CO2 and CO monolayers on MgO(100) : LEED experiments and potential energy calculations

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experiments and potential energy calculations

V. Panella, J. Suzanne, P. Hoang, C. Girardet

To cite this version:

V. Panella, J. Suzanne, P. Hoang, C. Girardet. CO2 and CO monolayers on MgO(100) : LEED

experiments and potential energy calculations. Journal de Physique I, EDP Sciences, 1994, 4 (6),

pp.905-920. �10.1051/jp1:1994235�. �jpa-00246954�

J. Phys. I Fiaiic.e 4 (1994) 905-920 _)UNE 1994, _PAGE 905

Classification Phy.ç<cs Abstiacts

67.70 68.20 61.14H

CO~ and CO monolayers

_on

MgO(100)

_:

LEED experiments

and potential _energy calculations

V. Panella

('),

J. Suzanne

('),

P. N. M.

Hoang (2)

and C. Girardet

(2)

(') CRMC2-CNRS (*), Département de

Physique,

Faculté des Sciences de

Luminy,

Case 901,

13288 Marseille Cedex 9, France

(~) Laboratoire de

Physique

Moléculaire

(*"),

Faculté des Sciences La Bouloie, Université de Franche Comté, 25030

Besançon

Cedex, France

(Received 29 Dec.ember 1993,

accepted

in filial foi-ni 4 Maic.h 1994)

Résumé. La _structure des monocouches C02 et CO adsorbées sur une surface monocristalline (100) de MgO _a été déterminée à la fois par diffraction d'électrons lents et par des calculs de potentiel d'interaction. La monocouche C02 forme _une phase commensurable

(2,5x,5)

tournée de 45° pour une température inférieure à 93 ± K. La monocouche CO _{a une structure} commensurable (4 _x 2) _pour Tw 40 K qui se transforme _{en une} (3 _x 2) dans le domaine de température 41-49K. Au-dessus de 50 K, cette monocouche évolue _{vers une}

phase

(n x 2) uniaxiale, avec une perte progressive de 1ordre lorsque T croît _encore. Les calculs

d'énergie

potentielle donnent _une très bonne interprétation de la géométrie stable de la monocouche CO~

ainsi que de la stabilité relative des différentes phases commensurables de CO.

Abstract. The _structure of CO~ and CO monolayers adsorbed _on MgO(100) single crystal

surfaces has been determmed by both LEED and

semi-empincal

potential calculations. The CO~

overlayer ^forms a commensurate

(2,é x,/5)R45°

_{sohd phase}

at Tw93 ±1K. The CO

monolayer forrns _a (4 x2) commensurate phase at Tw40K which is transformed into _a (3 _x 2

phase

when 41 K

w T _w 49 K. Above 50 K, the CO monolayer expands uniaxially toward

a in

x 2) phase with _a progressive loss of

long-range

order when temperature mcreases. Potential calculations interpret _very well the stable geometry ^for CO~ and the sequential

stability

of the

vanous commensurate phases for CO.

l. Introduction.

The

adsorption

of molecular species ^{onto iomc} substrates leads _{to a} wide vanety ^of

commensurate structures owmg ^to the

competition

between the adsorbate-substrate and the

adsorbate-adsorbate interactions. When molecules _are

beanng dipole, quadrupole

_or

higher

(~) Also associated with the Universities of Aix-Marseille 2 and 3.

(~*) URA CNRS 772.

moments, or

tions become substantial _{or even}

predominate

over the

dispersion

interactions. Structural

experiments

associated with model calculations grue a better

understanding

of the molecular behavior and

provide

_{a way} to

probe

the relative

magnitude

of each contribution.

CO~

is _one among the molecules which present no

dipole

but _a strong

quadrupole

moment

Q

=

4.3

DÀ.

_{One expects}

a substantial interaction of the molecule with the surface electric field

gradient

_{on one} hand

and,

on the other

hand,

a

quite

strong electrostatic molecule-

molecule interaction.

Experimental investigations using

laser induced thermal

desorption techniques

on air cleaved

Mgo samples

annealed in oxygen at 950K

[Ii

or

polarization

mfrared spectroscopy on in situ cleaved

samples [2, 3]

have been undertaken and

they

confirm that

CO~

is

molecularly

adsorbed _onto

MgO(100).

Infrared

experiments

have concluded that

CO~

forms _a

highly

ordered adsorbate structure at 82 K with _a

removing

of the fourfold

symmetry ^{of the} square

MgO

surface due _{to a}

preferential

orientation of atomic steps ^induced

by cleavage [2,3].

LEED

experiments

have shown that

CO~

_on

MgO(100)

_grues a

(2,à

x

,à)

_{R 45°}

commensurate

monolayer

structure

[4].

CO has _a small

dipole

moment p =

o-1D and _a medium

quadrupole

moment

Q

=

1.9

DÀ

_{which also lead to}

significant

electrostatic interactions with the ionic substrate and

within the adsorbed

monolayer.

LEED

experiments

have shown that CO _on

MgO(100)

surfaces is adsorbed in _a

(4

_x

2) phase,

at T _w 40 K

[5].

When

temperature

increases, this

commensurate

phase undergoes

_a uniaxial transition to a

(3

_x

2)

structure and then _{to a}

(n

x

2) phase.

These results _are consistent with

polanzation

mfrared spectroscopy measure- ments

[6].

