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Initial sticking coefficient of O2 on Ag (001)
Journal of Electron Spectroscopy and Related Phenomena, 54155
131
(1990) 131-141
Elsevier Science Publishers B.V.,Amsterdam
INITIAL STICKING COEFFICIENT OF 02 ON Ag (001)
M.Rocca, P.Traversaro and U.Valbusa Centro di Fisica delle Superfici e delle Basse Temperature de1 CNR, Dipartimento di Fisica, via Dodecaneso 33, 16146 GENOVA ITALY
Abstract
The paper describes a procedure for measuring the initial sticking coefficient of gases on clean surfaces. The method is based on the determination of the rate of increase of coverage with total exposure to impinging particles using a combination of an Electron Energy Loss Spectrometer with a nozzle beam source. The adsorption of 0, on Ag (001) has been studied. For a crystal temperature of 300 K and for an Oz beam impinging on the crystal at normal incidence and at an energy of 88 meV, the adsorption is dissociative and _re find an initial sticking coefficient of ( 7.9 + 2.8 ) 10 .
l.INTRODUCTION The determination of the initial sticking coefficient So of a molecule impinging on a surface is of particular interest in adsorption studies since it is directly associated with the interaction of a single particle with the clean surface. So is the rate of increase of coverage (n,) with total exposure (q) to impinging particles at zero-coverage Bn So= t Bn 1.
(1) q=o
Several experimental methods are currently used for its determination. The volumetric method is often employed in systems with a high sticking probability.' So is alternatively
0368-2048/90/$03.50
0 1990-Elsevier Science Publishers B.V.
132
determined by measuring the surface coverage by several of the spectroscopic techniques of surface science, such as Auger spectroscopy (AES),2 LEED,3 work function,' thermal desorption I and He scattering .s'6 The paper describes a method for measuring So, which is very useful for systems having low So values because of
its
high sensitivity to the coverage determination. It is based on the use of an EEL spectrometer in connection with a supersonic molecular beam. Adsorption of 0, on Ag (001) has been investigated. The system has been chosen being a typical example where the initial sticking coefficient is strongly dependent upon crystal face 3 and where other spectroscopic techniques are of little help being the adsorption disordered and 0 difficoult to be detected on Ag with AES. The present data on the (001) surface offer new information on the low Miller index faces of silver and at the same time, test the capability of the method to measure extremely low initial sticking coefficient.
2. APPARATUS
The data we report in this paper have been obtained
using
a
combination of an EEL spectrometer with a nozzle beam source in order to determine the initial sticking coefficients for 02 adsorbed on Ag (001). Since the apparatus will be described in details elsewhere only the characteristic features of the system and of the method will be discussed here. For further details we refer the 7 reader to this source. The experimental apparatus, shown schematically in Fig. 1, consists of an EEL spectrometer and a supersonic nozzle source. The main chamber is pumped by an ion pump and a titanium sublimator. After bake out for 24 h at 200 OC the typical base pressure is 1 lo-'* mbar. The molecular beam source is located in chamber 1 and is pumped by a turbomolecular pump of 2000 l/set. The supersonic
133
Figure
1. Schematic view of the experimental and 4 are the vacuum chambers.
molecular
beam is formed by the continuous
high stagnation expansion of
470
is skimmed pm
differentially chamber.
pressure
the exposure chamber.
and
orifices
before
speed. A
collimated entering
shutter
pumped collimation time
Typical
by a sharp-edge
further
UHV
during
chamber
which
the
base line pressure
is
in
to
in chamber
0.55 cm. The main chamber spectrometer.
contains
The molecular
the
control
lO%bar,
at the crystal position
crystal
beam passes
main
1,2,3,4 with the
0, beam on are Pi=8.3 10-gmbar,Pz=4.6 10-6mbar,Pg=1.4 Pd= 4.9 10eembar. The beam diameter
main first
the
enters
two
pumps
the
used
beam
at The
orifice by
the
located
1,2,3
of gas
diameter.
