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Design, construction and use of a fast atom scattering spectrometer
Surface and Coatings Technology, 35 (1988) 125
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DESIGN, CONSTRUCTION AND USE OF A FAST ATOM SCATTERING SPECTROMETER N. S. XU’~’,S. 0. SAIED and J. L. SULLIVAN Applied Physics, Aston University, Birmingham B4 7ET (U.K.) (Received December 16, 1987)
Summary Atom-surface scattering can provide information on surface composition, surface structure and surface dynamics without many of the problems associated with charged particle beams. The technique of ion-scattering spectrometry (ISS) is now well established and this paper describes a development of this technique involving the use of an uncharged fast atom beam (FAB) source in place of an ion source. The particular problem associated with the use of neutral particles in this application is in the accurate measurement of particle energy where conventional electrostatic methods cannot be employed. This was overcome by developing a time-of-flight (TOF) system consisting of a nanosecond pulsing system for the source together with detection and data acquisition and processing systems. The design, development and construction of the system is described in detail. The use of the TOF system to characterize the source fully is described, and the results of some initial atom-surface scattering experiments are given. 1. Introduction Low-energy ion-scattering spectrometry (ISS) is now a well established technique for surface studies and surface analysis [1 4]. The technique is unique for its extreme surface sensitivity [3, 4] with the majority of ions being scattered from the first one or two monolayers of a solid surface. Within this range of the first or second monolayer, the technique provides information on chemical composition, surface crystal structure and atomic arrangement [3, 4] and surface electronic structure and bonding [51. The technique is also of major importance in more fundamental studies of low energy ion—surface interactions: for example, neutralization mechanisms, shadowing effects and scattered ion and neutral yield [6]. Standard commercial ISS systems employing ions as the primary bombarding particle and in general an electrostatic analyser system for the -
*On leave from Physics Department, Zhongshan University, China. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
126
measurement of scattered ion energy have some disadvantages. The use of ions is known to produce problems when examining insulating samples because of sample charge build-up [7]. The surface charging effect is due to charge exchange with the incident ions and to secondary electron emission, the former of these two processes being the most important. The surface charge causes changes in surface potential which lead to shifts in the spectra detected by electrostatic analysers. Since this potential cannot easily be determined and changes during measurement, it can confuse results, particularly when one is attempting to measure small energy shifts. In addition to this, primary ion beams are known to result in significant damage to surfaces causing both physical and chemical change [8, 9]. Positively charged surfaces can be neutralized by the use of a lowenergy electron flood gun, but this could give rise to surface changes such as beam decomposition and desorption. Successful neutralization also relies on precise feedback and control and thus introduces further technological difficulties. A second disadvantage of using the standard commercial system is the use of the electrostatic analyser itself. This type of analyser, of course, detects only the charged portion of the scattered beam. Thus a great deal of the available information is lost as scattered ion represents less than 10% of the total particle content in the beam, the remainder being neutrals. We have recently designed and constructed a fast atom scattering spectrometer (FASS) which overcomes the majority of the problems mentioned above. By using fast atoms (in the energy range 100 eV 5 keV) as the primary projectile instead of ions, charging of non-conducting material is significantly reduced. It has also been reported that both physical and chemical surface damage to non-conducting samples caused by particle bombardment is significantly reduced if neutrals rather than ions are used in the primary beam [10, 11]. The measurement of the energy of the scattered neutrals requires a time-of-flight (TOF) spectrometer and this enables the collection of the total scattered yield, thus gaining all available information. The system may be operated in either ion or fast atom mode and will be an important tool for studying the fundamental interaction between fast atoms and surfaces and for comparisons between similar ion—surface interaction. It is believed that no systematic fast atom—surface interaction study has yet been attempted. This paper briefly describes the FASS system and reports on the initial results of both fast atom and ion scattering measurements. -
2. Experimental background The fast atom scattering spectrometry is based on the same principle as that of ISS, according to which a monoenergetic and well-collimated fast atom beam bombards a surface and the energy distribution of atoms (and
127 STOP DETECTOR ROTATABLE SAMPLE
SOURCE DETECTED SIGNAL PROCESSING AND TIMING PICK-OFF
PULSE GENERATOR TAC DELAY AMPLIFIER MU LT ICHA N NE L ANALYSER
MICROCOMPUTER
PLOTTER OR PRINTER
Fig. 1. Schematic diagram illustrating the time-of-flight system.