From the above mentioned

experimental investigations,

it appears that the

adsorption

of CO and

CO~

on

MgO

surfaces is _a molecular

adsorption

_process. In this paper, we want to

analyse

further the

previous

LEED results _on

CO~ [4]

^{and CO}

[5]

adsorbed _on

MgO(1où)

and compare (hem to

semi-empirical potential

calculations _m order _to obtain _a better

understanding

of the

mechanisms govemmg the molecular arrangements ^{in the}

layer.

2.

Experiments.

2,1 EXPERIMENTAL _SET-uP. The

experiments

have been

performed

in _a UHV chamber

equipped

with _a LEED

apparatus having

_a low electron beam

intensity II w10~~ _A)

_which

reduces any

possible perturbation

of the adsorbed molecules. The set-up ^has

already

been

descnbed elsewhere

[7],

let _us

just

recall here that the LEED pattem is recorded _{in a}

MacIntosh 2 CX usmg a video _camera PANASONIC Wu BL 600/G.

MgO crystals

of punty

99.99 fb and dimensions 4 _x 2 x10

mm~

_from

Spicer

Co. Ltd hâve been used.

They

are

cleaved in situ with _a

special

cleaver

[8]

mounted _on the

sample manipulator.

The

sample temperature,

^that can be lowered down to 30K

using

_a closed

cycle cryocooler (CTI- cryogemc),

is measured

by

a

platinum

resistor

(100

Q at 273

K).

A temperature ^controller

provides

a temperature

stability

AT

=

0.05 K. We estimate the absolute temperature calibration

at ±1K.

The CO and

CO~

_gases are 99.995 fb pure. A

supplementary

_gas

cleamng procedure

analogous

to that used for ethane

[7]

has been

applied.

The gas

punty

is checked

by

_{a mass} spectrometer

(BALZERS QMG 31Ii

For both gases the

background impunties

consist

mainly

of molecular

hydrogen

^which does not

physisorb

onto

MgO

at temperatures ^{above 30 K.}

Before _an expenment, ^the

MgO crystal

_is cooled down to the temperature

required

and then cleaved in _a

background

_pressure lower than 10~ ^'° _torr. The first expenment on a

freshly

cleaved

sample

is

perforrned

on a

perfectly

clean surface about 10 _mm after

cleaving.

An

Auger analysis,

made _at this stage ^with a CMA spectrometer

(RIBER

OPC

105)

in the 10- l 500 eV energy range, does not show any contaminant within the spectrometer sensitivity.

N° 6 C02 AND CO MONOLAYERS ON

MgO(100)

907

CO _or

CO~

condensation is followed

by measuring

^LEED kinetic isotherrns _at the temperature

required,

that is

monitoring

the decrease of

intensity

the four

(10) MgO

_{spots as a} function of time.

2.2 RESULTS.

2.2.1

CO~ monolayer.

LEED kinetic isotherms _at 60 K and 80 K hâve been

reported

in _a

previous paper

[4].

We will focus here _on the _structure

analysis.

Extra spots are

appearing

after about 6 min of condensation _{at a constant}

C02

_{gas pressure} of 4 _x 10~ ^~ torr.

They

_{appear very}

dim _at

first,

then their mtensity mcreases until it reaches _{a constant} value at

monolayer saturation,

_givmg the LEED pattems ^shown m

figure

1. Successive expenments,

following

the

one carried out

nght

after

cleavage,

show LEED pattems ^{of less}

good quality.

An expenment lasts about _to 3 hours. At the end of each

experiment,

the

MgO sample

is warrned up ^to room

temperature ^{and then} cooled down

again

the _next

day

for _{a new}

experiment. Auger

spectroscopy was

performed

^{in between} two successive

expenments

^{in order} to check the

MgO

surface cleanliness. After about three expenments, ^carbon contamination was observed and LEED pattems ^{from the}

CO~ monolayer

were

barely

visible. At this stage, ^it was

necessary to use a new

freshly

cleaved

crystal.

Measurements made _on LEED pattems ^{hke those shown m}

figure (see

also

Fig. 2)

indicate

a

(2,à xl)

_{R 45°}

commensurate

CO~ overlayer,

that is _a unit cell _m real space with

A

=

8.42

À

_{and B}

=

4.21

À.

_The

accuracy of _our measurements is estimated to fG. This unit

cell, shown _m

figure 3,

has _{an area} of 35.5

À2

and _{contours two} molecules _as

expected

from the molecule _size. The strong

quadrupolar

moment of the

CO~

molecule lets _us think that there

is

probably

a

herrmgbone packing

similar to that calculated

[9]

for

C02

_on

graphite

or that

found

expenmentally

for

CO~

_on Nacl

[10,

Il

]. Normally,

two

equivalent

domams at 90°

should exist above the

MgO

surface givmg use to spots m the LEED pattem as shown

schematically

in

figure

2. The

il1)

and

(20)

reflections have

appreciable

intensities in both

figures

la and 16. On the other hand, in

figure16

there _are spots which _are not visible _m

figure

la. This feature

corresponds

to the presence of

only

_one domain _in

figure

la, while _two

equivalent

domams appear m

figure16.