2 and 3 are pumped by two turbomolecular
of 400 l/set pumping differentially
at 1 cm downstream
pumped
expansion
from a nozzle of 40 pm
diameter
Chamber
apparatus.
holder directly
and
is
the
EEL
through
the
center of the spectrometer. The spectrometer electron
energy
consists analyzer
of an electron of
single
pass
monochromator 127O
and
cylindrical
134 design. Typical resolution is 7 meV with a current of
5 10-'r' A. The loss signal detected by the EEL spectrometer is used to monitor the 0, coverage on the surface, as will be shown in the next chapter. The Ag single crystal, used in the present experiment, is the same utilized in our previous work. The procedure for cleaning the crystal by ion bombardment and annealing has been previously described.' The measurements have been carried out with the crystal at room temperature (293 K). The holder can be lifted above the molecular beam line in order to measure the flux of particles which are impinging on the crystal surface. As shown in figure 1, in front of the beam is located the small volume ( 49.5 cm') chamber which is collimated by an orifice ( 0.75 cm diameter) having the same size of the beam. A spinning rotor viscosity gauge is located in the chamber in order to measure the pressure. The pressure rise in this chamber, due to the beam, determinates of the particle flux i. With the O2 beam at room temperature and a stagnation pressure of 20 Atm, the measured particle flux reads @ = (1.03 T 0.03) lo+- cm+sec-'
3.METHOD According to eq. (11, a measurement of Se requires a knowledge of both no and n. n, is easily determined from the particle flux I and the Lexposure time t since n.= \ % t cos 8.. B is determined by the method described in the previois chapter, t is the opening time of the shutter of chamber 2 and ~9~is the angle between the beam and the normal to the surface. Since O2 adsorption is dissociative, the value of % for molecular oxygen must be multiplied by a factor 2 in order to obtain nL. Figure 2 shows a typical loss spectrum taken after a surface exposure of 190 sec. The spectrum reports the elastic peak and
135 shows a single loss peak at 32 meV. This loss is do to the exci\tationof the stretching mode of the adsorbed oxygen. The presence of a single dipole active vibration suggests an adsorption site with Cl" symmetry. No other loss peaks were observed ruling out the presence of other adsorption sites on Ag(001). No ordered structure was observed by LEED. An investigation of 0 adsrption on Ag(001) has been performed by C. S. Ares Fang p who finds a frequency of 30 meV at
of 0 EELS also
room temperature and of 37 meV after cooling the sample with liquid nitrogen when a weak ~(2x2) structure is formed. He assigns therefore these vibrations to 0 atoms in ~(1x1) and ~(2x2) sites respectively. The occupation of both sites in our case could explain the disagreement of 2 meV between our result and that of Fang.
AgCOOll-0 ul IH
E,=8.leV
32 I
8,=78.3' t =190sec
5
E,=88meV 8, =o"
:2000
0 ENERGY
Fig.
2.
100
50 LOSS
150
CmeV)
Typical energy loss spectrum recorded after exposure of faint an$* broad structure at high 190 sec. The frequency is a ghost peak.
136 According to Ibach and Mills I1 the surface coverage n is related to the integrated intensity of the energy loss 'peak, ILWL' divided by the elastic intensity, IeL, according to n= s
--
A
LL
2
Id
(3)
A is a constant which depends from spectrometer parameters such as incident energy Ee and angle Be of electrons, acceptance angle a of the spectrometer, loss frequency os and physical constants. ,u is the expectation value of the perpendicular component of the dynamical dipole moment of the adsorbed molecule. In the case of present experiment, given that Ee=8.1 eV, ee = 78.3x, a = 1.5x, and ws= 32 meV, one obtains A= 2.33 10-22 esu'. ILn_L/IaLcan be determined from the spectra (see Fig.21 from the heights of elastic and loss peak intensity. From eq. 3 this ratio is proportional to the coverage at least for low coverages where depolarizing effects can be neglected and ~1 is constant. Several checks were carried out in order to test the reliability of this determination of ILnrL/I.,. As the surface disorder increases due to coverage, the elastic peak broadens. It is therefore necessary to choose an acceptance angle of the spectrometer larger than the specular elastic beam in order to avoid this apparent increase in ILnol/IoL." For the present results the validity of this choice has been demonstrated by measuring angular distributions of both Irnrl see and IoL, as reported in figure 3. From this figure one can that the angular width of both ILno,andI., are equal to the angular acceptance of the spectrometer. Similar measurements have been carried out for all the data reported in this paper and all the distributions have angular widths determined by a. Figure 3 shows that, by increasing the coverage, the peak positions of both the loss and the elastic peaks shift by 5 0.5" from the zero-coverage peak position. This effect has been
137
taken into account by carrying out the measurements after changing the scattering angle of the electrons in order to obtain the maximum of the peak. Figure 4 reports the ratio of IrneL/ I., taken in two different ways. The circles give the ratio of peak heights associated with ILnrLand IeL, while the squares are the ratio between the intensities IL**\ and angular distribution. The figure coincide
within
the
I., integrated over the shows that the results
experimental
errors, indicating that effects caused by surface disorder are negligible and that Irn.L'I*, as determined from the peak heights associated with the loss and elastic peaks is an accurate measurement of n0’
0 0
t,,I.,‘,I,,,‘I...,‘.“,j
-1
0
2
3
A0 (c&g)
Fig. 3. Angular distribution of elastic peak (circles) and loss peak (squares) at an exposure time at 720 set and incidence angle ei= 41.5 .Aei indicates the difference in angle between covered and clean surface
138
f
Exposure
(set)
Fig. 4. Ratio between loss and the elastic intensity taken from ratios between the maximum of the angular a) distribution (circles) and b) ratios between the integrated intensity of the angular distribution (squares).