possibly ions) scattered at a definite angle is measured. Through analysis of the spectrum recorded, information about atomic mass and surface structure may be obtained. As shown in Fig. 1, to measure the energy distribution of the neutral particles, a time-of-flight system is employed; this consists essentially of a flight tube 1.3 m long, a specimen chamber and an electronic system. An electron impact fast atom source is mounted on a port at 90° from the axis of the flight tube and a detector is placed at the end o:f the tube. The scattering angle is thus fixed in this system at 90°. The flight tube and the specimen chamber are evacuated by a system based on a diffusion pump (not shown here). In addition, the source is differentially pumped to maintain chamber pressure. The whole vacuum system is unfortunately unbakable at present. However, with this arrangement, the specimen chamber pressure can reach about 8 X iO~ mbar with a cold trap fully filled with liquid nitrogen. When the source is operating, noble gas pressures in the specimen chamber increase. Nevertheless, the differential pump evacuates the main flow of the noble gas and thus can keep the pressure in the chamber about two orders of magnitude lower than that of the source. It is also found that in the pressure range of this experiment, the pressure at the end of the flight tube is better than 2 X I 06 mbar. The electronic system also shown in Fig. 1 consists of three parts: the pulsing, the timing pick-off arid the storage and output. The pulsing system is designed to pulse the source so that a pulse of particles (atom or ion and atom) can be obtained (this pulsing system is subject to a patent application). The pulses used in the experiment have widths from a few nanoseconds to a few tenths of nanoseconds and frequencies in the range of 10 kHz 1 MHz. The pulse of particles generated is focused to strike the surface of a target in the specimen chamber. Particles scattered from the surface at a known laboratory angle travel freely along the flight tube and are finally detected by the detector. The electron current signal of the detector generated by the bombardment -
128
of the particle is sent to the timing pick-off circuit. The data are stored in a multichannel analyser and can either be directly printed out through a printer or a graphic plotter or be sent to a computer for further data processing. The system is designed so that on average only one scattered particle strikes the detector for each pulse. The source which is manufactured by the Kratos company and is commercially available is a very important part of the TOF system and thus experiments have been carried out to characterize the source before any scattering measurements were made. Originally, the source could be operated only under d.c. conditions and thus only characterization in terms of ion or atom current, neutral production efficiency and beam divergence etc. can be carried out. However, the system mentioned above, which has been developed in this experimental programme for pulsing the source, provides us with the means of measuring the energy distribution of ions and/or atoms with the existing TOF system. Details of the characterization will be published elsewhere [12] and only a summary of the results is given here. (a) The neutral production efficiency of the source is rather low, e.g. less than 10% even with a source pressure of iO~ mbar, and the neutral current is small, e.g. about 10 nA at a chamber pressure of about 106 mbar for atoms of energy 3 keV. However, with our TOF system, it is possible to operate with the source working in the very low current mode because of the high transmission coefficient of such a system. (b) The source is capable of giving a pure neutral beam because of the deflection plates within the source. This is important for this experiment since it eliminates the possibility of confusing atom with ion scattering. (c) Both ion and atom beams produced by the source have small beam diameters, i.e. about 350 /.tm, and small divergence, i.e. about 1°.Both features ensure very good resolution for the FASS. (d) The energy spread of both ions and atoms are measured to be about 1% for ions and about 5% for atoms. These are illustrated in Figs. 2 and 3: Fig. 2 is a typical TOF spectrum of the total beam, and Fig. 3 is a typical TOF spectrum of the neutral beam, both measured with the source directed into the flight tube. The main peaks correspond to Ark, and the small peaks to Ar2~.It should be mentioned that peaks of the residual gas have also been found in the total beam spectra and, under some working conditions, hydrogen peaks may appear in the neutral beam spectrum, although they are not shown in these two examples.
3. Experimental procedure and results Initial measurements of energy distribution of scattered particles have been made with gold targets bombarded by helium. The target consisted of a gold film a few thousands of ângstroms thick, coated onto a cleaned glass substrate. Before inserting the specimen into the chamber, all the
129 1000 900 800 700
+
600 50C~
300
4
200
~
~c0~
0
100
200
300
400
500
600
‘OO
SOC
900
1000
800
1000
Fig. 2. Time-of-flight spectrum of 1 keV argon total beam. 500
400
r-
-.
