Helium atom scattering studies of thermal energy vibrations on the clean and adsorbate-covered NI(100) surfaces

Journal of Electron Spectroscopy and Related Phenomena, 44 (1987) 183-196

183

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

HELIUM ATOM SCATTERING STUDIES OF THERMAL ENERGY VIBRATIONS ON THE CLEAN AND ADSORBATE-COVERED NI(100) SURFACES

R. Berndt, J.P. Toennies and Ch. W~II

Max-Planck-Institut fSr StrSmungsforschung, Bunsenstra@e i0 D-3400 G~ttingen, Federal Republic of Germany

ABSTRACT Vibrations at a Ni(100) surface have been studied by Helium atom scattering. Adsorption of various adsorbates (C0,H,O) leads to characteristic changes and new features in the phonon spectrum of the clean surface. The dispersion curves for the clean substrate are compared to recent EELS data. Experimentally determined values for the frequency of the hindered translation of adsorbed C0-molecules strongly deviate from theoretical predictions.

I. Introduction

In this article we will present new data obtained with helium atom beam scattering

HAS

for the

clean and with various

adsorbates

(H,O,CO)

covered

surface of Ni(100). He atom scattering has previously been used for studying the phonon spectrum of various clean and adsorbate covered metals *) as well as semiconductors 2) and

insulators ~) . In

the past vibrations

at surfaces have

been mainly a domain of low energy (= i0 eV) electron energy loss spectroscopy EELS and infra-red spectroscopy IRS. By extending EELS to higher energies of 100-200 eV (impact scattering) it has become possible to study, in addition to the vibrations of adsorbed species, also the vibrations of the substrate. Most of this recent work has been devoted to Ni(100) 3) , where it has been shown that

by

taking

reference,

the

the

phonon

frequency

dispersion

shifts

curves

of

the

of surface-locallzed

clean

surface

Rayleigh

phonons

as

a

upon

adsorption of C *) , S 5) , N 6) and 0 ?) reveal interesting information concerning the local bonding between an adsorbate and the surface atoms. with

EELS,

HAS provides

order of magnitude

a considerable

from ca 5 meV

(EELS)

increase

in resolution

to typically 0.5 meV

When compared of about an (HAS) and is

therefore able to detect modes with energies down to 1 meV. Its major drawback comes from the fact, that HAS is so far limited to energy transfers of less than about 30 meV

0368-2048/87/$03.50

by a large multlphonon background.

© 1987 Elsevier Science Publishers B.V.

184 II. Clean Ni(lO0)

In Fig.

i we compare

EELS energy

loss

and HAS

time of flight spectra

showing structures due to surface phonons of clean Ni(100).The Ni(lO0) crystal used in the HAS experiments was cleaned so that the residual carbon concentration was less than 1% as determined by a cylinder mirror Auger analyzer, which is the same as the cleanliness

achieved in the EELS study. This is the first

time that HAS and EELS data could be compared for the same system. the

Although

structure in the EELS and HAS spectra cannot be compared directly due to

the fact

that in the latter case the scan-curves

are parabolas

rather than

lines at constant Q as in the case for EELS, the differences in resolution are clearly apparent.

Fig. 2 shows a comparison of the dispersion curves measured

by EELS with the

HAS data. The two sets of results agree within the scatter

of the EELS

experimental

ments is attributed

points.

The lowest observed

to the transverse polarized Rayleigh mode S 4 whereas the

other modes are surface resonances tions projected

mode in both experi-

embedded

onto the surface.

in the continuum of bulk vibra-

Note that HAS reveals

a mode not seen in

EELS and is sensitive to different wave vector regions of the same mode seen in

EELS.

These

scattering

differences

are

related

to

the

more

interaction of EELS which also probes

vibrations,

whereas HAS is sensitive

complicated

multiple

the second and third layer

only to vibrations

at the surface and

couples only weakly to the surface atoms via the charge density at distances of 3 to 4 A from the nuclei.

The EELS data for Ni(lO0) the

framework

constant,

where

of

a only

lattice the

were analyzed by several dynamical

nearest

model

neighbors

with are

a

authors 8),9),*°) single

coupled.

radial

This

in

force

model

was

justified by the fact that with one force constant the bulk dispersion curves can be fitted to within about 4%. The EELS zone boundary were found to be which was attributed to a

15% greater

surface phonon frequencies at the than

these

theoretical

results,

stiffening of the surface interlayer force constant

by 10%.

