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Determination of the gas-surface potential from hyperthermal atom scattering
Vacuum/volume 33Inumbers Printed in Great Britain
1 O-l P/pages 665 to 667/l
Determination hyperthermal A D Tenner. K T Gillen’, 407. 1098 SJ Amsterdam,
0042-207X/83$3.00+ .OO @ 1983 Pergamon Press Ltd
983
of the gas-surface atom scattering A W Kleyn and J Los, FOM-Institute The Netherlands
potential
from
for Atomic and Molecular Physics, Kruislaan
The scattering of potassium ions and atoms (It%100 eV energy) from W( 110) shows angular and energy distributions which are independent of the charge state of the incoming particles. This is due to ionization of the neutral atom at a distance of -5 A above the surface. The energy distributions of scattered ions shows a strong dependence on scattering and azimuthal angles. At low incident energies the spectra are relatively simple, whilst for higher energies the spectra are more complex with up to four peaks at different energy losses.
The understanding of processes like ionization or chemisorption of particles on metals is the aim of many investigations in surface science at the present time. Apart from studying the (static) result of these processes, many investigations have been carried out in order to obtain insight into the dynamics of these processes. This insight can be obtained from scattering experiments. Scattering experiments at thermal energies show in case of He projectiles (very weak) diffraction phenomena and in the case of heavier atoms and molecules these experiments show generally complicated trapping/desorption phenomena’. Scattering experiments with collision energies of more than roughly 200 eV, on the other hand, show scattering of the projectile with individual surface atoms’. In either case no clear information is obtained about the dynamics of the collision of an atom at the various sites of a single crystal metal surface. This information can be obtained from scattering experiments in the intermediate energy range, it large enough to prevent trapping/desorption phenomena occurring and small enough to observe collisions with the ‘entire surface’ without penetration. Experiments of this type have been carried out using alkali ions as projectiles2*‘, because these ions are easily prepared and neutralization does not occur. In addition these ions have a closed shell noble gas configuration. Nevertheless, to obtain a potential between the alkali ion and the surface from ion scattering experiments is far from simple. In the present study our aim is to obtain scattering profiles as detailed as possible in order to constrain the choice of the potential. This has been achieved in two ways: (1) The incoming beam is always normal to the surface and detection can occur along two independent polar angles 6 and 4. Thus there are no blocking effects. Because there is no incident velocity parallel to the surface plane there is also no preferential scattering direction. This facilitates the analysis because the surface is fully exposed to the beam and particles can be detected in any direction. (2) Energy analysis of the scattered ions provides additional information concerning the scattering process. However, this is
l
SRI International, 333 Ravenswood Avenue, Menlo Park, CA, USA.
only true if the mass ratio is not unfavourable. Therefore potassium ions have been chosen as projectiles. The mass ratio is 0.21 in this case and various scattering paths are not only resolved in angle (8, 4) but also in energy (E’). From the triple differential cross section presented in this contribution very detailed information about the collision dynamics and the interaction potential can be extracted. The data and analysis will be described in more detail elsewhere*. The experimental apparatus consists of a UHV chamber with a base pressure of about lo-* Pa The target is a W(110) surface which is cleaned in the usual way (annealing in oxygen and flashing to 2500 K). The crystal is mounted on a two-axis goniometer which allows all possible incident angles and azimuthal angles. In the UHV chamber a potassium ion source with a lens system is mounted. A differentially pumped beamline with a charge exchange source provides a neutral potassium atom beam. The detector, a 90” electrostatic analyzer, is rotatable around two axes, in and out of the scattering plane. Figures 1 and 2 give energy distributions for scattered potassium ions as a function of the scattering angle for two diNerent azimuthal angles (4 = 0”, 90”) for an incident ion beam energy of 35 eV. The unit cell of a W(110) surface has the shape of a diamond with a short and a long axis. In Figure 1 the scattering plane lies along the short axis [OOl] and in Figure 2 along the long axis [ liO] of the diamond. The difference between Figures 1 and 2 shows that there is a large dependence on the azimuth. For lower incident energies the spectra become more simple with a single peak, while for higher incident energies the spectra become even more complex. The same experiments have been performed with a neutral potassium beam of 35 eV. Both angular and energy distributions obtained very closely resemble the spectra obtained with the ion beam. Consequently the scattering phenomena are independent of the initial charge state of the projectiles. This is to be expected because the potassium atoms ionize at a distance of about 5 A in front of the surface5. To explain the origin of the different peaks in Figures 1 and 2 it is necessary to take the potential of all surface atoms in a unit cell together to give one surface potential. The projectile can be 665
A D Tenner, K T Gillen. A W Kleyn and J Los:
I
I 7.0
, 14.0 ENERGY
Gas-surface potential from hyperthermal
0-
57.4’
0=
52.4’
0-
42.4’
e-
37.4’
0=
22.4’
0-
17.4’
I
I
21 .o (EV)
28.0
0= 22.4
0s 1
35.0
Figure 1. Energy spectra measured for K+ scattered from W(110) with normal incidence and an incident energy of 35 eV. 8 is the outgoing angle with respect to the surface normal. The scattering plane lies along the short axis, parallel to the [OOl] direction.