—----- ----
.~--—-~—---
-
-f
300 (0
200 4
0
100
200
300
400
500
500
700
800
Fig. 3. Time-of-flight spectrum of 1 keV argon neutral beam.
samples were cleaned with distilled water and then ultrasonically cleaned in methanol for about 10 mm. The samples were mounted onto a specimen holder which could be rotated from outside the vacuum system, to change the impact angle. The angle was read from a meter on the manipulator. In this experiment, scattering spectra were recorded for impact angles of 45°, 35°and 20°with incident beams consisting of both total beam (mainly ions) and neutral beam. As mentioned previously, the scattering angle was always 90°.
130 - . —~
/
~I~L
—
-11~
..-~
~
\~
~
/\ .~i:fl_ -
_.~~____(
C--45
B-35°
~\ A--20
-)
Fig. 4. Comparison of time-of-flight spectra for helium total beam scattered from a gold surface for impact angles 20°, 350 and 45°
Figure 4 is a sequence of typical TOF spectra of helium scattered from the gold sample with an incident beam of ions and atoms (source characterization shows that ion content is greater then 95% of the total). It is found that the major peak in the spectrum corresponds to that of helium scattered from carbon owing to surface contamination. The second largest distinct peak, shown in spectra 4B and 4C but not in 4A, is due to helium scattered from gold. As can be seen from the spectra, the gold signal disappears as the impact angle decreases to 20°.This indicates that at a smaller impact angle only particles scattered from the outermost few monolayers are detected, but as the impact angle increases, the beam penetrates deeper so that gold substrate is revealed. Figure 5 is a similar sequence of typical TOF spectra of helium scattered from the gold sample, but where the incident beam is wholly neutral. Only one peak is apparent and that corresponds to helium scattered from carbon. In these spectra there is no trace of gold for all impact angles measured. This suggests that the neutral species does not penetrate the samples to the same extent as ions of the same energy, which seems to be contrary to what one might expect. A further difference in the behaviour of scattered ions and fast atoms is that shown in Fig. 4 where the peak positions, and hence the energies of the scattered particles, shift with impact angle. No such shift is observed when atoms are used as the incident projectile, as can be seen from Fig. 5. Figures 4 and 5 also show that for both ions and atoms the scattering yield decreases with decreasing impact angle. All spectra are normalized to a counting time of 3600 s.
131 5000
1 000
0
______________________________________________
Fig. 5. Comparison of time-of-flight spectra for helium neutral beam scattered from a gold surface for impact angles of 20°, 35°and 45°.
4. Conclusions We have designed and constructed a time-of-flight scattering spectrometer which can operate with either ions or fast atoms as the incident projectile. The initial results show that good spectra are produced by the system for both types of scattered particle. These can be explained, in general, in terms of elastic collision between the incident beam and the first few monolayers of the surface. The results also show, however, that there are distinct differences between spectra obtained from scattered ions and those obtained from scattered fast atoms. Preliminary spectra suggest that ions penetrate the surface atoms and that ion interactions are not totally elastic, since scattered ion peak energy depends on the angle of impact and hence possibly on penetration depth. Thus, as in a number of other situations, the charge on the incident particle appears to have a significant effect on the particle—surface interaction mechanisms. This spectrometer provides a unique method of studying these mechanisms.
Acknowledgments We wish to thank the British Technology Group for supporting this work and C. G. Pearce for invaluable assistance in processing the data.
132
References 1 D.P.Smith,Surf.Sci., 25(1979)171. 2 D. G. Armour and G. Carter, J. Phys. E, 5 (1972) 2 - 8. 3 T. M. Buck, Methods and phenomena, in S. P. Wolsley and A. W. Czanderna (eds.), Methods of Surface Analysis, McGraw-Hill, New York, 1975. 4 W. L. Baun,SurfacelnterfaceAnal., 3(6) (1981) 243- 250. 5 R. L. Erickson and D. P. Smith, Phys. Rev. Lett., 34 (6) (1975) 297 - 299. 6 W. Heiland and E. Taglauer, Nuci. Instrum. Methods, 132 (1976) 535 - 545. 7 R. F. Goff,J. Vac. Sci. Technol., 10(2) (1973)355-358. 8 R. Kelly, Nucl. Instrum. Methods, 149 (1978) 553 - 558. 9 J. Franks, Vacuum, 34 (1 - 2) (1984) 259 - 261. 10 A. Brown, J. A. Van den Berg and J. C. Vickerman, J. Chem. Soc., Chem. Commun., (1984) 1684 - 1686. 11 AppI. Phys. Lett., 49 (4) (1986) 188 - 190. 12 N. S. Xu, S. 0. Saied and J. L. Sullivan, unpublished work.
