However two factors have to be considered:

to a

R. Riwan et al. / Cs and 0 on Mo(100)

399

First, the back scattering factor for the Cs 47 eV and 558 eV Auger transitions increases when 0, is adsorbed (fig. 8). This effect was also observed for 0 on Cs/W(lOO) [7] and 0 on Cs on Si(lll) [45]. It affects also the amplitudes of the oxygen transitions which have to be corrected. Second, the measured intensity of the 0 KL,,L,, transition, fig. 7, is weaker than that observed on Mo(100) up to 1 L, but is identical to that observed for Cs on adsorbed attributed below

oxygen up to 0.5 L (fig. 7, curve III). This behaviour

in the case of Cs on W(100)

the

Cs

layer

O/Cs/Mo(lOO)

[7].

This

to the penetration

is consistent

with

has been

of the small 0 atoms

our

ARUPS

results

on

[17].

Therefore at these low exposures, the attenuation of the 0 large Cs atoms is higher than the backscattering increase.

signal

by the

For exposures between 1 and 3 L, the increase rate of the 0 signal is now higher than on the non-cesiated Mo(100) surface. The possible reasons are, beside the backscattering effect (fig. 8): _ the 0 atoms are now mainly above the Cs layer; - the sticking coefficient is slightly higher than on Mo(100); - some charge transfer occurs on 0 so that the effective electron on the 0 L,, shell is increased. If we assume that the oxygen sticking coefficient instead 0.72

of 0.7 L on a clean Mo(100)

ML instead

of 0.64

equal to 1.45 ML (after the apparent Above

ML.

surface

remains constant

[25], the coverage

At 3 L, the observed

backscattering

change

population

saturation

correction)

up to 1 L

is then equal to value is then

instead

of 1.5 ML,

value.

3 L, despite

the observation

of a saturation

value of the oxygen

signal, a small decrease of the MO Auger transitions (fig. 9b) indicates saturation is beyond this exposure and occurs approximately at 5 L. It results that an accurate 0 coverage from the AES results alone (fig. 7). An increase

determination

of the oxygen sticking coefficient

cannot

compared

surface has been reported for Cs adsorbed on metals such as W [7,19,46], Ni [47], Ag [48] and Fe [49].

that the

be obtained,

to that of the clean

less reactive

than MO

4.2.2. Cs on O/MO When Cs is adsorbed on a surface with chemisorbed oxygen, a mean reduction of the intensity of the oxygen signal of 12% is observed. Using the approximation X, a Ek/’ given by Seah and Dench [50], where

EA is the kinetic expect

energy of the Auger electrons and X, the escape length, we an attenuation of 22% by comparison with the 31.5% reduction of the

220 eV MO transition covered by a Cs monolayer. to the increase of the backscattering coefficient determine

in this case.

The difference is attributed which is more difficult to

400

R. Riwnn efal./ Cs ond 0 on Mo(100)

4.2.3. Origin of Cs 56 eV transition The presence of the 62 eV and 56 eV Cs Auger transitions has been identified by Desplat [51] as Cs N,,OZJV and Cs N,,O,V(O) transitions, with V designating the hybridized Cs 6s-W d surface state [I] in the valence band and V(0) the O,, level measured by ARUPS, 5-6 eV below the Fermi level [17]. Charge transfer on 0 has been postulated by Desplat [7], and by Lindgren and Wallden on Cs on Cu(ll1) [52]. This could explain the chemical shift observed for the Cs 5p electrons in UPS [7]. Unfortunately such a behaviour cannot be simply related to the ionicity because the distance of the Cs to the surface and/or the change in surface potential may modify the binding energy

[31.