considered to interact with all target atoms in the unit cell simultaneously. The peaks in the figures with energies higher than_ 5 eV can be explained with scattering from a single, additive K+-W potential. This has been checked by performing classical trajectory calculations. In the data two types of surface rainbows can be observed3. The data for the long axis show a rainbow for the large E’ peak around 8 = 52”. The data for the short axis show the same rainbow for the large E’ peak around 0 = 47”. In addition a second rainbow is observed at 6 = 32” and E’= 9 eV. For 0 c 25” the rainbow feature is split into two peaks in E’. The first rainbow is due to collisions in which the potassium ion undergoes two relatively grazing collisions with the surface whereas the second rainbow is due to more violent encounters. These violent encounters are only possible along rather close packed crystal directions. The structures with E’> 5 eV can be reproduced very well using classical trajectory calculations. The calculations reveal that only a few specific sites in the unit cell give rise to scattering into the specific detector positions as given in Figures 1 and 2 but these sites are not only situated along major axes of the unit cell. 666
atom scattering
3
7.0
I
14.0 ENERGY
I
21 .o
17.4
I
28.0
i.0
(EL’)
Figure 2. Energy spectra as in Figure 1. The scattering plane lies along the long axis, parallel to the [ liO] direction.
The peak with E’< 5 eV cannot be explained with the collision dynamics described, in which case the trajectories move along a single multidimensional potential surface. It might be due to electronic excitation, most likely of the potassium ion (level at 20.6 eV), or complex trajectories nearly trapped. Summarizing, we conclude that normal incidence low energy alkali ion scattering with a not too small mass ratio for projectile and target atom is a direct way to probe the interaction of a closed shell particle and a metal surface via the triple differential cross section: d3a/d8 d4 dE. From this cross section an expression for the interaction potential over the whole unit cell can be deduced. In addition inelastic excitation processes may be observable. Acknowledgement The authors thank R Hoep and C H van Oven for their technical assistance during the installation and testing of the experimental apparatus. This work is part of the research program of the Stichting voor Fundamental Onderzoek der Materie (FOM) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (ZWO).
A 0 Tenner, K T Gillen. A W Kleyn and J Los: Gas-surface
potential from hyperthermal atom scattering
Refereneea ’ M J Cardills Ann Reu Phys Chem, 32,331 (1981). ’ T von dem Hagcn and E Bauer, Phys Rev Lett, 47,579 (1981); T von dem Hagen, M Hou and E Bauer, Surface Sci, 117, 134 (1982). ’ U Gerlach-Meyer and E Hulpke, Topics in Surface Chemistry. (Edited by
E Kay and P S Bagus), p 195. Plenum Press, New York (1978); U GerlachMeyer, E Hulpkc and H-D Meyer, Chem Phys, 36,327 (1979). ’ A D Tenncr. K T Gillcn, A W Klcyn and J Los; Surface Sci (to be published). s E G Overbosch, B Rasser, A D Tenner and J Los, Surface Sci. 92,310 (1980).