&RESULTS
In Fig.5 we report Irnol/Ia, for different exposure times for an incident beam of Oz of energy E= incident L 88 meV and angle et= O". As previously shown, in the present conditions the exposure time t is related to ni and IineL/I., to n, by
(4)
where ns has been obtained from eq.(3) dynamic dipole moment as reported by Ag(Oll), /~=0.027D.
by assuming the Backs et al. "
same for
139
2 1o-3
1 1o-3
0
250
500
750
Exposure
1000
1250
(set)
Fig. 5. Relative intensity of the loss spectra as function of exposure. The beam has Ei= 88 meV and Bi = ox. Crystal temperature is 293 K
The results of figure 5 have been fitted with the equation I - ,nrL = D ~-emet] Irl
+ C
(5)
D, B and C have been obtained by a best fit
based on a Minuit routine. The best fit values are D= (2.45 f 0.35) lo--',B=(2.0 5 0.4) lOma set-'and C = (2.9 7 0.3) 10-4. D*A, i.e. 7.87 lOid cm-' is the 0 saturation coverage. Since on the (001) surface one has 1.20 lOas Ag
atoms
procedure
cm-= we
obtain
a
0 coverage of 66%. This relatively high value confirms are effectively measuring the 0 interaction with the surface and not with single defects present on it. It in agreement with the result of Fang who finds that at similar coverage, as inferred from the reported EELS intensity, a weak ~(2x2) structure forms when cooling the crystal to
maximum that we bare Ag is also
140 liquid nitrogen temperature. The initial sticking coefficient can be obtained by taking the derivative of equation (5) at zero-coverage
d(+] OL
=DB
(6)
dt t=o resulting in = SO
(
7.9 + 2.8) lo-*
(70)
which constitutes an upper limit for So. This value should be compared with that reported by Engelhard and Menzel ' for Ag(Oll) of So= 3 lo-' and by Spruit 6 for Ag(ll1) of So21 10-a. The error reported for the sticking coefficient takes into account only of the errors on D and B. We should point out that the value assumed for p is the major source of uncertainty on S0. With this method, however, we are intentioned to study the variation of So with particle energy and angle of incidence more than its absolute value.
S.REFERENCES 1 2 3 4 5 6
C.T. Rettner, L.A. De Louise, D.J.Auerbach, J. Chem. Phys. 85, 1131 (1986). C. T. Cambell, Surf. Sci. 157, 43 (1985). H.A. Engelhardt, D. Menzel, Surf. Sci. 57, 591 (1976). R. B. Grant, R. M. Lambert, Surf. Sci. 146, 256 (1984). B. Poelsema, R.L. Palmer, G. Comsa, Surf. Sci.'123, 152 (1982). M.E.M. Spruit, PhD thesis , University of Amsterdam (1989)
141
7 a 9 10 11 12
A.Gussoni, G.Maloberti, L.Racca, M.Rocca and U. Valbusa to be published. M.Rocca and U.Valbusa Phys. Rev. Lett 63, 2398 (1990) C.S. Ares Fang, Surf. SCi. Lett. 235, L291 (1990) H. Froitzheim, H.Ibach, S. Lehwald, Rev. Sci. Instr. 46, 1325 (1975) H. Ibach, D.L. Mills, "Electron energy loss spectroscopy and surface vibration", Academic Press, Londra (1982) W.M.H. c. Backx, C.P.M. de Groot, P. Biloen and Sachtler, Surf. Sci. 128, 81 (1983)
131
(1990) 131-141
Elsevier Science Publishers B.V.,Amsterdam
INITIAL STICKING COEFFICIENT OF 02 ON Ag (001)
M.Rocca, P.Traversaro and U.Valbusa Centro di Fisica delle Superfici e delle Basse Temperature de1 CNR, Dipartimento di Fisica, via Dodecaneso 33, 16146 GENOVA ITALY
Abstract
The paper describes a procedure for measuring the initial sticking coefficient of gases on clean surfaces. The method is based on the determination of the rate of increase of coverage with total exposure to impinging particles using a combination of an Electron Energy Loss Spectrometer with a nozzle beam source. The adsorption of 0, on Ag (001) has been studied. For a crystal temperature of 300 K and for an Oz beam impinging on the crystal at normal incidence and at an energy of 88 meV, the adsorption is dissociative and _re find an initial sticking coefficient of ( 7.9 + 2.8 ) 10 .