In the _case where

only

one domam _is present, ^the absence of the

(01)

and

(of

_spots indicates the _existence of _a

ghde plane perpendicular

to the

large

side of the real space ^unit cell

a) b)

Fig. I. LEED pattems of C02

monolayer

adsorbed _on MgO(100) at T 80 K and CO~ gas pressure 4x10~~torr for _two different

MgO crystals

cleaved _msitu.

(a)Only

_one domam _is observed (E ₌ 131.2 eV). lb) Two

eqwvalent

domains at 90° _are observed (E

= 130.8 eV ).

jÉ

(01)

,

~

o

@ Î~~

~ o ~

OE. ~ ,b

o

Fig.

2. Schematic representation ^{of the LEED}

~pattern

^of

C02/MgO(100).

The unit cells of the _two

domains

(Aj*,

B/) ^and

(Al.

Bf) of the (2,'2

x,fi)R45°

commensurate CO~ overlayer in the

reciprocal space are shown. In the figure,

large

_squares represent

MgO

diffracted beams small circles

are from domam of the overlayer large circles _are spots ^{from domain} 2 empty ^circles (small and large) are missing spots suggesting ghde

planes.

The MgO(100) surface unit cell _is indicated _as (a~, b~) Some of the

(Aj*,

Bj%) ^domain spots are indexed for clarity _purposes.

The presence of _two domains for the

CO~ monolayer

_on

MgO(100),

makes _it difficult _to

detect the presence of this

ghde plane.

Indeed, the second domain

(ho

spots, ^with n even, fait

exactly

at the positions ^{of the first} domam

(0 k)

_spots, with k odd,

masking

in this way

systematic

extinctions due _to

glide plane symmetries.

On _some

MgO crystals,

the two domains

presented

diffraction spots

having unequal

intensities due _to the

predommance

^of one domain

over the other. This observation agrees with the

analysis

of the

polanzation

^infrared

expenments ^of

Heidberg

and Meine

[2, 3].

Unlike the

(0 k),

beams, the _existence of two domams does not lead _to any

ambiguity

in

explaming

the absence of the (ho ) spots ^{with h odd.}

This _can be understood

by looking

at

figure

3.

Indeed,

the

(30

) and

(10)

_{spots are}

_only

_present

Q

Fig. 3.-Real space unit cell (A, B) ^{of the}

(2,<5x,'2)R45°

commensurate C02 monolayer

adsorbed _on

MgO(100).

The

MgO(100)

surface unit cell (a, hi is also shown. C02 molecules are

represented

as dark circles _on the large and small empty ^circles representing oxygen and magnesium atoms

respectively.

N° 6 CO~ AND CO MONOLAYERS ON

MgO(100)

909

at a few

energies

where the

intensity

of these

spots

^is very weak. Such _a weak

intensity

could be

explained by

the occurrence of

multiple scattering

when the incident electron beam _is not

exactly

at normal incidence

il 2].

The absence of the

(10

and

( loi

spots ^could not be checked since their

positions

_are too close to the inactive area of _our channel

plate

intensifier within the LEED energy range studied. Hence, ^{it is}

likely

that there is _a second

glide plane perpendicular

to the small side of the real space cell of

figure

3

[13].

The existence of two

glide planes implies

an

in-plane herringbone

structure similar to that calculated for

C02

on

graphite [91.

An

in-plane herringbone packing

seems to be reasonable from the _size of the molecule. The

surface areal

density

^which we found for the commensurate

CO~ layer

_on

MgO

is

17.7

À2/molecule,

_{that is,}

_larger

than the value calculated for

graphite

which is around 15

À2 [9].

It is also

larger

than the areal

density

of the densest

plane (111)

of bulk

C02>

that

is,

13.45

À2/molecule _(at

T

=

83

K) [14]

or than the value 15.85

À2/molecule

_{of the commensur-}

ate

(2

x

ii CO~ monolayer

on Nacl at Tw 80 K

[10, 1ii.

It is

interesting

to notice that

volumetric isotherm measurements at 156 K

[15]

have

given

an areal

density

^of

CO~

on

uniform

MgO powders

^{of about 11.9}

À2/molecule

_at

_monolayer completion.

This _{is a} rather

surprising

result since it

implies

a transition from the

(2 ,à

x

,à)

_{R 45°}

commensurate 2D sohd _{to a} much denser

phase

_at temperatures ^{above 93} K, the

highest

temperature at which _we have observed the commensurate structure.

We have also

investigated

a

possible change

of the _structure at T~ 60 K. After

having exposed during

14 _mm the

MgO surface,

held at 60 K, at a pressure of 4 _x 10~ ^~ _ton of

CO~,

we closed the gas valve. The gas pressure ^retums

rapidly

to the 10~ ^'° _ton range.

According

to

our kinetic isotherm measurements

[4],

we have saturated the commensurate

monolayer.

Then the temperature ^of the

sample

^{is lowered}

slowly

to 35 K

(about

1K per

mm).

No structural

change

is observed _{even at} the lowest temperature. ^{At 35}

K,

we have set

again

the

C02

_Pressure to 4 _x 10~ ^~ _torr for 12 min _in order to condense _{one more}

layer.

The

CO~

LEED superstructure does not

change

except ^for a strong attenuation of the spots. ^When two statistical

monolayers

of

C02

are condensed, the extra spots ^due to the commensurate

structure are

barely visible, indicatmg

either disorder in the

bilayer

film _or three dimensional

(3D) crystallite

formation.