667
1 O-l P/pages 665 to 667/l
Determination hyperthermal A D Tenner. K T Gillen’, 407. 1098 SJ Amsterdam,
0042-207X/83$3.00+ .OO @ 1983 Pergamon Press Ltd
983
of the gas-surface atom scattering A W Kleyn and J Los, FOM-Institute The Netherlands
potential
from
for Atomic and Molecular Physics, Kruislaan
The scattering of potassium ions and atoms (It%100 eV energy) from W( 110) shows angular and energy distributions which are independent of the charge state of the incoming particles. This is due to ionization of the neutral atom at a distance of -5 A above the surface. The energy distributions of scattered ions shows a strong dependence on scattering and azimuthal angles. At low incident energies the spectra are relatively simple, whilst for higher energies the spectra are more complex with up to four peaks at different energy losses.
The understanding of processes like ionization or chemisorption of particles on metals is the aim of many investigations in surface science at the present time. Apart from studying the (static) result of these processes, many investigations have been carried out in order to obtain insight into the dynamics of these processes. This insight can be obtained from scattering experiments. Scattering experiments at thermal energies show in case of He projectiles (very weak) diffraction phenomena and in the case of heavier atoms and molecules these experiments show generally complicated trapping/desorption phenomena’. Scattering experiments with collision energies of more than roughly 200 eV, on the other hand, show scattering of the projectile with individual surface atoms’. In either case no clear information is obtained about the dynamics of the collision of an atom at the various sites of a single crystal metal surface. This information can be obtained from scattering experiments in the intermediate energy range, it large enough to prevent trapping/desorption phenomena occurring and small enough to observe collisions with the ‘entire surface’ without penetration. Experiments of this type have been carried out using alkali ions as projectiles2*‘, because these ions are easily prepared and neutralization does not occur. In addition these ions have a closed shell noble gas configuration. Nevertheless, to obtain a potential between the alkali ion and the surface from ion scattering experiments is far from simple. In the present study our aim is to obtain scattering profiles as detailed as possible in order to constrain the choice of the potential. This has been achieved in two ways: (1) The incoming beam is always normal to the surface and detection can occur along two independent polar angles 6 and 4. Thus there are no blocking effects. Because there is no incident velocity parallel to the surface plane there is also no preferential scattering direction. This facilitates the analysis because the surface is fully exposed to the beam and particles can be detected in any direction. (2) Energy analysis of the scattered ions provides additional information concerning the scattering process. However, this is
l
SRI International, 333 Ravenswood Avenue, Menlo Park, CA, USA.
only true if the mass ratio is not unfavourable. Therefore potassium ions have been chosen as projectiles. The mass ratio is 0.21 in this case and various scattering paths are not only resolved in angle (8, 4) but also in energy (E’). From the triple differential cross section presented in this contribution very detailed information about the collision dynamics and the interaction potential can be extracted. The data and analysis will be described in more detail elsewhere*. The experimental apparatus consists of a UHV chamber with a base pressure of about lo-* Pa The target is a W(110) surface which is cleaned in the usual way (annealing in oxygen and flashing to 2500 K). The crystal is mounted on a two-axis goniometer which allows all possible incident angles and azimuthal angles. In the UHV chamber a potassium ion source with a lens system is mounted. A differentially pumped beamline with a charge exchange source provides a neutral potassium atom beam. The detector, a 90” electrostatic analyzer, is rotatable around two axes, in and out of the scattering plane. Figures 1 and 2 give energy distributions for scattered potassium ions as a function of the scattering angle for two diNerent azimuthal angles (4 = 0”, 90”) for an incident ion beam energy of 35 eV. The unit cell of a W(110) surface has the shape of a diamond with a short and a long axis. In Figure 1 the scattering plane lies along the short axis [OOl] and in Figure 2 along the long axis [ liO] of the diamond. The difference between Figures 1 and 2 shows that there is a large dependence on the azimuth. For lower incident energies the spectra become more simple with a single peak, while for higher incident energies the spectra become even more complex. The same experiments have been performed with a neutral potassium beam of 35 eV. Both angular and energy distributions obtained very closely resemble the spectra obtained with the ion beam. Consequently the scattering phenomena are independent of the initial charge state of the projectiles. This is to be expected because the potassium atoms ionize at a distance of about 5 A in front of the surface5. To explain the origin of the different peaks in Figures 1 and 2 it is necessary to take the potential of all surface atoms in a unit cell together to give one surface potential. The projectile can be 665
A D Tenner, K T Gillen. A W Kleyn and J Los:
I
I 7.0
, 14.0 ENERGY
Gas-surface potential from hyperthermal
0-
57.4’
0=
52.4’
0-
42.4’
e-
37.4’
0=
22.4’
0-
17.4’
I
I
21 .o (EV)
28.0
0= 22.4
0s 1
35.0
Figure 1. Energy spectra measured for K+ scattered from W(110) with normal incidence and an incident energy of 35 eV. 8 is the outgoing angle with respect to the surface normal. The scattering plane lies along the short axis, parallel to the [OOl] direction.