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DESIGN, CONSTRUCTION AND USE OF A FAST ATOM SCATTERING SPECTROMETER N. S. XU’~’,S. 0. SAIED and J. L. SULLIVAN Applied Physics, Aston University, Birmingham B4 7ET (U.K.) (Received December 16, 1987)
Summary Atom-surface scattering can provide information on surface composition, surface structure and surface dynamics without many of the problems associated with charged particle beams. The technique of ion-scattering spectrometry (ISS) is now well established and this paper describes a development of this technique involving the use of an uncharged fast atom beam (FAB) source in place of an ion source. The particular problem associated with the use of neutral particles in this application is in the accurate measurement of particle energy where conventional electrostatic methods cannot be employed. This was overcome by developing a time-of-flight (TOF) system consisting of a nanosecond pulsing system for the source together with detection and data acquisition and processing systems. The design, development and construction of the system is described in detail. The use of the TOF system to characterize the source fully is described, and the results of some initial atom-surface scattering experiments are given. 1. Introduction Low-energy ion-scattering spectrometry (ISS) is now a well established technique for surface studies and surface analysis [1 4]. The technique is unique for its extreme surface sensitivity [3, 4] with the majority of ions being scattered from the first one or two monolayers of a solid surface. Within this range of the first or second monolayer, the technique provides information on chemical composition, surface crystal structure and atomic arrangement [3, 4] and surface electronic structure and bonding [51. The technique is also of major importance in more fundamental studies of low energy ion—surface interactions: for example, neutralization mechanisms, shadowing effects and scattered ion and neutral yield [6]. Standard commercial ISS systems employing ions as the primary bombarding particle and in general an electrostatic analyser system for the -
*On leave from Physics Department, Zhongshan University, China. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
126
measurement of scattered ion energy have some disadvantages. The use of ions is known to produce problems when examining insulating samples because of sample charge build-up [7]. The surface charging effect is due to charge exchange with the incident ions and to secondary electron emission, the former of these two processes being the most important. The surface charge causes changes in surface potential which lead to shifts in the spectra detected by electrostatic analysers. Since this potential cannot easily be determined and changes during measurement, it can confuse results, particularly when one is attempting to measure small energy shifts. In addition to this, primary ion beams are known to result in significant damage to surfaces causing both physical and chemical change [8, 9]. Positively charged surfaces can be neutralized by the use of a lowenergy electron flood gun, but this could give rise to surface changes such as beam decomposition and desorption. Successful neutralization also relies on precise feedback and control and thus introduces further technological difficulties. A second disadvantage of using the standard commercial system is the use of the electrostatic analyser itself. This type of analyser, of course, detects only the charged portion of the scattered beam. Thus a great deal of the available information is lost as scattered ion represents less than 10% of the total particle content in the beam, the remainder being neutrals. We have recently designed and constructed a fast atom scattering spectrometer (FASS) which overcomes the majority of the problems mentioned above. By using fast atoms (in the energy range 100 eV 5 keV) as the primary projectile instead of ions, charging of non-conducting material is significantly reduced. It has also been reported that both physical and chemical surface damage to non-conducting samples caused by particle bombardment is significantly reduced if neutrals rather than ions are used in the primary beam [10, 11]. The measurement of the energy of the scattered neutrals requires a time-of-flight (TOF) spectrometer and this enables the collection of the total scattered yield, thus gaining all available information. The system may be operated in either ion or fast atom mode and will be an important tool for studying the fundamental interaction between fast atoms and surfaces and for comparisons between similar ion—surface interaction. It is believed that no systematic fast atom—surface interaction study has yet been attempted. This paper briefly describes the FASS system and reports on the initial results of both fast atom and ion scattering measurements. -
2. Experimental background The fast atom scattering spectrometry is based on the same principle as that of ISS, according to which a monoenergetic and well-collimated fast atom beam bombards a surface and the energy distribution of atoms (and
127 STOP DETECTOR ROTATABLE SAMPLE
SOURCE DETECTED SIGNAL PROCESSING AND TIMING PICK-OFF
PULSE GENERATOR TAC DELAY AMPLIFIER MU LT ICHA N NE L ANALYSER
MICROCOMPUTER
PLOTTER OR PRINTER
Fig. 1. Schematic diagram illustrating the time-of-flight system.