After

having

condensed _one

monolayer

at T

=

80

K,

if _{we mcrease} the substrate temperature

above 90

K,

the diffraction pattem

gradually disappears

within K around T 93 K. This

result _is the signature ^of an order-disorder transition. If, after

increasing

the temperature _up to 100 K, _we lower _it down to values below that of the

transition,

_we fait _to observe the ordered

structure.

Similarly,

the ordered structure is not observed if _we condense the

CO~ monolayer

at

temperatures ^{above the} transition,

namely

100

K,

and then cool down the

MgO

below the transition

temperature.

^The results _are the _same for different

cooling

rates, the slowest _one

being

0.5 K per minute.

During

the _increase of temperature, care was taken _to avoid the

evaporation

of the

C02 monolayer by increasing

the gas pressure

accordingly.

It is

interesting

to compare this transition temperature

Tj(2D)

to that of the

melting

of bulk solid carbon dioxide

T~(3D)

=

216.6 K

[16] giving

a ratio

T~(2D)/T~(3D)

= 0.43. This value _is appreci-

ably

smaller than that

usually

found for

physisorbed monolayers

which is close to, or

larger

than, ^0.6. It shows that the

C02

commensurate

layer

_{is not}

strongly

stabilized

by

the

MgO

substrate unlike

CH4

for instance, which presents an ordered commensurate

phase

_up to 80 K with _a

corresponding

ratio of 0.89

[17].

2.2.2 CO

monolayer. Figure4

exhibits the LEED patterns ^{of the}

(4x2)

and

(3

_x 2 HOC _structures of the CO

monolayer

at T

=

39 K and T

=

45

K, respectively.

The

double spots between _two

MgO

first order

spots

can be

explamed by

the presence of _two

domains rotated 90° apart above the

(100)

surface. In

figure

5 _we show the

reciprocal

lattices of

JOURNAL OE PHYSIQUE T 4 N'6 )UNE J994 11

a) b)

Fig.

4. LEED pattems ^{of the} commensurate monolayers ^of

CO/MgO(100).

(ai (4 x 2) structure at T 39 K and CO gas pressure 4 _x 10~^~ torr ;E~ =

112.7 eV (b) (3 _x 2) structure at T

= 45 K and CO

gas pressure 10~^? torr E~ ^{115.0 eV. In both} cases, two equivalent ^domams at 90° exist above the MgO(100) ^surface.

©

~

~

~~

io21

, ,

,~ °

,

' '

' ', ',

,

, , ,

, ,,

o o

' ,

R O o p .

,' ,

,

' .

o ,

'

o O

~ o o ' (21)

(31)

~

o o

~

a*

Q

^a*

g3

. o

~ ~ (401 1301

(a) (b)

Fig.

5. Schematic

representation

of the LEED pattems ^of

CO/MgO(100).

The _unit cells

(Al, BT

and

(Al, BÎ)

of the two equivalent domains of the (a) (4 _x 2 structure and (b) (3 _x 2 structure, _in the reciprocal _{space are} shown. In both _cases, (a*, b*) represents the reciprocal _{space unit} cell of the

MgO(100)

surface,

large

_{squares represent} MgO ^diffracted beams large empty circles _are domam1 spots ; large filled circles _are domain 2 spots : small empty ^circles represent ^double diffraction spots.

Some of the (Al,

Bf

domain spots are indexed for

clanty

_purposes.

the

(4

_x 2) and

(3

_x 2)

phases.

We _see that the spots ^with

appreciable

mtensites are the

(31), (11), (31)

and

()Î)

_spots for the

(4

_x

2)

structure and

(21), (il

_),

(21)

and

(ii )

for the

(3

x 2

).

The faint doublets doser _to the

specular

spot are at the position ^{of the}

(11)

reflections from the CO

overlayer.

Part of _or ail the intensity ^{of these} spots may also be due to double diffraction from

MgO

diffracted beams. Under favourable conditions of

crystal

surface

quality

and electron energy, we also observe very dim

(20)

_spots. The

(4

_x 2 solid structure is stable from 40 K down to the lowest

temperature

^that our

cooling _system

_can

reach,

that is 30 K.

Figure

^{6 shows the real} space ^unit cells of the two commensurate structures.

Unhke _m the

analysis

_we have made _{in our} previous paper [51> we do not represent the

N° 6 CO~ AND CO MONOLAYERS ON

MgO(100)

⁹¹¹

Mg O

,

(4x2)

a)

--,Mg

,-

o

jCO/Mgo (3x2)

b)

Fig.

6. Real space unit cell (A, B of the commensurate CO monolayer adsorbed _on

MgO(100).

(a)

(4 _x 2) structure : A Il.92

À

_{and B}

=

5.96

À

_{(b) (3}

x 2) structure A 8.94

À

_{and B 5.96}

À.

(a, b) represents the MgO(100) unit cell. The CO molecules _are located along Mg _rows, in both _cases.

primitive

cell with _one molecule per cell, _smce we now believe that the 6 CO molecules

contained _m the

(4

_x

2) phase

_or the 4molecules in the

(3

_x

2) phase

have different

orientations above the

surface,

_as we will _{see m} section 3. The surface areas of the two unit cells are

71À2

_{and 53.3}

_À2, respectively,

and _we have thus for the CO molecular surface

area : Il.8

À2/molecule

_in the

(4

x

2) phase,

and 13.3

À2/molecule

in the

(3

x 2

phase.

