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N2O system by secondary ion mass spectrometry
Volume
26, number 2
15 ?Jay 1974
CHEMICAL PHYSICS LETTERS
AN INVESTIGATION BY SECONDARY
OF THE Ni/N20
SYSTEM
ION MASS SPECTROMETRY
M. BARBER and J.C. VICKERMAN Departmrnt
of Chemistry.
Tire Uniwnity of Blattci~estcr Znstinttc of Science and Techtology. dJattcItesrer 11160 JQD, UA’ Received
25 February
1974
A study of the interaction of N20 with a nickel foil at room temperature by secondary ion mass spectrometry reveals the formation of a number of relatively labile and reactive surface species. The most striking feature is tha: In the presence of N20 gas, NzO will adsorb to form the surface species NiaNa, NiaO and NiaN in preference to NiN2, NiO and NiN. When the NlO atmosphere is removed the NiZNZ, Ni20 and N&N species are Irss strongly held than NiO+. The mode of NtO adsorption on Ni is discussed in the light of the data presented.
1. Introduction
The decomposition of nitrous oxide on metals has been
little
studied,
although
there is an extensive
liter-
ment of the specimen, the energy of the bombarding beam being 3 kV and of sufficiently low current density ( 1Omg- 10-l o A/cm2) to minimise surface destruction during the time scale of the experiment. Mass anal-
ature on its interaction, with oxides [l-3] _For example NiO is known to be a good catalyst for the decomposition of N20 [2], and furthermore a large proportion of the oxygen so formed is retained by the catalyst. Recently, studies have appeared in which the interaction of N2q with Al and Cu have been described [4--61. On clean metal films only dissociative adsorption was observed at room-temperature. Bulk oxidation does occur above about 350°K but there is little evidence of this at room temperature. It was therefore considered that a study by using secondary ion mass spectrometry of the surface species formed when N20 adsorbs on Ni metal should provide not only interesting information about the precursors in the oxidation of the metal, but also data on the mechanism of decoinposition of the N,O molecule.
ysis of the secondary ions was carried out using a VG QS quadrupole mass spectrometer, modified with an off axis channeltron electron multiplier detector to improve signal to background. Base pressures of lo-lo torr were routinely attaiiable in the analysis chamber, and sample preparation was carried out in.a separately pumped vessel in which similar base pressures could be obtained_ The specimen can be cleaned by argon ion etching in this chamber and suitable inlet systems allow dosing of the surface with measured quantities of gas. Connection to the main analysis chamber is made via-a bakeable isolating gate valve and specimen transfer from one region to another by a UW bellows system. In this series of experiments high purity polycty~talline Ni foil was used (Johnson Matthey) and 99.9% : pure N,O employdd in the oxidation. Care was taken to ensure; as far as possible, that the primary argon ion b&am current at the sample was
2. Experimental
kept constant from run to.run (1.5 X 10~_10~A/~m2)~ Two investigations were carried out. The. first.wa‘s to study the ixitertiction of N,O v&h the Ri foil in situ..:. at various dquilibrium pies&es between 10-7 .and :. : 5 X 10-S torr, analyois being @-ied out as the‘Ni0. -; was s&amed over-the Ni ipecimeri in th(? IT&? $iam~ ‘I’
The secondary ion’mass spectrometer (SIMS) )vas] co&ucted by Vacuum Generators, and-wtis‘simila;
in principle td that described by B&i&oven [7j. : .Secondary ions.were.produced by argon io_nbomdard:
._ :
. .
~. :
~-
:. _
:.:.
.. ..-..
_-.... ~.277‘:. .. .-._‘I-^.
CHEUICAL PHYSICS LETTERS
Volume 26. number 2
15 May 1974
120-
Ni,N:
Niz
(xyz/j)
[x1/,)
Ni20+(x’/31 NiOf
. I c
58 52 Eg.
0
II
20
10
30
+
40
50
60
(1
at
I)
23
1. Secondary ion spectrum of an argon
etched nickel foil.
Fig. 2. Variation of the peak intensity at mass 74 (NiO+) =, (Nif) n, mass 132 (NizO+) v. and mass 144 (NizNr) a, as a function of NzO dose rate (points marked 1 atm indicate the amplitude of the peak after esposure to 1
mass116 ber. The second study was to admit N,O to the Ni foil at various
pressures
between
1O-6
torr and 1 at-
mosphere in the sample preparation chamber for varying periods of time. The gas was then removed and the specimen returned to the main chamber for anal-
atm N20 in the preparation chamber and subsequent removal of the gas). Ordinate: peak height, abscissa: dose rate (arbitrary units)_
ysis.