l.INTRODUCTION The determination of the initial sticking coefficient So of a molecule impinging on a surface is of particular interest in adsorption studies since it is directly associated with the interaction of a single particle with the clean surface. So is the rate of increase of coverage (n,) with total exposure (q) to impinging particles at zero-coverage Bn So= t Bn 1.
(1) q=o
Several experimental methods are currently used for its determination. The volumetric method is often employed in systems with a high sticking probability.' So is alternatively
0368-2048/90/$03.50
0 1990-Elsevier Science Publishers B.V.
132
determined by measuring the surface coverage by several of the spectroscopic techniques of surface science, such as Auger spectroscopy (AES),2 LEED,3 work function,' thermal desorption I and He scattering .s'6 The paper describes a method for measuring So, which is very useful for systems having low So values because of
its
high sensitivity to the coverage determination. It is based on the use of an EEL spectrometer in connection with a supersonic molecular beam. Adsorption of 0, on Ag (001) has been investigated. The system has been chosen being a typical example where the initial sticking coefficient is strongly dependent upon crystal face 3 and where other spectroscopic techniques are of little help being the adsorption disordered and 0 difficoult to be detected on Ag with AES. The present data on the (001) surface offer new information on the low Miller index faces of silver and at the same time, test the capability of the method to measure extremely low initial sticking coefficient.
2. APPARATUS
The data we report in this paper have been obtained
using
a
combination of an EEL spectrometer with a nozzle beam source in order to determine the initial sticking coefficients for 02 adsorbed on Ag (001). Since the apparatus will be described in details elsewhere only the characteristic features of the system and of the method will be discussed here. For further details we refer the 7 reader to this source. The experimental apparatus, shown schematically in Fig. 1, consists of an EEL spectrometer and a supersonic nozzle source. The main chamber is pumped by an ion pump and a titanium sublimator. After bake out for 24 h at 200 OC the typical base pressure is 1 lo-'* mbar. The molecular beam source is located in chamber 1 and is pumped by a turbomolecular pump of 2000 l/set. The supersonic
133
Figure
1. Schematic view of the experimental and 4 are the vacuum chambers.
molecular
beam is formed by the continuous
high stagnation expansion of
470
is skimmed pm
differentially chamber.
pressure
the exposure chamber.
and
orifices
before
speed. A
collimated entering
shutter
pumped collimation time
Typical
by a sharp-edge
further
UHV
during
chamber
which
the
base line pressure
is
in
to
in chamber
0.55 cm. The main chamber spectrometer.
contains
The molecular
the
control
lO%bar,
at the crystal position
crystal
beam passes
main
1,2,3,4 with the
0, beam on are Pi=8.3 10-gmbar,Pz=4.6 10-6mbar,Pg=1.4 Pd= 4.9 10eembar. The beam diameter
main first
the
enters
two
pumps
the
used
beam
at The
orifice by
the
located
1,2,3
of gas
diameter.