The densest

plane

of the bulk sohd CO

(corresponding

_to the

(111) plane)

has _a molecular surface

area of 13.8

À2 il 8].

The

highest density

of the _most stable

(4

_x

2)

structure is due to the strong adsorbate-substrate interaction _as confirrned

by

_our calculations. For companson, the

commensurate

(2

_x

1)

and

(1

_x structures of

CO/Nacl(100)

have _{an area} per molecule of

15.7

À _{il 9].}

Upon increasing

the temperature at constant pressure, the

monolayer undergoes

a uniaxial

phase

^transition

along

the

Mg troughs ([01]

surface

direction).

The intense

(31)

spots ^of the

(4

_x 2 _{structure move} doser _to each other and

they finally

become the

(21)

_spots of _{a new}

(3

x 2

phase

which appears ^at T

-

41 K. The _new

phase

is stable up to about T

=

49 K. The diffraction spots ^{associated with the}

(3

x 2

) phase

are less resolved than those

charactenzing

the

(4

_x 2 _structure. Above 50

K,

the

background intensity

_mcreases and the

spots broaden,

mdicating

a decrease of the

monolayer coherency

as shown in

figure

7. A contmuous transition to different

(n

_x 2

phases

is still detected. However, the uncertamty on the measurements of

Fig.

7. -LEED pattern ^{of the} (n _x 2)

phase

of

CO/Mgo(100)

_ai T 52 K and CO ga~ pressure

x 10~ ^? _torr, E~ ^{15.0 eV. Due} to the increasing disorder, the

quahty

of the LEED pattern is poor and

does _net allow _us to determine _n

the molecule-molecule distance becomes

larger

due to the poor

quality

of the LEED pattern which prevents us from

determining

_n with accuracy. The results _seem to indicate _a

possible

teck-in of the adsorbate into _{a new}

phase (n

x 2) in the range of temperatures between 51 K and 54 K. Above 55 K, the LEED spots are broad and

barely visible, suggesting

either that the CO

monolayer

reaches _a one-dimensional

liquid-like

state _or that the size of the ordered

domains has decreased

dramatically.

The variation of the molecule-molecule nearest

neighbor

distance _{as a} function of

temperature at constant CO pressure

Pc~

=

10~~

_torr has been

reported

in reference

[5].

The

observed _succession of teck-in

phases separated by sharp

transitions

(width

AT w1K)

between them is _a

good

illustration of the

incomplete

^devil s staircase

[20].

3. Calculations.

3,1 INTERACTION _POTENTIAL. We determine the interaction

potential

V between the

adsorbate and the substrate _on the basis of _a

semi-empincal description

of the interactions connected either with the

layer-substrate

contributions _or with the

in-layer

molecule-molecule

contributions. This

description

takes into account the

dispersion

and

repulsion

between _atoms

pertaining

to the molecule _{or to} the substrate

by

_{assuming a}

pairwise

Lennard-Jones form with parameters s and _«. The electrostatic interactions between molecules _or between _a molecule and the substrate

charges

are calculated _on the basis of a distributed

multipolar analysis [2 ii.

For the diatomic molecule,

charges, dipole

and

quadrupole

moments borne

by

the C and O

atoms and

by

the _center of the bond

represent [22] accurately

the _most important contributions ID the electrostatic

potential

between _two CO molecules _or between the molecule and the

Mg

_or O effective substrate

charges equal

to ± 1.2 electronic unit

[23].

For the tnatomic

molecule,

the

charge

and

dipole

contributions

nearly

cancel each other due _to the molecular symmetry and _a

point description

of the molecular electrical property ^of

CO~

_in terms of _a

single

quadrupolar

moment seems to be _more suitable

[24].

The other contributions to the interaction

potential,

I.e. induction terms, substrate-mediated terms.. remain in

general

weak

enough

ID be

disregarded

here.

3.2

EQUILIBRIUM

STRUCTURE. The

potential

_energy V of the adsorbate _is minimized with

respect

to the position R and orientation n of every molecule _in the

layer

_; the substrate _is

assumed _to be

ngid

and undeformable with _{a nearest}

neighbor

distance _a

=

2.98 between

N° 6 CO~ AND CO MONOLAYERS ON

Mgo(100)

⁹¹³

Mg

or O atoms. We consider

only

commensurate structures of the type

(n

_{x m} R 4l which _can

be non-rotated

(4l

=

0)

_or rotated with respect to the substrate frame

(X, Y).

The X axis is chosen

along

a

Mg

row. The minimization

procedure [7]

consists in _a numencal search for the

potential

minimum

V[][~,~~~(R, ⁿ⁾

connected with the

(n xm)

unit cell

containing

s-

molecules,

with respect to the 5 _s

degrees

of freedom three for the

position (X,

Y, Z and _two for the orientation

(à,

_~g of each molecule. The

cyclic

conditions _are

applied

to the other cells of the

monolayer by assuming

that

equivalently

adsorbed molecules

rriove similarly

in every cell. Reasonable values for the numbers _n and _m have been selected, which

correspond

either

to lower order commensurate

geometries (n

and _{m w}

2)

_or to

higher

order commensurate

phases

(m or ii

reaching

values

equal

to

4, 5...).