3, Results The SIMS spectrum of a clean Ni film after extensive argon ion etching is shown in fig. l_ The charac&&ii Ni isotope pattern at masses 58,60 and 62 can be clearly seen, together with intense impurity peaks at masses 23,27 and 52 due to Na, Al and Cr, the high intensity of these peaks being due to the miich greater sputtering rates of these elements in comparison to Ni [73 _ Admission to the sample of lop7 torr of N20 in the first series of experiments ci?rried an approximately ten-fold increase in the N? signal. Little change was noted in the intensity of the impurity peaks. Much more significant however was the appearance of newsignals at masses 74 and 76 (NiO+); 116, 118, 120 @Ii;); 1?0,132,134,136,138 (NiZw and Ni20+) anti 144, !46,148 (N$%). As the.pressure of N20 was increased the intensity. of these species changed, the variation is shown graphically in fig. 2 and further new species appeared; a group at masses 174- 180 : : 278 ._
(Ni?) and 190-196 (Ni30f) together with a group at 88-90 due to NiNO+. Finally, at a pressure of 5 X 10e5 torr the spectrum shown in fig. 3 was obtained. The presence and relative strengths of the NixNz and NiNO+ signals compared to the NiO+ and Ni,O+ is oI” some considerable interest especially in view of the absence of an NiNz or NiN+ signal. The only negative ion species detected were O- , NT and 0~. In the second series of experiments, exposure of the clean Ni foil to 1W6 torr of N20 for 30 set again increased the Ni+ signals by about tenfold. Very weak signals only were observed for NiO+, Ni$, O- and 0%. A further exposure to low6 torr for 30 set reduced the.signaIs drasticaIIy_ This curious phenomenon was found to be reproducible. Un!ess later exposures were longer or at a higher pressure the NiO+, etc.,,signals were reduced markedly compared with the immediately preceding spectrum_ Furthermore, prolonged analysis or exposure to higb vacuum caused the species to disappear, One explanation of this phenomenon is that the. chemisorbed species fomied are fairly weakly held an< reactive_ Pnother is that diffusion of the oxygen into the metal-takes place. Such a rapid diffusion at I .;
CHEMICAL PHYSICS LETTERS
Volume 26, number 2
15 May 1974
Fig. 4. Variation of the peak intensity of mass 74 (NiO+) as a function of dose rate for specimens examined after removal of the gas. Ordinate: peak height, abscissa: dose rate (arbitrary units).
6%ii&
144 134 116
74
56
Fig. 3. Secondary ion spectrum of a nickel foil in the presence of 5 x lo-’ torr N20.
298°K is probably not likely, but further studies are required to elucidate this. However, increasing dose rates do result in increases in intensity of NiG+ (see fig. 41, Ni$ and Ni20+, and weak signals at the higher dose rates do occur corresponding to NiNO+, NiNZ@, Ni2N+ and Ni,Ns. After exposure to 1 atmosphere of N20 for 15 minutes, the spectrum shown.in fig_ 5 was obtained. The sensitivity scale is the same as for fig. 3. As can be seen the NiO+ signal is much stronger than that in fig. 3 with the other complex species being smaller and in order of intensity N$O+ > NC!.> Ni,Ns, whereas in fig_ 3, the ordering was Ni2Ns > Nis > Ni,O+ _The negative ion spectrum only revealed O- and NT.
that the secondary ions observed may in fact bear little relation to the chemisorbed entities. The mechanism of the sputtering process [7-lo] involves penetration of high energy Ar+ into the bulk of the solid where resulting cascades of collisions between the atoms of the solid both dissipate the kinetic energy and may result
R= 3x103c/sec-’
4. Discussion There are very few investigations of adsorbed species by secondary ion mass spectrometry. Sinde the sputtering process is utilised it may be thought that fragmentation of the adsorbed speciescould result so
I
I
144 134 116
,
89
,‘I
75
I
56 44
Fig. 5. Secondary ion spectrum of’a nick?1 foil exposed to 1 atmosphere of N20 for 15 minutes after.which the m &s rem&d before analysis.