2 and 3 are pumped by two turbomolecular
of 400 l/set pumping differentially
at 1 cm downstream
pumped
expansion
from a nozzle of 40 pm
diameter
Chamber
apparatus.
holder directly
and
is
the
EEL
through
the
center of the spectrometer. The spectrometer electron
energy
consists analyzer
of an electron of
single
pass
monochromator 127O
and
cylindrical
134 design. Typical resolution is 7 meV with a current of
5 10-'r' A. The loss signal detected by the EEL spectrometer is used to monitor the 0, coverage on the surface, as will be shown in the next chapter. The Ag single crystal, used in the present experiment, is the same utilized in our previous work. The procedure for cleaning the crystal by ion bombardment and annealing has been previously described.' The measurements have been carried out with the crystal at room temperature (293 K). The holder can be lifted above the molecular beam line in order to measure the flux of particles which are impinging on the crystal surface. As shown in figure 1, in front of the beam is located the small volume ( 49.5 cm') chamber which is collimated by an orifice ( 0.75 cm diameter) having the same size of the beam. A spinning rotor viscosity gauge is located in the chamber in order to measure the pressure. The pressure rise in this chamber, due to the beam, determinates of the particle flux i. With the O2 beam at room temperature and a stagnation pressure of 20 Atm, the measured particle flux reads @ = (1.03 T 0.03) lo+- cm+sec-'
3.METHOD According to eq. (11, a measurement of Se requires a knowledge of both no and n. n, is easily determined from the particle flux I and the Lexposure time t since n.= \ % t cos 8.. B is determined by the method described in the previois chapter, t is the opening time of the shutter of chamber 2 and ~9~is the angle between the beam and the normal to the surface. Since O2 adsorption is dissociative, the value of % for molecular oxygen must be multiplied by a factor 2 in order to obtain nL. Figure 2 shows a typical loss spectrum taken after a surface exposure of 190 sec. The spectrum reports the elastic peak and
135 shows a single loss peak at 32 meV. This loss is do to the exci\tationof the stretching mode of the adsorbed oxygen. The presence of a single dipole active vibration suggests an adsorption site with Cl" symmetry. No other loss peaks were observed ruling out the presence of other adsorption sites on Ag(001). No ordered structure was observed by LEED. An investigation of 0 adsrption on Ag(001) has been performed by C. S. Ares Fang p who finds a frequency of 30 meV at
of 0 EELS also
room temperature and of 37 meV after cooling the sample with liquid nitrogen when a weak ~(2x2) structure is formed. He assigns therefore these vibrations to 0 atoms in ~(1x1) and ~(2x2) sites respectively. The occupation of both sites in our case could explain the disagreement of 2 meV between our result and that of Fang.
AgCOOll-0 ul IH
E,=8.leV
32 I
8,=78.3' t =190sec
5
E,=88meV 8, =o"
:2000
0 ENERGY
Fig.
2.
100
50 LOSS
150
CmeV)
Typical energy loss spectrum recorded after exposure of faint an$* broad structure at high 190 sec. The frequency is a ghost peak.
136 According to Ibach and Mills I1 the surface coverage n is related to the integrated intensity of the energy loss 'peak, ILWL' divided by the elastic intensity, IeL, according to n= s
--
A
LL
2
Id
(3)
A is a constant which depends from spectrometer parameters such as incident energy Ee and angle Be of electrons, acceptance angle a of the spectrometer, loss frequency os and physical constants. ,u is the expectation value of the perpendicular component of the dynamical dipole moment of the adsorbed molecule. In the case of present experiment, given that Ee=8.1 eV, ee = 78.3x, a = 1.5x, and ws= 32 meV, one obtains A= 2.33 10-22 esu'. ILn_L/IaLcan be determined from the spectra (see Fig.21 from the heights of elastic and loss peak intensity. From eq. 3 this ratio is proportional to the coverage at least for low coverages where depolarizing effects can be neglected and ~1 is constant. Several checks were carried out in order to test the reliability of this determination of ILnrL/I.,. As the surface disorder increases due to coverage, the elastic peak broadens. It is therefore necessary to choose an acceptance angle of the spectrometer larger than the specular elastic beam in order to avoid this apparent increase in ILnol/IoL." For the present results the validity of this choice has been demonstrated by measuring angular distributions of both Irnrl see and IoL, as reported in figure 3. From this figure one can that the angular width of both ILno,andI., are equal to the angular acceptance of the spectrometer. Similar measurements have been carried out for all the data reported in this paper and all the distributions have angular widths determined by a. Figure 3 shows that, by increasing the coverage, the peak positions of both the loss and the elastic peaks shift by 5 0.5" from the zero-coverage peak position. This effect has been
137
taken into account by carrying out the measurements after changing the scattering angle of the electrons in order to obtain the maximum of the peak. Figure 4 reports the ratio of IrneL/ I., taken in two different ways. The circles give the ratio of peak heights associated with ILnrLand IeL, while the squares are the ratio between the intensities IL**\ and angular distribution. The figure coincide
within
the
I., integrated over the shows that the results
experimental
errors, indicating that effects caused by surface disorder are negligible and that Irn.L'I*, as determined from the peak heights associated with the loss and elastic peaks is an accurate measurement of n0’
0 0
t,,I.,‘,I,,,‘I...,‘.“,j
-1
0
2
3
A0 (c&g)
Fig. 3. Angular distribution of elastic peak (circles) and loss peak (squares) at an exposure time at 720 set and incidence angle ei= 41.5 .Aei indicates the difference in angle between covered and clean surface
138
f
Exposure
(set)
Fig. 4. Ratio between loss and the elastic intensity taken from ratios between the maximum of the angular a) distribution (circles) and b) ratios between the integrated intensity of the angular distribution (squares).