possibly ions) scattered at a definite angle is measured. Through analysis of the spectrum recorded, information about atomic mass and surface structure may be obtained. As shown in Fig. 1, to measure the energy distribution of the neutral particles, a time-of-flight system is employed; this consists essentially of a flight tube 1.3 m long, a specimen chamber and an electronic system. An electron impact fast atom source is mounted on a port at 90° from the axis of the flight tube and a detector is placed at the end o:f the tube. The scattering angle is thus fixed in this system at 90°. The flight tube and the specimen chamber are evacuated by a system based on a diffusion pump (not shown here). In addition, the source is differentially pumped to maintain chamber pressure. The whole vacuum system is unfortunately unbakable at present. However, with this arrangement, the specimen chamber pressure can reach about 8 X iO~ mbar with a cold trap fully filled with liquid nitrogen. When the source is operating, noble gas pressures in the specimen chamber increase. Nevertheless, the differential pump evacuates the main flow of the noble gas and thus can keep the pressure in the chamber about two orders of magnitude lower than that of the source. It is also found that in the pressure range of this experiment, the pressure at the end of the flight tube is better than 2 X I 06 mbar. The electronic system also shown in Fig. 1 consists of three parts: the pulsing, the timing pick-off arid the storage and output. The pulsing system is designed to pulse the source so that a pulse of particles (atom or ion and atom) can be obtained (this pulsing system is subject to a patent application). The pulses used in the experiment have widths from a few nanoseconds to a few tenths of nanoseconds and frequencies in the range of 10 kHz 1 MHz. The pulse of particles generated is focused to strike the surface of a target in the specimen chamber. Particles scattered from the surface at a known laboratory angle travel freely along the flight tube and are finally detected by the detector. The electron current signal of the detector generated by the bombardment -
128
of the particle is sent to the timing pick-off circuit. The data are stored in a multichannel analyser and can either be directly printed out through a printer or a graphic plotter or be sent to a computer for further data processing. The system is designed so that on average only one scattered particle strikes the detector for each pulse. The source which is manufactured by the Kratos company and is commercially available is a very important part of the TOF system and thus experiments have been carried out to characterize the source before any scattering measurements were made. Originally, the source could be operated only under d.c. conditions and thus only characterization in terms of ion or atom current, neutral production efficiency and beam divergence etc. can be carried out. However, the system mentioned above, which has been developed in this experimental programme for pulsing the source, provides us with the means of measuring the energy distribution of ions and/or atoms with the existing TOF system. Details of the characterization will be published elsewhere [12] and only a summary of the results is given here. (a) The neutral production efficiency of the source is rather low, e.g. less than 10% even with a source pressure of iO~ mbar, and the neutral current is small, e.g. about 10 nA at a chamber pressure of about 106 mbar for atoms of energy 3 keV. However, with our TOF system, it is possible to operate with the source working in the very low current mode because of the high transmission coefficient of such a system. (b) The source is capable of giving a pure neutral beam because of the deflection plates within the source. This is important for this experiment since it eliminates the possibility of confusing atom with ion scattering. (c) Both ion and atom beams produced by the source have small beam diameters, i.e. about 350 /.tm, and small divergence, i.e. about 1°.Both features ensure very good resolution for the FASS. (d) The energy spread of both ions and atoms are measured to be about 1% for ions and about 5% for atoms. These are illustrated in Figs. 2 and 3: Fig. 2 is a typical TOF spectrum of the total beam, and Fig. 3 is a typical TOF spectrum of the neutral beam, both measured with the source directed into the flight tube. The main peaks correspond to Ark, and the small peaks to Ar2~.It should be mentioned that peaks of the residual gas have also been found in the total beam spectra and, under some working conditions, hydrogen peaks may appear in the neutral beam spectrum, although they are not shown in these two examples.
3. Experimental procedure and results Initial measurements of energy distribution of scattered particles have been made with gold targets bombarded by helium. The target consisted of a gold film a few thousands of ângstroms thick, coated onto a cleaned glass substrate. Before inserting the specimen into the chamber, all the
129 1000 900 800 700
+
600 50C~
300
4
200
~
~c0~
0
100
200
300
400
500
600
‘OO
SOC
900
1000
800
1000
Fig. 2. Time-of-flight spectrum of 1 keV argon total beam. 500
400
r-
-.