4.2.4. Structural effects The formation of the c(2 X 2)-Cs-0 at the expense of the hexagonal structure has already been observed by Desplat [7] for Cs on W(100). In absence of reconstruction of the substrate, this structure corresponds to a coverage of 0.5 ML Cs and 0 atoms and requires displacement of Cs atoms probably on MO sites with four-fold symmetry. Reduction of the Cs radius from 2.57 to 2.23 A is also necessary, caused either by partial ionization or polarization or both. The c(2 x 2)-Cs-0 is also observed when an amount of 4.3 X lOI Cs atoms/cm2 (the coverage for a Cs monolayer on Mo(100)) is evaporated on a Mo(100) surface with 0.5 ML of oxygen. Similarities are found for these two structures from AES (curve 111.1, fig. 7) and ARUPS results [17] but work function measurements and EELS results indicate that they are not completely identical. The difference is probably due to the strength of the bond between 0 and the substrate. As adsorbates induce reconstruction [ll] and particularly 0, on Mo(100) [25,27] we propose the models shown in fig. 13 in which the reconstructed substrate model is obtained either by place exchange or by a shift of the surface atoms.

4.2.4.1. p(4 X 4) structure. We assume that the p(4 x 1) structure found on a flat surface is very close to the c(2 x 2) one. The oblique mesh (fig. 14) derives from the c(2 x 2) structure by a small lateral shift of the Cs atoms in the second row with a correlative shift of the underlying oxygen and MO atoms. This structure is probably inhibited by the unsufficient extent of the terraces on the stepped surface. A similar effect has been reported by King and Thomas [32] who found that a terrace width of approximately 20 A may inhibit the formation of the W-c(2 x 2)-H structure. It is interesting to note that the numerous structures reported by Desplats [7], and Papageorgopoulos and Chen [5] for 0, on Cs on W(100) due to continuous distortion of the

hexagonal mesh, are not observed in our case. This may be related to the fact that the Mo(100) substrate presents a great ability to reconstruct at room temperature in the presence of 0 [25-271, whereas for 0, on W(lOO), heating up to 700 K is necessary to observe the reconstruction of the surface [l&53].

Fig. 13. Mo(lOO)-Cs-0 (~(2 x2)) models (views from the bulk): (a) with the MO substrate reconstructed by a PLD similar to that on the clean surface; (b) r~onsteuction by place exchange.

Fig. 14. Model for Mo(lOO)-Q-0

(4 x 1).

402 5. Conclusions

In the present work we showed that Cs adsorption on clean Mo(100) causes the substrate to be reconstructed. At low coverage a four-fold symmetry is observed, while at 1 ML coverage a domain selection occurring for both the hexagonal overlayer and the underlying substrate surface produces a two-fold symmetry. This selective growth allows the determination of the p2mg space group of the ~6 x 6 R45 o reconstructed surface mesh. This structure is interpreted as the result of a periodic lattice distortion with the k vector along [ll] and the wavelength equal to &a (a = parameter of the MO mesh). It is not established if the four-fold symmetry at low coverage results from a similar structure with a complete domain degeneracy. The presence of a high density of up and down steps parallel to one [loo] direction has been found to hinder the preferential growth of both reconstructed substrate and overlayer domains. But while the overlayer domains are greater than the electron diffraction coherence length, the substrate domains are limited by the terrace width. From these observations it is deduced that the orientation and the populations of the hexagonal overlayer domains and of the recontructed substrate domains are strongly correlated. Inclusion of the reconstruction in theoretical calculations on the electronic structure of a Cs monolayer on Mo(l~) is expected to improve the agreement between experiment [3] and theory [8]. This is already the case for Cs on W(100) where an electronic state at 2,6 eV below the Fermi level at V is attributed to the substrate reconstruction. The structure observed for 0, adsorption on a Cs monolayer is also a function of the surface microgeometry. On the stepped surface an intense c(2 x 2)-Cs-0 structure is stabilized in opposition to the p(4 X 1)-Cs-0 one obtained on the flat surface. Due to the ability of,the Mo(100) surface to reconstruct in the presence of several adsorbates, the reconstruction is assumed by place exchange for 0 and MO or shift of the MO surface atoms.

Acknowledgement

The authors

are grateful

to A. Besnard

for technical

assistance.

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