Volume 26, number 2
CHEMICAL PHYSICS LETTERS
in emission of a secondary ion from the surface. The majority of secondary ions detected have kinetic energies in the range l-10 eV [&lo]. this suggests that the final collision which ejects a species from the surface is comparatively gentle. Detailed consideration of this process does suggest that little Gagmentation of chemisorbed entities will occur. Furthermore, the work of Fogel and Benninghoven [7] as well as our experimental data, yet to be published, on a number of simple systems, characterised by other techniques, lends adequate support to this thesis. In the case of oxide catalysed decomposition of N,O the mechanism usually postulated [ 121 is N20 (8) * N20 (ads), N,O (ads) + e * N,O- (ads), NzO- (ads) =+N, (g) + O- (adi). From the first set of experimeits it would appear that this is not the mechanism over clean Ni foil. There is no evidence for the.presence of N20 adsorbed as such except on the specimen exposed to 1 atm of N,O. This is in complete agreement with the observation that N,O adsorbs dissociatively on cleaning [6]. From the secondary ion spectra produced, it would also appear rllat the N20 molecule will decompose by breaking either of its two bonds, yielding adsorbed N?, NO and N. What is perhaps more interesting is the fact that the spectra indicate that strongly associated with N,, 0 and N are two nickel atoms. Whereas with NO only the NiNO+ species is observed. Of considerable interest also is the complete absence of NiNf and NiNs. A possible explanation of these results is as follows. If we assume two modes of adsorption, then the first and most predominant way is that the linear N,O molecule will adsorb perpendicular to the surface and between two Ni atoms, either N down or 0 down, this mode giving rise to such species as Ni, O+, Ni,N+ and Nix%;. The second mode is for N,O to adsorb only via its 0 atom to a simple Ni atom, resulting in adsorbed 0 and NO, thus explaining the NiNO+ entity and the absence.of NiN+. The majority of the adsorbed species are only weakly bound, however, and this is clearly revealed by the second set of experiments where the nitrogen species do not even survive the removal of the gas phase’. The oxygen species, although more strongly held, disappear very rapidly if a small dose of I$0 .is ,280.
1
15 May 1974
admitted. This may be due to the mechanism suggested for oxide decomposition of N20 when there is a reasonable coverage of adsorbed oxygen [ 12,131 namely: O-(ads) + N# (g) + N2 (8) + 05 (ads), 05 (ads) + 0, (g) + e (to catalyst)_ The disappearance of the oxygen species after prolonged outgassing at 1O-1o torr may also demonstrate their labile nature_ However, a possible explanation for this would be diffusion and incorporation of the oxygen into the metal. This view is possibly supported by the maintenance of very high intensity Nif peaks. The large increase in sputtering rates for clean versus reacted metal surfaces is a general phenomenon [7] brought about possibly by the weakening of metalmetal bonds in the surface when adsorption takes place. Thus if the oxygen in this system migrated just below the surface, the surface Ni-Ni bonds would still be weakened, giving rise to the high intensity Nif peak, whilst no signal due to the oxygen species would
be determined by this technique. Further work is in progress to investigate this possibility. Acknowledgement Our thanks are due to Dr. J.D. Waldron (Chairman) of the Board of Directors of Vacuum Generators Ltd.! East Grinstead, for their generous loan of equipment on which this work was carried out.
References [l] E.R.S. Winter, Advan. Catalysis 10 (1958) 196. [2] E.R.S. Winter, Discussions Faraday Sot. 28 (1959) 183. [3] E.R.S_ Winter, J. Catalysis 19 (1970) 32_ [4] C.L. Hunt and I.M. Ritchie, J. Chem. Sot. Faraday I 68 (1972) 1413. IS] J.J.F. Scholten and J.A. Konvalinka, Trans. Faraday Sot. 65 (1969) 2465. [6] S.A. Isa and J.M. Saleh. I_ Phys. Chem. 76 (1972) 2530. [7] A. Benninghoven. Surface Sci. 35 (1973) 427. [8] M.W. Thompson. Phil. Mag. 18 (1968) 415. [9] V.I. Veksler, Fiz. Tverd. Tela 11 (1969) 3132. [lOI 2. JureIa, Radiation Effects 19 (1973) 175. [ll] Ya.M. Fogel, Kin$ics Catalysis 5 (1964) 431. [lZ] R.M. Dell, F.S. Stone and P.F. l%y, Trans. Faraday sot. 49 (1953) 201. . j13] C.B. Amphlett, Trans. Faraday sot. 50 (1959) 273.