In analogy

to the dependence

of force constants

diatomic

molecules

(Badgers Rule11)),

the stiffening was attributed

on bond lengths

of

to a 4%

inward relaxation of the distance between the first and second layers at the surface as measured by ion backscatterlng

12). In the analysis of HAS data for

the surfaces of Ag 13), Au I~), Pt 15), Pd 16) and especially A1 I?) it was found however that a

very precise fit of the bulk phonon data is required

better than 1%) in order to correctly describe

(for A1

all the surface phonon data

within the high resolution afforded by the HAS time of flight spectra.

185

Ni ( 1 0 0 ) < 1 1 0 > ( 1 ~ - k )

Electrons Ei = 115eV E)f = 55.1°

Helium Atoms E i =/.0./. meV

-9.9

-9.1

I,

Q = 0./-,9 A -1

Qe

J'~T

/)\

O = 0.50 A-1

E) = 39.0 °

J

I I•

-12.3 Q =0.76 A-1 I

"41-'

u}

/ ~.

r0,1

o= 076 a' , _ , ~ L ~

E) =36"0°

4..m I-.4

#

-~55

'~",%

1- ::°:::;o.

••

Q=I.01A -I

,a I

20

,

I

0

,

,

,

= 33.5 °

,I °

J ,

I

-50

I

2O

,

I

0

I

I

I

I -50

Energy Transfer [meV]

FiE. i: Comparison between EELS-data (Ref. 3) and HAS time of flight spectra showing enerEy loss peaks due to surface phonons of clean Ni(100) alon E the r-~ direction.

186

25.0

I

II

> 20.0 o

E

,_~ 15.0

I

/

~6~

I

I J

le~i-~

J

O

eU.I ¢-

o 10.0

r.o n

5.0

0O 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Phonon Wave Vector [ A-']

Fig. 2: Comparison between surface phonon dispersion curves of clean Ni(lO0) measured by EELS (Ref. 3, filled circles) and HAS (open circles) along the r-~ direction.

187 The possible pitfalls in fitting surface phonon data with a non-converged set of force constants for the bulk is nicely illustrated for the Ni(100) surface. In an attempt to improve the calculations of surface phonons we used the set of force constants published by Black et. interactions

up

to second nearest neighbors

between pairs of nearest neighbors

ELI.*s) in which central

and a bending

force

constant

(to account for many body forces) were

fitted to the neutron data and elastic constants. Fig. 3 shows a comparison of the

experimental

discrepancy Raylelgh

phonon

between

mode

S4

data

the is

with

such

experimental

much

smaller

a

calculation.

and and

has

In

theoretical

this

position

the opposite

case

the

for

the

sign apparently

implying that the surface force constant is weakened and not stiffened.

Before drawing such conclusions, however, the convergence of the surface Rayleigh-mode frequency at the high symmetry points should be checked with respect to a further

increase in the number of force constants. Surprisingly

in the case of Ni(100) the

results obtained by Bortolani and coworkers ,~)

with a lattice-dynamical model involving central interactions nearest

neighbours

neighbours

do

and

angle-bend

give nearly

interactions

the same

frequencies

up

to

the

(within

0.i

surface phonons as obtained by the inn model discussed above.

up to the 6-th 4-th meV)

nearest for

the

So far Ni(100)

is the only system for which a single central force constant works so well. Apparently there is an oscillatory dependence of the Rayleigh-mode frequency on

the number

of

shells

of nearest

neighbors,

to which

interactions

are

extended, and, luckily, the converged values are close to those obtained for the simple inn model. Thus in concluding this part of the paper we would like to emphasize that a very precise description of bulk phonon data and converged surface phonon frequencies with increase in interaction range is essential before it is possible to arrive at reliable conclusions concerning surface induced effects on interatomic force constants.

III. C0 on Ni(lO0)

In the case of CO-adsorption on metal surfaces the study of the frequencies of the various CO normal modes with EELS and IRS has provided results on the symmetry of the bond-slte as well as on the different force constants. The high resolution of IRS has made it also possible to study the line-shifts and -widths of these modes in dependence of surface temperature. In the case of CO

adsorbed

occupied.