&RESULTS
In Fig.5 we report Irnol/Ia, for different exposure times for an incident beam of Oz of energy E= incident L 88 meV and angle et= O". As previously shown, in the present conditions the exposure time t is related to ni and IineL/I., to n, by
(4)
where ns has been obtained from eq.(3) dynamic dipole moment as reported by Ag(Oll), /~=0.027D.
by assuming the Backs et al. "
same for
139
2 1o-3
1 1o-3
0
250
500
750
Exposure
1000
1250
(set)
Fig. 5. Relative intensity of the loss spectra as function of exposure. The beam has Ei= 88 meV and Bi = ox. Crystal temperature is 293 K
The results of figure 5 have been fitted with the equation I - ,nrL = D ~-emet] Irl
+ C
(5)
D, B and C have been obtained by a best fit
based on a Minuit routine. The best fit values are D= (2.45 f 0.35) lo--',B=(2.0 5 0.4) lOma set-'and C = (2.9 7 0.3) 10-4. D*A, i.e. 7.87 lOid cm-' is the 0 saturation coverage. Since on the (001) surface one has 1.20 lOas Ag
atoms
procedure
cm-= we
obtain
a
0 coverage of 66%. This relatively high value confirms are effectively measuring the 0 interaction with the surface and not with single defects present on it. It in agreement with the result of Fang who finds that at similar coverage, as inferred from the reported EELS intensity, a weak ~(2x2) structure forms when cooling the crystal to
maximum that we bare Ag is also
140 liquid nitrogen temperature. The initial sticking coefficient can be obtained by taking the derivative of equation (5) at zero-coverage
d(+] OL
=DB
(6)
dt t=o resulting in = SO
(
7.9 + 2.8) lo-*
(70)
which constitutes an upper limit for So. This value should be compared with that reported by Engelhard and Menzel ' for Ag(Oll) of So= 3 lo-' and by Spruit 6 for Ag(ll1) of So21 10-a. The error reported for the sticking coefficient takes into account only of the errors on D and B. We should point out that the value assumed for p is the major source of uncertainty on S0. With this method, however, we are intentioned to study the variation of So with particle energy and angle of incidence more than its absolute value.
S.REFERENCES 1 2 3 4 5 6
C.T. Rettner, L.A. De Louise, D.J.Auerbach, J. Chem. Phys. 85, 1131 (1986). C. T. Cambell, Surf. Sci. 157, 43 (1985). H.A. Engelhardt, D. Menzel, Surf. Sci. 57, 591 (1976). R. B. Grant, R. M. Lambert, Surf. Sci. 146, 256 (1984). B. Poelsema, R.L. Palmer, G. Comsa, Surf. Sci.'123, 152 (1982). M.E.M. Spruit, PhD thesis , University of Amsterdam (1989)
141
7 a 9 10 11 12
A.Gussoni, G.Maloberti, L.Racca, M.Rocca and U. Valbusa to be published. M.Rocca and U.Valbusa Phys. Rev. Lett 63, 2398 (1990) C.S. Ares Fang, Surf. SCi. Lett. 235, L291 (1990) H. Froitzheim, H.Ibach, S. Lehwald, Rev. Sci. Instr. 46, 1325 (1975) H. Ibach, D.L. Mills, "Electron energy loss spectroscopy and surface vibration", Academic Press, Londra (1982) W.M.H. c. Backx, C.P.M. de Groot, P. Biloen and Sachtler, Surf. Sci. 128, 81 (1983)