:
26, number 2
15 ?Jay 1974
CHEMICAL PHYSICS LETTERS
AN INVESTIGATION BY SECONDARY
OF THE Ni/N20
SYSTEM
ION MASS SPECTROMETRY
M. BARBER and J.C. VICKERMAN Departmrnt
of Chemistry.
Tire Uniwnity of Blattci~estcr Znstinttc of Science and Techtology. dJattcItesrer 11160 JQD, UA’ Received
25 February
1974
A study of the interaction of N20 with a nickel foil at room temperature by secondary ion mass spectrometry reveals the formation of a number of relatively labile and reactive surface species. The most striking feature is tha: In the presence of N20 gas, NzO will adsorb to form the surface species NiaNa, NiaO and NiaN in preference to NiN2, NiO and NiN. When the NlO atmosphere is removed the NiZNZ, Ni20 and N&N species are Irss strongly held than NiO+. The mode of NtO adsorption on Ni is discussed in the light of the data presented.
1. Introduction
The decomposition of nitrous oxide on metals has been
little
studied,
although
there is an extensive
liter-
ment of the specimen, the energy of the bombarding beam being 3 kV and of sufficiently low current density ( 1Omg- 10-l o A/cm2) to minimise surface destruction during the time scale of the experiment. Mass anal-
ature on its interaction, with oxides [l-3] _For example NiO is known to be a good catalyst for the decomposition of N20 [2], and furthermore a large proportion of the oxygen so formed is retained by the catalyst. Recently, studies have appeared in which the interaction of N2q with Al and Cu have been described [4--61. On clean metal films only dissociative adsorption was observed at room-temperature. Bulk oxidation does occur above about 350°K but there is little evidence of this at room temperature. It was therefore considered that a study by using secondary ion mass spectrometry of the surface species formed when N20 adsorbs on Ni metal should provide not only interesting information about the precursors in the oxidation of the metal, but also data on the mechanism of decoinposition of the N,O molecule.
ysis of the secondary ions was carried out using a VG QS quadrupole mass spectrometer, modified with an off axis channeltron electron multiplier detector to improve signal to background. Base pressures of lo-lo torr were routinely attaiiable in the analysis chamber, and sample preparation was carried out in.a separately pumped vessel in which similar base pressures could be obtained_ The specimen can be cleaned by argon ion etching in this chamber and suitable inlet systems allow dosing of the surface with measured quantities of gas. Connection to the main analysis chamber is made via-a bakeable isolating gate valve and specimen transfer from one region to another by a UW bellows system. In this series of experiments high purity polycty~talline Ni foil was used (Johnson Matthey) and 99.9% : pure N,O employdd in the oxidation. Care was taken to ensure; as far as possible, that the primary argon ion b&am current at the sample was
2. Experimental
kept constant from run to.run (1.5 X 10~_10~A/~m2)~ Two investigations were carried out. The. first.wa‘s to study the ixitertiction of N,O v&h the Ri foil in situ..:. at various dquilibrium pies&es between 10-7 .and :. : 5 X 10-S torr, analyois being @-ied out as the‘Ni0. -; was s&amed over-the Ni ipecimeri in th(? IT&? $iam~ ‘I’
The secondary ion’mass spectrometer (SIMS) )vas] co&ucted by Vacuum Generators, and-wtis‘simila;
in principle td that described by B&i&oven [7j. : .Secondary ions.were.produced by argon io_nbomdard:
._ :
. .
~. :
~-
:. _
:.:.
.. ..-..
_-.... ~.277‘:. .. .-._‘I-^.
CHEUICAL PHYSICS LETTERS
Volume 26. number 2
15 May 1974
120-
Ni,N:
Niz
(xyz/j)
[x1/,)
Ni20+(x’/31 NiOf
. I c
58 52 Eg.
0
II
20
10
30
+
40
50
60
(1
at
I)
23
1. Secondary ion spectrum of an argon
etched nickel foil.
Fig. 2. Variation of the peak intensity at mass 74 (NiO+) =, (Nif) n, mass 132 (NizO+) v. and mass 144 (NizNr) a, as a function of NzO dose rate (points marked 1 atm indicate the amplitude of the peak after esposure to 1
mass116 ber. The second study was to admit N,O to the Ni foil at various
pressures
between
1O-6
torr and 1 at-
mosphere in the sample preparation chamber for varying periods of time. The gas was then removed and the specimen returned to the main chamber for anal-
atm N20 in the preparation chamber and subsequent removal of the gas). Ordinate: peak height, abscissa: dose rate (arbitrary units)_
ysis.