The

normal modes,

on Ni(lO0), on-top ~t*

at low coverages

site has

on-top

and bridge

the C4v-Symmetry , giving rise

to ~t~ whereas

sites

are

to only four

the brldge-bonded site has Czv symmetry,

resulting in six different normal modes, ~bl to ~bS. For the on-top position

188

the dipole-active modes

at higher energies,

corresponding to the internal

stretch mode (~t,) and the carbon-nickel stretch (~t2) have been measured by EELS 2 0 ) The ~t3-mode (frustrated rotation) has not been observed by EELS for CO on Ni(100)

and until recently no experimental data for this or similar

systems was available for the ~t* mode, corresponding to a hindered translation. The frequency of this ~t~-mode has however recently been estimated

by

two groups using a model involving the interaction of the CO molecule with a cluster consisting of either n=5 atoms 2:) or n=9 atoms 22) of the metal. The various force constants in both cases were taken from a normal mode analysis of the free molecule Ni(CO),. These predictions differ strongly yielding 10.2 meV and 5.5 meV, respectively.

In order

to provide a direct

comparison between

the theoretical pre-

dictions for Ni(100) and experimental data we have recently investigated the adsorption of CO on Ni(lO0). Fig. 4

shows HAS energy transfer spectra for a

clean Ni(100) surface and the same surface covered with 0.4 L of CO. A broad loss peak centered at 3.5 meV is clearly observed on the CO covered surface. According

to the predictions

discussed

above

this new

feature

has

to be

assigned to the hindered translation of the molecule although the discrepancy to the theoretical values (of 10.2 and

5.5 meV) is surprisingly large. The

spectra show a considerable width of the loss peak which is much larger than the experimental resolution of about 0.3 meV. We attribute this to the fact that in the case of CO on Ni(100) even at low coverages and crystal temperatures of 140E both on-top an___ddbridge sites are occupied ~°). Thus in fact modes of both sites are probably observed and we explain the width of the peak by an incoherent superposition of the two

types of energy losses. If we use

the theoretical calculations as a guide of the relative energies we estimate that probably the on-top site frequency is at about 3.2 meV and the bridge site frequency at about 3.7 meV.

By increasing the exposure we were also able to produce the c(2x2) CO superstructure on Ni(lO0), for which an angular distribution is shown in Fig. 5.

In

Fig.

6

a time-of-flight

spectrum

is displayed

for

the CO

c(2x2)

surface. The CO energy loss is now even broader and shifted to somewhat higher energies

indicating

now

a

frequency

of

about

3.8

meV

for

the

hindered

translation in the on-top position. This value is in fairly good agreement with a value of 4.6 meV obtained by Black 23) for the c(2x2) superstructure of CO or Ni(lO0) with the help of a Greens function technique.

If the explanation for the considerable width of the loss peaks in the HAS-spectrum of Fig. 4

is right, the observation of a comparable width

in

189

r 3n

2 >

E 21

%

C~ f..,..

C III

C O C O J~

$I 1,

a_

0 0.2 0.4 0.6 0.8 1.0 1.; Phonon Wave .Vector [ A -I]

Fig. 3: HAS-data f o r the phonon dispersion curves of the clean surface compared with the results of a lattice dynamical calculation using the force constant scheme of Model II in Ref. 18. The S,-mode is of shear horizontal character and cannot be seen by HAS or EELS.

190

m

=1.20

NITNoKE 1°01°° 122meVco /

.

.

.

.

U In in t-

o

15

0.4CO ~-~x100

~Phonon

L

10

t)

c5

~n U9 . m

in

~:~20 U

I

I

I

I

I

I

,

i

i

m

I

I

Ni (100) -100=, Ei = 12.2 meV TNi:lS0K clean

In

"~ in 15 t-

o

10

O

O

c

xl00

5

U3

0 I -10.0 -8.0 "6.0 -Z,.0 -2.0

o

~o ~,o ~o ~OlOO

Energy [meV]

Fig. 4: Helium time of flight spectra for the clean (lower part) Ni(lO0) surface and after exposure to 0.4L CO (upper part). The spectra were taken at an He incidence angle e i of 50.0". Note, that the intensity of the loss peak at ~3.8 meV, which corresponds to an annihilation of a Rayleigh phonon on the clean surface, strongly decreases upon adsorption of CO.