Such _a selection is based _on the necessary

optimization

of

computational

times

(which

become

prohibitive

for

larger cells)

and _on

experimental

evidence which prevents a

systematic

search of the cell sizes and

shapes.

3.3 RESULTS.

3.3.1

CO~ monolayer.

Calculations of the most stable geometry of the

CO~ layer

have

already

been

performed

and discussed _m another paper

[24].

The charactenstics of this

structure are

given

in table I. We

give

here

only

the main results which will be useful _to

determine the structure factors

required

for _a direct companson with LEED

experiments (see

Tab.

Il).

The non-rotated commensurate

C02 Phases

_{are net} stable _on

MgO

when the _n and _m values remain within the reasonable limits discussed before. Indeed, the talerai interactions _are

highly repulsive

ai the

layer completion

in the

(1

_x

1), (2

_x

1)

and

(2

_x 2 structures with _{one, two}

and four molecules per

primitive cell, respectively.

In contrast, the rotated

(2,/2

x

,,/2)

_{R 45°}

_phase,

_{which contains two molecules per cell,}

_lying

_fiai

_along

a

Mg

row and with

their _axes

mutually ~perpendicular,

is very stable

(Fig. 3).

This structure is much _more stable than the

(,à

x

,/2)

_R 45° and

(2 ,,/2

x 2

,à)

_{R 45°}

_phases.

_The

_adsorption

energy per molecule for the

equilibrium

structure is

equal

to 479 mev, with _a contribution of the lateral energy

equal

to 22 % of the total interaction. The molecule-surface distance is

equal

to 2.53

À

and the lateral distance between _nearest

neighbor

molecules

(d

=

4.20

À)

is close to the distance observed in the solid (> 4

À) _[14].

3.3.2 CO

mvnolayer.

The minimization

procedure

^of V has been

applied

to several

primitive

cells for the CO

layer.

Low-order commensurate structures such _as

(1xl ), (2

_x

1)

and

(2

_x 2 geometnes are net stable _on

MgO(10 j.

^Similar conclusions _are reached

from the consideration of rotated

(,à

x

,fi)

_R

_{45°, (2}

, 2 _x

,,~)

_{R 45°..}

_{phases leading}

to

higher

energy structures.

The most stable calculated geometry

corresponds

to the non-rotated

(4x2) phase

containing ^{6 molecules} per

primitive

cell

(Fig. 6a),

with _an average

adsorption

_{energy per}

molecule

equal

to 2 il mev. Table shows that the molecular _centers of _{mass are}

along

two

adjacent Mg

_rows

~yla

_> ⁰ and m1) ^and that the

adsorption

sites in two

adjacent

_{rows are}

equivalent.

In _{a unit}

cell,

two molecules _are located in

Mg

sites with their _axes

perpendicular

ID

the surface (à

0°),

two others stand between _two

adjacent Mg

_atoms and

they

are flat

(à 90°

)

above the substrate and the last two are tilted

by

about 30° from the normal. The

main feature _is the strong localization of the molecular centers of _mass and of the molecular

axes

along

the

Mg troughs.

A

study

^{of the} relative contributions to the

potential

shows that the lateral interactions account for about 19 % of the total interaction. The molecule-substrate

contributions are dominated

by

the

charge-charge

and

charge-dipole

electrostatic interactions described within the distnbuted

multipolar approach.

Other

geometries (n

_x

2)

are found but

they

are less stable than the

(4

_x 2 geometry. ^For

of

most stable structures

for CO~ monolayers

adsorbed _on

MgO(1où),

X, Y, Z _ai-e the coordinates

of

the _cerner

of

_mass

of

the molecl~les _in the unit cell. ôis the

angle

betmJeen the molecl~le axis and the normal to the

surface

and

~g ^is the

angle

betmJeen the

pi-ojection of

the molecl~le _{axis in} the X, Y

plane

and the X _a-us-

Phase Molecule X

(À) Y(À) Z(À)

_à

_(deg)

~g

(deg)

CO~

^{3.16 1.05 2.53 90 135}

(2 ,fi

x

,fi)

_{R 45°}

2 7.37 3.16 2.53 90 45

CO 0.00 0.00 3.07 0 0

(4

_x

2)

2 3.75 0.00 3.01 30 0

3 8.02 0.00 2.65 90 0

4 1.91 2.98 2.71 90 0

5 5.96 2.98 3.10 0 0

6 9.71 2.98 3.04 30 0

CO 0.03 0.06 3.07 0 0

(3

_x

2)

2 5.00 0.09 2.98 40 180

3 3.25 3.07 3.01 10 20

4 7.03 3.04 2.95 45 0

these structures, with _n

=

3, 5, 6..,

the molecular centers of _{mass are} located

along

two

parallel Mg troughs,

in order to form commensurate

phases

consistent with the _two- dimensional arrangement ^{of the CO} molecules _in the

monolayer. Increasing

the value of _n leads to the _occurrence of

phases

with additional molecules located _on

non-equivalent

sites.

The molecular orientations in the

(n

_x

2)

structures are more or less tilted with respect to the normal,

depending

_on the

adsorption

site. On top ^{of the}

Mg

site, the molecule stands

upright (à

= 0°

),

but ils axis becomes _more and _more tilted when the _center of _mass is

displaced

from

this site; at the middle of the

Mg-Mg distance,

the molecular _axis becomes flat

(à

=

90°

).