3, Results The SIMS spectrum of a clean Ni film after extensive argon ion etching is shown in fig. l_ The charac&&ii Ni isotope pattern at masses 58,60 and 62 can be clearly seen, together with intense impurity peaks at masses 23,27 and 52 due to Na, Al and Cr, the high intensity of these peaks being due to the miich greater sputtering rates of these elements in comparison to Ni [73 _ Admission to the sample of lop7 torr of N20 in the first series of experiments ci?rried an approximately ten-fold increase in the N? signal. Little change was noted in the intensity of the impurity peaks. Much more significant however was the appearance of newsignals at masses 74 and 76 (NiO+); 116, 118, 120 @Ii;); 1?0,132,134,136,138 (NiZw and Ni20+) anti 144, !46,148 (N$%). As the.pressure of N20 was increased the intensity. of these species changed, the variation is shown graphically in fig. 2 and further new species appeared; a group at masses 174- 180 : : 278 ._
(Ni?) and 190-196 (Ni30f) together with a group at 88-90 due to NiNO+. Finally, at a pressure of 5 X 10e5 torr the spectrum shown in fig. 3 was obtained. The presence and relative strengths of the NixNz and NiNO+ signals compared to the NiO+ and Ni,O+ is oI” some considerable interest especially in view of the absence of an NiNz or NiN+ signal. The only negative ion species detected were O- , NT and 0~. In the second series of experiments, exposure of the clean Ni foil to 1W6 torr of N20 for 30 set again increased the Ni+ signals by about tenfold. Very weak signals only were observed for NiO+, Ni$, O- and 0%. A further exposure to low6 torr for 30 set reduced the.signaIs drasticaIIy_ This curious phenomenon was found to be reproducible. Un!ess later exposures were longer or at a higher pressure the NiO+, etc.,,signals were reduced markedly compared with the immediately preceding spectrum_ Furthermore, prolonged analysis or exposure to higb vacuum caused the species to disappear, One explanation of this phenomenon is that the. chemisorbed species fomied are fairly weakly held an< reactive_ Pnother is that diffusion of the oxygen into the metal-takes place. Such a rapid diffusion at I .;
CHEMICAL PHYSICS LETTERS
Volume 26, number 2
15 May 1974
Fig. 4. Variation of the peak intensity of mass 74 (NiO+) as a function of dose rate for specimens examined after removal of the gas. Ordinate: peak height, abscissa: dose rate (arbitrary units).
6%ii&
144 134 116
74
56
Fig. 3. Secondary ion spectrum of a nickel foil in the presence of 5 x lo-’ torr N20.
298°K is probably not likely, but further studies are required to elucidate this. However, increasing dose rates do result in increases in intensity of NiG+ (see fig. 41, Ni$ and Ni20+, and weak signals at the higher dose rates do occur corresponding to NiNO+, NiNZ@, Ni2N+ and Ni,Ns. After exposure to 1 atmosphere of N20 for 15 minutes, the spectrum shown.in fig_ 5 was obtained. The sensitivity scale is the same as for fig. 3. As can be seen the NiO+ signal is much stronger than that in fig. 3 with the other complex species being smaller and in order of intensity N$O+ > NC!.> Ni,Ns, whereas in fig_ 3, the ordering was Ni2Ns > Nis > Ni,O+ _The negative ion spectrum only revealed O- and NT.
that the secondary ions observed may in fact bear little relation to the chemisorbed entities. The mechanism of the sputtering process [7-lo] involves penetration of high energy Ar+ into the bulk of the solid where resulting cascades of collisions between the atoms of the solid both dissipate the kinetic energy and may result
R= 3x103c/sec-’
4. Discussion There are very few investigations of adsorbed species by secondary ion mass spectrometry. Sinde the sputtering process is utilised it may be thought that fragmentation of the adsorbed speciescould result so
I
I
144 134 116
,
89
,‘I
75
I
56 44
Fig. 5. Secondary ion spectrum of’a nick?1 foil exposed to 1 atmosphere of N20 for 15 minutes after.which the m &s rem&d before analysis.