191 the c(2x2) phase would imply that both bridge and on-top sites are occupied. The same conclusion, which is in disagreement with the work of Andersson z°) , has been reported by Bertolini and Tardy 2~) . The shift in energy probably is due to the enhanced interaction between CO-molecules

in the quite densely

packed c(2x2) phase. Time of flight spectra taken at different angles were not able to detect any dispersion of this mode.

It should be noted,

that additional complications may

arise from the

high amount oF short range disorder of the c(2x2) structure of C0 on Ni(100). This is indicated by the broad triangular base of the specular peak in the angular distribution of Fig.5. Therefore the large width of the CO-loss peak at higher

coverages

between neighbouring

can possibly

also

be explained by

a local coupling

CO molecules. Because of the very dense packing of CO in

the c(2x2) structure on Ni(lO0) we expect the inter-molecular interaction to be very strong,

which should further be enhanced by the low frequency and

resulting high amplitude of the frustrated translations.

250

_F

I

I

I

I

I

I

I

I

I

I

I

I

- -

Ni(100) <100> CO c(2x2)<100> Ei =1&4 meV TNi=150K

150

% "~100

5O

0

I

-4.0

I

I

I

I

-3.0 -2.0 -1.0 0 1.0 20 3.0 Parallel Momentum Transfer AK [~1]

I

4.0

Fig. 5: Angular distribution of helium atoms scattered by a c(2x2) superstructure of CO on Ni(lO0). Note the considerable width of the diffraction peaks which indicates a large amount of disorder.

192 a

i

-="3

I

I

I

I

I

I

!

t

~,0

6.0

8.0

Nili00) CO c(2x21 Ei =14.4 meV TNi=150K

U

Ul

~2 0

e-

0 I

I

I

-10.0 -8.0 -60 -&O -2.0 0 2.0 Energy [meV]

~

Fig. 6: Helium time of flight spectrum for the c(2x2) on Ni(lO0) at an incidence angle of 39.0".

10.0

superstructure of CO

IV. CO on Pt(lll)

With

~%S

it

has

also

been

possible

to

also

measure

directly

the

frequency of the frustrated translation ~t~-mode of CO on Pt(lll) z5). For the on-top site a frequency of 6 meV and for the bridge site a frequency of 7 meV was obtained. In the low coverage regime of CO on Pt(lll) the ~t4 peak is very sharp, This

in accordance with the fact that only on-top sites are occupied 2~)

observation

supports

our

explanation

given

above

for

the large

width

observed for CO on Ni(lO0). For CO bonded in on-top or bridge sites on Pt(lll) the HAS-data 25) together with the EELS-data z6) provide all expected frequencies with the possible exception position,

of the frustrated

rotations

in the bridge

which may also have been observed in the HAS spectra 25) .

Persson

and Ryberg z?) have developed

a theory,

which

for some systems

gives a relation between the damping of the internal stretch mode (~,) of CO adsorbed on a substrate and the energy of the frustrated translation.

193

It is gratifying to note that the measured frequency of this mode for the c(4x2) phase of CO on Pt(lll) gives a very good agreement with the IRS data for the dependence of the ~1-11ne width on surface temperature 2s) .

In the case of C0 adsorbed on Pt it has also been possible to study the dispersion of the hindered translations in the c(~x2) overlayer. As in Ni(100) this mode does not apparatus

(<0.3

show any dispersion within

meV).

This

is indicative

of

the resolution of the HASeither

a weak purely

radial

coupling between the C0-molecules at neighbouring sites or some compensating effects which are not understood at the present time.

Finally we note that the ~ - m o d e

is thermodynamically very relevant, as

it is the only mode which is occupied at thermal energies. The extremely small value of the frequency suggests a very flat valley of the potential hypersurface at both the on-top and bridge positions

and would seem to imply a

very small activation barrier for diffusion.

Obviously the frequencies of based on

force-constants

from molecular data

rough approximation. Thus it ab

initio

concerning

calculations the

modes

normal-modes on metal surfaces which are

on at

apparently it is not yet

(like Ni(C0)~)

can only be a

would be very interesting to have results from these

higher

systems.