considered to interact with all target atoms in the unit cell simultaneously. The peaks in the figures with energies higher than_ 5 eV can be explained with scattering from a single, additive K+-W potential. This has been checked by performing classical trajectory calculations. In the data two types of surface rainbows can be observed3. The data for the long axis show a rainbow for the large E’ peak around 8 = 52”. The data for the short axis show the same rainbow for the large E’ peak around 0 = 47”. In addition a second rainbow is observed at 6 = 32” and E’= 9 eV. For 0 c 25” the rainbow feature is split into two peaks in E’. The first rainbow is due to collisions in which the potassium ion undergoes two relatively grazing collisions with the surface whereas the second rainbow is due to more violent encounters. These violent encounters are only possible along rather close packed crystal directions. The structures with E’> 5 eV can be reproduced very well using classical trajectory calculations. The calculations reveal that only a few specific sites in the unit cell give rise to scattering into the specific detector positions as given in Figures 1 and 2 but these sites are not only situated along major axes of the unit cell. 666
atom scattering
3
7.0
I
14.0 ENERGY
I
21 .o
17.4
I
28.0
i.0
(EL’)
Figure 2. Energy spectra as in Figure 1. The scattering plane lies along the long axis, parallel to the [ liO] direction.
The peak with E’< 5 eV cannot be explained with the collision dynamics described, in which case the trajectories move along a single multidimensional potential surface. It might be due to electronic excitation, most likely of the potassium ion (level at 20.6 eV), or complex trajectories nearly trapped. Summarizing, we conclude that normal incidence low energy alkali ion scattering with a not too small mass ratio for projectile and target atom is a direct way to probe the interaction of a closed shell particle and a metal surface via the triple differential cross section: d3a/d8 d4 dE. From this cross section an expression for the interaction potential over the whole unit cell can be deduced. In addition inelastic excitation processes may be observable. Acknowledgement The authors thank R Hoep and C H van Oven for their technical assistance during the installation and testing of the experimental apparatus. This work is part of the research program of the Stichting voor Fundamental Onderzoek der Materie (FOM) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (ZWO).
A 0 Tenner, K T Gillen. A W Kleyn and J Los: Gas-surface
potential from hyperthermal atom scattering
Refereneea ’ M J Cardills Ann Reu Phys Chem, 32,331 (1981). ’ T von dem Hagcn and E Bauer, Phys Rev Lett, 47,579 (1981); T von dem Hagen, M Hou and E Bauer, Surface Sci, 117, 134 (1982). ’ U Gerlach-Meyer and E Hulpke, Topics in Surface Chemistry. (Edited by
E Kay and P S Bagus), p 195. Plenum Press, New York (1978); U GerlachMeyer, E Hulpkc and H-D Meyer, Chem Phys, 36,327 (1979). ’ A D Tenncr. K T Gillcn, A W Klcyn and J Los; Surface Sci (to be published). s E G Overbosch, B Rasser, A D Tenner and J Los, Surface Sci. 92,310 (1980).
667