—----- ----
.~--—-~—---
-
-f
300 (0
200 4
0
100
200
300
400
500
500
700
800
Fig. 3. Time-of-flight spectrum of 1 keV argon neutral beam.
samples were cleaned with distilled water and then ultrasonically cleaned in methanol for about 10 mm. The samples were mounted onto a specimen holder which could be rotated from outside the vacuum system, to change the impact angle. The angle was read from a meter on the manipulator. In this experiment, scattering spectra were recorded for impact angles of 45°, 35°and 20°with incident beams consisting of both total beam (mainly ions) and neutral beam. As mentioned previously, the scattering angle was always 90°.
130 - . —~
/
~I~L
—
-11~
..-~
~
\~
~
/\ .~i:fl_ -
_.~~____(
C--45
B-35°
~\ A--20
-)
Fig. 4. Comparison of time-of-flight spectra for helium total beam scattered from a gold surface for impact angles 20°, 350 and 45°
Figure 4 is a sequence of typical TOF spectra of helium scattered from the gold sample with an incident beam of ions and atoms (source characterization shows that ion content is greater then 95% of the total). It is found that the major peak in the spectrum corresponds to that of helium scattered from carbon owing to surface contamination. The second largest distinct peak, shown in spectra 4B and 4C but not in 4A, is due to helium scattered from gold. As can be seen from the spectra, the gold signal disappears as the impact angle decreases to 20°.This indicates that at a smaller impact angle only particles scattered from the outermost few monolayers are detected, but as the impact angle increases, the beam penetrates deeper so that gold substrate is revealed. Figure 5 is a similar sequence of typical TOF spectra of helium scattered from the gold sample, but where the incident beam is wholly neutral. Only one peak is apparent and that corresponds to helium scattered from carbon. In these spectra there is no trace of gold for all impact angles measured. This suggests that the neutral species does not penetrate the samples to the same extent as ions of the same energy, which seems to be contrary to what one might expect. A further difference in the behaviour of scattered ions and fast atoms is that shown in Fig. 4 where the peak positions, and hence the energies of the scattered particles, shift with impact angle. No such shift is observed when atoms are used as the incident projectile, as can be seen from Fig. 5. Figures 4 and 5 also show that for both ions and atoms the scattering yield decreases with decreasing impact angle. All spectra are normalized to a counting time of 3600 s.
131 5000
1 000
0
______________________________________________
Fig. 5. Comparison of time-of-flight spectra for helium neutral beam scattered from a gold surface for impact angles of 20°, 35°and 45°.
4. Conclusions We have designed and constructed a time-of-flight scattering spectrometer which can operate with either ions or fast atoms as the incident projectile. The initial results show that good spectra are produced by the system for both types of scattered particle. These can be explained, in general, in terms of elastic collision between the incident beam and the first few monolayers of the surface. The results also show, however, that there are distinct differences between spectra obtained from scattered ions and those obtained from scattered fast atoms. Preliminary spectra suggest that ions penetrate the surface atoms and that ion interactions are not totally elastic, since scattered ion peak energy depends on the angle of impact and hence possibly on penetration depth. Thus, as in a number of other situations, the charge on the incident particle appears to have a significant effect on the particle—surface interaction mechanisms. This spectrometer provides a unique method of studying these mechanisms.
Acknowledgments We wish to thank the British Technology Group for supporting this work and C. G. Pearce for invaluable assistance in processing the data.
132
References 1 D.P.Smith,Surf.Sci., 25(1979)171. 2 D. G. Armour and G. Carter, J. Phys. E, 5 (1972) 2 - 8. 3 T. M. Buck, Methods and phenomena, in S. P. Wolsley and A. W. Czanderna (eds.), Methods of Surface Analysis, McGraw-Hill, New York, 1975. 4 W. L. Baun,SurfacelnterfaceAnal., 3(6) (1981) 243- 250. 5 R. L. Erickson and D. P. Smith, Phys. Rev. Lett., 34 (6) (1975) 297 - 299. 6 W. Heiland and E. Taglauer, Nuci. Instrum. Methods, 132 (1976) 535 - 545. 7 R. F. Goff,J. Vac. Sci. Technol., 10(2) (1973)355-358. 8 R. Kelly, Nucl. Instrum. Methods, 149 (1978) 553 - 558. 9 J. Franks, Vacuum, 34 (1 - 2) (1984) 259 - 261. 10 A. Brown, J. A. Van den Berg and J. C. Vickerman, J. Chem. Soc., Chem. Commun., (1984) 1684 - 1686. 11 AppI. Phys. Lett., 49 (4) (1986) 188 - 190. 12 N. S. Xu, S. 0. Saied and J. L. Sullivan, unpublished work.