There

energies

(especially

has

been the

much

progress

~1-mode),

possible to obtain theoretical results

but

also for

these low-energy modes at the present time.

V. Atomic Adsorbates on Ni(100)

We have also studied the effect of various chemisorbed atomic species on the surface phonons of the Ni(lO0) surface. Hydrogen adsorption provides an ideal way to change the electronic density of states at the surface since it forms a lxl layer and because of its small mass it does not appreciably change theeffective mass loading of the surface oscillators. Previously we had found a noticeable hydrogen

softening

adsorption,

of

surface

whereas

in

modes

W(lO0)

in

Si(111) 29)

hydrogen

leads

and to

Pt(111) l~) a

by

significant

stiffening of the Raylelgh mode by about 50% ~°) . In Ni(lO0) we observed that the highest frequency mode on the clean surface (see Fig. 2b), which we had assigned to a longitudinal mode, disappears almost completely and that the Rayleigh mode is slightly shifted downwards by 0.7 meV at the ~-point and 1.5 meV at R. It is interesting to note that the hydrogen adsorption restores the phonon frequencies to the values predicted from calculations using bulk force constants (see part If). This is in accord with the observations on Pt(111) .5)

194 where

the

anomalous

mode

disappears

altogether

and

the

Rayleigh

mode

is

shifted downwards by 1.5 meV. We attribute this phenomenon to a stiffening of the lateral

forces in the surface layer at the expense of the transverse

bonding between

the

first

two layers

resulting

from

a

saturation

of

the

dangling d-orbitals at the surface.

For oxygen adsorption we find good agreement with the EELS data for the 0 p(2x2)

structure 7) . However,

coverages we have

for

the 0 c(2x2)

found a splitting of

structure

at higher

the Rayleigh mode

peak

oxygen

along

the

r-~direction at large wave vectors which increases towards the zone boundary to about 1.5 meV. We interpret this as a breaking of the symmetry at the four-fold hollow site where the 0 atoms are thought to reside. This agrees with a model originally proposed by Demuth et.al. ~I) in order to explain LEED I(V) curves for this system. Strong and Erskine ~2) have recently investigated the effect of this symmetry breaking on the Rayleigh model n the context of a lattice

dynamical

qualitatively

calculation

similar

to

our

and

predict

observations.

a

splitting

Another

at

the

explanation

~-point could

be

related to the high amount ~3) of defects within the 0 c(2x2) phase on Ni(lO0). This seems unlikely, since we expect defects to give rise to a broadening of loss-peaks,

rather than the creation of a sharp,

new mode. However,

Banse

et.al. 34) have demonstrated for this system by a theoretical approach, that a high amount of disorder can give rise to

rather sharp additional peaks

in

the density of states above 55 meV. Unfortunately they did not investigate the density of states at lower energies.

Nevertheless c(2x2)

the

surface nicely

observed

splitting

illustrates

the

at

new

the

~-polnt

information

of

the Ni(100)+0

resulting

from

the

EELS on

the

higher resolution of HAS compared to EELS.

VI. Summary

Surface

phonon

dispersion

curves

previously

measured

by

Ni(lO0) surface have been shown to agree with recent He atom measurements with a substantially higher resolution on the Ni(100) surface. This is the first reported test of both methods for measuring surface phonon dispersion curves. Recent He atom inelastic scattering experiments have measured the hindered translations for CO

on Pt(111) both the on top (6 meV) and bridge sites (7

meV) as well as for CO on Ni(100) (=B meV) indicating that these modes have a significantly lower frequency than previously predicted. Our experimental data for the dispersion curves of Ni(lOO)+H(lxl) indicate, that hydrogen adsorption restores the force-constants at the surface to

their bulk values. Finally new

195 evidence for a C4v symmetry breaking in the 0 c(2x2) overlayer on Ni(100) is provided by a small 1.5 meV splitting of the Rayleigh mode at

the zone

boundary ~-point. These experiments illustrate how HAS experiments can extend the information on vibrations at higher frequencies available from EELS and IRS and observe new features at thermal energies and below.

Acknowledgements: We thank D.J. Auerbach for

comments on an early draft and H.J. Ernst for

carefully reading the manuscript.

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