In ail situations, the molecular _axes

(for

à #

0°)

_are oriented

parallel

to the

Mg

row, taken _as the X _axis. When

changing

_n, the

adsorption

_{energy per} molecule is modified

by

a few mev

only

but the molecule orientations are much _more sensitive due to the

change

of the site.

The characteristics of the

(3

_x

2) phase

_are

presented

in table I. This

phase

^{contains four}

molecules _{per unit} cell located

along

the

Mg troughs.

Two molecules stand

nearly upright,

and the others _{two are} tilted

(145°

with respect to the normal

(Fig. 6b).

The average

adsorption

energy per molecule _is 207meV, that _is

slightly

smaller than for the most stable

(4

_x

2) phase

with _a lateral contribution of about 15 %.

N° 6 CO~ AND CO MONOLAYERS ON Mgo(100) ⁹¹⁵

Table II. Sri"l~ctl~i"e

factoi"s for CO~/MgO(100)

and

CO/&IgO(100)

ut T

= 0 K calcl~lated

fi.om

the

eql~ilibiil~m monolayei"

geomen"les ^{deteimined in section 3}

(sec

Sect. 4

for

_moi-e

detailed

explanations).

hk

CO~/MgO CO/MgO CO/MgO

(2 fi

x

,fi)

_{R 45°}

⁽⁴

x

2) (3

_x

2)

00 36.0 144.0 64.0

01 0.8 o-o o-o

02 0.5 144.0 64.0

03 1.4 o-o o-o

04 1.o 144.0 63.3

10 o-o o-o 7.1

11 10.2

,0.7

5.0

12 2.3 o-o 7.3

13 1.9 0.7 5.2

14 4.2 o-ù 7.4

20 10.1 2.1 7.0

21 12.0 ù-1 22.5

22 2.9 2.1 7.6

23 IA ù-1 22.1

24 11.5 2.1 8.2

30 ù-ù 0.2 5.7

31 2.8 l17.9 31.2

32 7.6 0.2 5.5

33 6.4 l17.9 30.1

34 7.1 0.2 5.2

40 1.7 12.5 12.1

energy

(3

_x

2) (4

_x

2) phases

is the _a

competition

between the lateral energy which increases

by

10 mev per molecule when _n

increases from 3 _to 4 and the

layer-substrate potential which,

_on the contrary, ^decreases

by

6 mev per molecule. This fact _can be

easily explained

if _we note that the average distances

between molecules _in

adjacent Mg troughs

are similar

(di

3.70

À)

_{for the}

₍₄

x

2)

and

(3

_x

2)

structures. In contrast,

along

the

Mg troughs,

the average distances between CO

molecules _are

significantly

different:

d=4.47À

_and

3.97À

_{for the}

_{(3 x2)}

_{and the}

(4

_x

2) phases, respectively.

Note that this second value is very close _to the _nearest

neighbor

distance in the CO solid

(d

=

3.99

À)

_{and ii tends to}

_optimize

_{the lateral}

energy in the

(4

_x

2)

structure.

The _reverse situation _can be _seen for the

(5

_x

2) phase

with 8 molecules per unit cell. The

magnitude

of the adsorbate/substrate contribution tends to decrease _{as a} result of the

density

increase but the

corresponding

talerai interactions also decrease because the intermolecular

distance becomes _too small

(d

> 3.73

À)

_{and tends to favour}

_repulsive

interactions. Once

again,

the energy variation from the

(4

_x

2)

to the

(5

_x

2) phase

represents a few mev _per

molecule, but the

(4

_x

2)

structure is

clearly

found to be the most stable.

4.

Comparison

between

experimental

results and calculations discussion.

The structure factors for the calculated

equilibrium geometries

^of

CO~

and CO

monolayers

(see

3.3)

have been determined for ail

(hk)

reflections accessible _{to our} LEED

experiments,

and the

corresponding

relative

mtensity

values _are

reported

in table II.

Ii should be

pointed

Dut that the calculated intensifies _{assume a} kinematical behavior of the diffracted beams which is

certainly

net the _case for LEED.

Furthermore,

the calculations have been done

by considering

^the same atomic form factor for carbon and oxygen ^atoms and

disregarding

the influence of the

Debye-Waller

factor.

Hence,

the

comparison

with the LEED results _is

only qualitative.

Nevertheless, _we expect the calculations _to

interpret

1) systematic

extinctions of the LEED pattem ^due to

glide plane symmetries,

and

ii)

_any

intensity

value close to zero related _to the absence of the

corresponding

LEED reflection _ai ail electron energies. A

more

quantitative dynamical study, involving

the

analysis

of LEED spots intensities _i>eisus electron energy curves, is aise in progress

[25].

This

analysis

will

provide

us with _more details about molecular orientations and

positions

above the

MgO(100)

surface.

4.1

C02

MONOLAYER. The calculated relative intensities of the

il 0)

and

(30)

reflections

are

equal

to zero

(see

Tab.

Il).

This _is in

perfect

agreement ^{with the LEED results} (see

Figs.

l and

2)

and it

corresponds

to the existence of _a

glide plane perpendicular

_to the small side of the real space ^unit cell

(see Fig.

3). The

loi )

and

(03)

reflections _are absent from the LEED

pattems ^{while the calculated} intensities for the

(01)

and

(03

reflections _are not

equal

to _zero but have small values.

Strictly speaking,

this

implies

that there _{is no}

glide plane perpendicular

to the

large

side of the real space ^unit cell unlike what _we have deduced from _our LEED observation.

Indeed,

the calculations indicate _a

slight displacement

of the molecule _center of

mass with respect to the mid distance between _{two magnesium atoms} which _can

produce

a

small tilt

(w

5° of the molecular _axis

responsible

for the absence of the second

glide plane (see Fig. 3).

However, the fact that the calculated intensifies of the

loi

and

(03

spots are weak

with respect to that of the _intense visible spots may account, within the kinematical

approximation,

for the non-observation of these spots in our LEED patterns. The main visible spots

correspond

to the

il Il

and

(20)

reflections, which have indeed the

largest

calculated

intensifies besides those

coinciding

with

MgO

reflections like the

(21).

N° 6 C02 AND CO MONOLAYERS ON Mgo(100) ⁹¹⁷

4.2 CO _MONOLAYER.

.

(4 x2) phase.

Table II shows that the calculated intensifies for the

(31)

and

(33)

reflections _are

large compared

to ail other reflections. This is also the _case for the

(02)

and

(04)

reflections but

they

_are in the _same

position

_as the

MgO

spots. ^{The LEED}

experiments

_agree with the

(3 il

_spots

having

_a

large intensity

since

they

are the main visible spots

(doublets

in

Fig. 4a).

At energies

larger

than 150 eV, the size of the

reciprocal

_space

investigated

in LEED allows _{us to} observe the presence of the

(33)

reflections but _a

comparison

with the calculated intensities of table II is

meaningless

since these reflections may

come from double diffraction from the

MgO (02)

diffracted beams as well. We _can conclude that the calculated intensities _are in

good qualitative

_agreement with the LEED observations.

.

(3

x

2) phase.

As stated

above,

the LEED

pattern

^{of the}

(3

x

2) phase

_is less well defined than that of the

(4

_x 2

) phase. However,

the calculated intensities of table II _are in fair agreement with the LEED observations. The

(21)

and

(23 peak

reflections have

appreciable

intensities and the _{same comments} hold _as for the

(31)

and

(33)

reflections of the

(4

_x

2) phase. However,

other reflections have also

non-negligible

calculated intensities.

Particularly,

the

(31)

and

(33)

reflections of the

(3

x

2) phase

are

comparable

to the

(21)

and

(23)

reflections

although they

_are not visible _m the LEED pattern. ^{This result}

means that the calculated geometry ^{of the molecule} in the

(3

_x

2) phase

^contains

probably

some inaccuracies connected _{on one} hand, with the determmation of the

potential

coefficients and, _on the other hand, with the

neglect

of _some contributions in the interaction

potentials (Sect. 3.1).

Moreover, the calculations show that several different

(3 x2) geometries corresponding

to close values

(within

a few

mev)

of the

adsorption

_energy are found,

depending

_on the sites

along

the

Mg

_rows. These

findings

_agree with the fact that

experiments give

less clear results for the

(3

x

2) phase

than for the

(4

_x

2) phase. Finally,

it should be

recalled that the calculations _are

performed

at 0K whereas the LEED

experiments

are

performed

above 30K. The

entropic

effects _can have _a

non-negligible

influence _on the orientational

configurations

of the admolecules

and,

as a

result,

_on the

intensity

of the reflections. More

specially,

^it is

quite possible

that the transition from the

(4

_x

2)

to the (3 _x

2) phase

around 41K be

triggered by

_a rotational

disordering

transition of the CO

molecules. The model used in this work is unable _to show evidence of such _a transition.

4.3 GENERAL DISCUSSION. The

herringbone

_geometry with almost flat molecular _orien-

tations obtained for the

CO~ monolayer

_agrees

fairly

well with the LEED results and with the

existence of _one

glide plane perpendicular

to the small side of the real space unit cell. It _is

however in

partial disagreement

with the tilt

(-

60°

[2, 3]

with respect to the normal _to the surface determined from the

intensity

ratio of the

parallel

and

perpendicular polarization

mfrared bands. This

discrepancy questions again

the relative

sensitivity

of the _two

techniques

with

regard

to the orientation of the

monolayer

molecules. A

general

conclusion for that _{case is} nevertheless the _occurrence of _a very stable rotated

(2 ,à

x

,,fi)

_{R 45°}

_phase

_{containmg two}

molecules per unit cell, which is found in both LEED and

polanzation

FTIR expenments ^and

is furthermore

supported by

the present calculations _at T

=

0 K. These calculations _are however unable _to

explain

the

phase

transition from this commensurate

phase

to a disordered

state between 90K and 100 K, and _more

particularly

the

irreversibihty

of this transition.

Simulation calculations could give information _on this

pecuhar

behavior.

For the CO

monolayer

structures, the LEED pattern ^exhibits the existence of _a stable (4 _x

2)

dense _structure at low temperatures, a

phase

transition into a

(3

_x 2) commensurate

geometry ^{and then mto} a uniaxial disordered

phase

when T increases. This _succession of

transitions

that,

unlike

CO~,

showed to be

reversible,

_can be

mterpreted,

from _our calculations