Volume 26, number 2
CHEMICAL PHYSICS LETTERS
in emission of a secondary ion from the surface. The majority of secondary ions detected have kinetic energies in the range l-10 eV [&lo]. this suggests that the final collision which ejects a species from the surface is comparatively gentle. Detailed consideration of this process does suggest that little Gagmentation of chemisorbed entities will occur. Furthermore, the work of Fogel and Benninghoven [7] as well as our experimental data, yet to be published, on a number of simple systems, characterised by other techniques, lends adequate support to this thesis. In the case of oxide catalysed decomposition of N,O the mechanism usually postulated [ 121 is N20 (8) * N20 (ads), N,O (ads) + e * N,O- (ads), NzO- (ads) =+N, (g) + O- (adi). From the first set of experimeits it would appear that this is not the mechanism over clean Ni foil. There is no evidence for the.presence of N20 adsorbed as such except on the specimen exposed to 1 atm of N,O. This is in complete agreement with the observation that N,O adsorbs dissociatively on cleaning [6]. From the secondary ion spectra produced, it would also appear rllat the N20 molecule will decompose by breaking either of its two bonds, yielding adsorbed N?, NO and N. What is perhaps more interesting is the fact that the spectra indicate that strongly associated with N,, 0 and N are two nickel atoms. Whereas with NO only the NiNO+ species is observed. Of considerable interest also is the complete absence of NiNf and NiNs. A possible explanation of these results is as follows. If we assume two modes of adsorption, then the first and most predominant way is that the linear N,O molecule will adsorb perpendicular to the surface and between two Ni atoms, either N down or 0 down, this mode giving rise to such species as Ni, O+, Ni,N+ and Nix%;. The second mode is for N,O to adsorb only via its 0 atom to a simple Ni atom, resulting in adsorbed 0 and NO, thus explaining the NiNO+ entity and the absence.of NiN+. The majority of the adsorbed species are only weakly bound, however, and this is clearly revealed by the second set of experiments where the nitrogen species do not even survive the removal of the gas phase’. The oxygen species, although more strongly held, disappear very rapidly if a small dose of I$0 .is ,280.
1
15 May 1974
admitted. This may be due to the mechanism suggested for oxide decomposition of N20 when there is a reasonable coverage of adsorbed oxygen [ 12,131 namely: O-(ads) + N# (g) + N2 (8) + 05 (ads), 05 (ads) + 0, (g) + e (to catalyst)_ The disappearance of the oxygen species after prolonged outgassing at 1O-1o torr may also demonstrate their labile nature_ However, a possible explanation for this would be diffusion and incorporation of the oxygen into the metal. This view is possibly supported by the maintenance of very high intensity Nif peaks. The large increase in sputtering rates for clean versus reacted metal surfaces is a general phenomenon [7] brought about possibly by the weakening of metalmetal bonds in the surface when adsorption takes place. Thus if the oxygen in this system migrated just below the surface, the surface Ni-Ni bonds would still be weakened, giving rise to the high intensity Nif peak, whilst no signal due to the oxygen species would
be determined by this technique. Further work is in progress to investigate this possibility. Acknowledgement Our thanks are due to Dr. J.D. Waldron (Chairman) of the Board of Directors of Vacuum Generators Ltd.! East Grinstead, for their generous loan of equipment on which this work was carried out.
References [l] E.R.S. Winter, Advan. Catalysis 10 (1958) 196. [2] E.R.S. Winter, Discussions Faraday Sot. 28 (1959) 183. [3] E.R.S_ Winter, J. Catalysis 19 (1970) 32_ [4] C.L. Hunt and I.M. Ritchie, J. Chem. Sot. Faraday I 68 (1972) 1413. IS] J.J.F. Scholten and J.A. Konvalinka, Trans. Faraday Sot. 65 (1969) 2465. [6] S.A. Isa and J.M. Saleh. I_ Phys. Chem. 76 (1972) 2530. [7] A. Benninghoven. Surface Sci. 35 (1973) 427. [8] M.W. Thompson. Phil. Mag. 18 (1968) 415. [9] V.I. Veksler, Fiz. Tverd. Tela 11 (1969) 3132. [lOI 2. JureIa, Radiation Effects 19 (1973) 175. [ll] Ya.M. Fogel, Kin$ics Catalysis 5 (1964) 431. [lZ] R.M. Dell, F.S. Stone and P.F. l%y, Trans. Faraday sot. 49 (1953) 201. . j13] C.B. Amphlett, Trans. Faraday sot. 50 (1959) 273.
: