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XPS and STM study of carbon deposits at the surface of platinum (110)
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appleo surface ELSEVIER
science
Applied Surface Science 120 (1997) 239-242
XPS and STM study of carbon deposits at the surface of platinum (110) R.I. Kvon *, A.I. Boronin, Sh.K. Shaikhutdinov, R.A. Buyanov Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia
Received 19 May 1997; accepted 21 June 1997
Abstract The deposition of carbon due to high temperature ethylene decomposition at Pt(110) was studied by XPS and STM techniques. In the temperature range 700-1400 K no graphite species were observed. Instead, two carbon states were distinguished by XPS. At temperatures 700-850 K chemisorbed carbon layer is formed with BE(Cls)= 284.2 eV, this carbon state reacting readily with both oxygen and hydrogen. At T > 850 K carbon layer with BE(Cls) = 284.6-284.9 eV is formed. Further study showed this carbon species to be stable up to 1150 K and to be inert towards both hydrogen and oxygen up to 1000 K. This state was attributed to diamond-like carbon (DLC). STM study of DLC on Pt(110) revealed the patched pseudo-C-(3 × 1) structure. This reconstruction is believed to account for the DLC formation at platinum (110) surface. © 1997 Elsevier Science B.V. PACS: 82.80.Pv; 81.05.Tp; 61.16.Ch Keywords: XPS; Carbon; Platinum
1. Introduction The nowadays petrochemical industry exploits alumina supported Pt catalysts with the high activity and selectivity. But the serious problem still to be solved is the catalyst degradation due to the process of coke formation because of side hydrocarbons decomposition reactions. Such carbonaceous deposits are usually considered to have graphite nature. However this question is still not clear since it is quite difficult to study the supported catalysts by the modem physical methods of surface analysis. As for the
* Corresponding author. Tel.: + 7-3832-352269; fax: + 7-3832355756; e-mail: [email protected].
investigation of carbon on massive Pt metal samples there is plenty of works ([1-4] and references therein). However the nature of carbon on Pt has not been understood in details. In our previous papers [4,5], ethylene adsorption at P t ( l l 0 ) and its decomposition up to elementary carbon were studied, and 3-dimensional graphite islands were supposed to form at elevated temperatures. But our recent investigation of carbon at Pt(100) surface has not confirmed this proposal: neither graphite layers nor 3D structures were found. It should be noted that Belton and Shmieg have shown the formation of thin layers of diamond at the surface of polycrystaUine platinum with aid of activated C H a / H 2 mixture [6]. This result also stimu-
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)0025 1-1
240
R.I. Kvon et al. // Applied Surface Science 120 (1997) 239-242
lated us to investigate thoroughly the C / P t ( l l 0 ) system where carbon is produced by simple catalytic way of ethylene decomposition. We have assumed that not only sp 2 (graphite) but also sp 3 (diamondlike) carbon species could be formed at platinum surface. Some results confirming this assumption are presented in this paper.
~
1 i
i
284.2
i~
i
i
i
i
I
i
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shake up 6 5 4
2. Experimental details XPS experiments were carried out with photoelectron spectrometer VG ESCALAB HP. XPS spectra were recorded using AIK,, irradiation and calibrated against Au 4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.7 eV) lines. STM images were taken by home-made microscope as described elsewhere [7]. The calibration of the images was confirmed by atomic resolution of the Pt(110)-(1 x 1) surface. Both sides of polished Pt single crystal were cleaned by standard procedures until no impurities were detected by XPS and Pt(ll0) plane had clear (1 × 2) LEED pattern.
3. Results and discussion Temperature-programmed reaction shows that adlayer formed by ethylene adsorption at T > 670 K contains no hydrogen. Thus the temperature range of 700-1400 K was chosen to study hydrogen-free carbon layers. Fig. 1A presents C is spectra for saturated carbon adlayers at Pt(110) formed at different temperatures. Two peculiarities become obvious as the temperature rises. First, the XPS peak position shifts to higher BE values from 284.2 eV to 284.6 eV, and second, the peak intensity grows by factor of about 1.5, indicating the formation of denser carbon adlayer, but the latter one could hardly be graphite, since the binding energy of C ls for graphite is 284.0-284.4 eV [8]. To identify the state of carbon more precisely, C Is spectrum of the adlayer formed at 1000 K was compared with C ls spectra of perfect graphite and pure microcrystalline diamond samples. Fig. 1B demonstrates that graphite (C with sp 2 hybridization) is characterized by the main peak with BE = 284.0 eV and satellite peak at BE = 290-291 eV which is
3 ,4¢v~ 2 1
280
285
290
Binding Energy, eV
295
280
285
290
295
Binding Energy, eV
Fig. 1. (A) C ls spectra of carbon adlayers formed by ethylene adsorption at Pt(110) at: curve 1:750 K; curve 2:800 K; curve 3: 850 K; curve 4 : 9 5 0 K; curve 5 : 1 0 5 0 K; curve 6 : 1 1 5 0 K, (B) C ls spectra recorded from: curve 1: graphite; curve 2: carbon adlayer at Pt(110) formed at 950 K; curve 3: diamond.
related with 'shake-up' process in the course of C ls photoionization [9]. Since the diamond (sp 3) structure contains no 7r bonds, its C ls spectrum (Fig. 1B, curve 3) has not the satellite peak. Within the accuracy of measurements C ls peak of carbon adlayer on Pt(100) formed at T = 950 K does not demonstrate shake-up satellite. The value of BE C ls is substantially higher than that of graphite (see Fig. 1B, curve 2). The analysis of Auger spectra shows also that C-KLL structure of this carbon state at Pt(110) does not correspond to the graphite state of carbon [10]. On the contrary, it is clear from Fig. 2 that C KLL spectrum is quite similar to that of diamond sample rather than of graphite. Finally, our attempts to observe C-ring LEED patterns have failed. So one can conclude that at high temperatures ( T > 850 K) carbon adlayer at Pt(ll0) surface is very likely to form diamond-like carbon (DLC) species. The binding energy value for DLC is lower than that one measured for diamond probably because of the strong influence of the electron system of platinum. To characterize the properties of carbon adlayer mentioned above, we have used non-traditional application of XPS technique. To find out the tempera-
241
R.L Kvon et al. /Applied Surface Science 120 (1997) 239-242
1
240
260
280
300
Kinetic Energy, eV Fig. 2. C KLL spectra recorded from: curve 1: graphite; curve 2: carbon adlayer at Pt(110) formed at 950 K; curve 3: diamond. ture range where D L C / P t ( 1 1 0 ) is stable, slow heating up to 1400 K of the adlayer formed at 700 K was performed in the flow of ethylene (see Fig. 3A). Both C ls and P t 4 f peaks intensity were recorded in the same experiment run, so that C 1s / P t 4f intensity ratio was free from systematic experimental error. One can see that between 850 K and 1100 K carbon coverage increases by 40%, from T > 1150 K drops just to the starting value and from 1250 to 1400 K remains constant. Thus DLC state is stable in the temperature range of 850-1150 K (darker square in Fig. 3A). Fig. 3B illustrates some kinetic features of the formation of carbon adlayer at Pt(110) surface at 750 K and 1150 K. Curve 1 corresponds to chemisorbed carbon accumulation and curve 2 is for DLC formation. The initial parts of both curves (up to ~ 15 L of ethylene marked by asterisk in Fig. 3B) exactly coincide. Then, the rate of chemisorbed carbon formation retards substantially while the DLC growth is still quite rapid. It was also established that chemisorbed carbon adlayer is quite active in the reactions with both hydrogen and oxygen, while DLC is inert towards these gases. Fig. 3C shows that C Is spectrum obtained after 15 L ethylene exposure at 1150 K gives the BE value which is characteristic for diamond-like carbon (284.6 eV). Thus even at initial steps of ethylene adsorption the DLC growth occurs. On the basis of these data the nucleation mechanism of the DLC formation could be proposed.
LEED study of both chemisorbed ( 7 0 0 - 8 5 0 K) and diamond-like (850-1150 K) carbon layers does not reveal any patterns, but strong background only. So one may conclude that both carbon species are formed by small islands with no long ordered structures. To obtain structural information STM technique was used in the other machine. Thus the main drawback of the procedure used was the sample transfer through the atmosphere. Since DLC is absolutely stable towards oxygen, STM images were taken only for DLC/Pt(110). Indeed, after pumping of STM machine Auger spectra were recorded and no O KLL peaks were observed. STM images of this sample showed step-like surface without any threedimensional islands. A large-scale high resolution image is presented in Fig. 4. It is interesting to see the 8.4 A modulation in (110) direction (especially in the upper-right part of image) which is almost equal to 8.34 A calculated for Pt(ll0)-(3 × 1) reconstruction. The (110) rows are slightly 'diffused' but also could be found if one looks at glancing angle in (110) direction. A close inspection of this structure showed that along the (110) rows the interatomic distances are much shorter than for Pt lattice which, in turn, obscured the whole structure. We attributed
o
"~
0.22
"~ ~u
0.21
0
1000 2000 Time, sec
0.20
0.19 750 1000 1250 Temperature, K
................ 280 285 290 BindingEnergy,eV
Fig. 3. Carbon formation at Pt(110): (A) carbon coverage during the programmed(/3 = 0.2 K/s) heating of the sample in 5 x 10-s mbar flow of ethylene; (B) kinetics of carbon deposition in the 5 × 10-s mbar flow of C2H 4 at: curve I: 750 K; curve 2:1150 K; *: c=15 L; (C) Cls spectra recorded at initial stage of carbon deposition: curve 1: at 750 K; curve 2: at 1150 K.
242
R.I. Kvon et al./ Applied Surface Science 120 (1997) 239-242
diamond-like c a r b o n ( s p 3 hybridization) forms. DLC is stable up to 1150 K and is inert both to hydrogen and to oxygen treatments. According to STM data the structure of the DLC adlayer is (3 × 1) with the extra rows of Pt atoms in the normal direction to original rows in well-established 'missing row' structure.
Acknowledgements
Fig. 4. STM image of the Pt(110) surface after ethylene treatment at 1150 K. Size 150 × 150 A. Tunnel parameters: bias = 30 mV, current = 2 hA.
This work was supported by Russian Foundation of Basic Research No. 95-03-96676.
References this structure to the pseudo-C-(3 X 1)-Pt(110) reconstruction. However, further STM and XPS investigations are needed.
4. Conclusions The state and the structure of carbon adlayers formed by ethylene decomposition at Pt(110) plane were studied by XPS, LEED and STM. The different carbon species were proved to form depending on the temperature of C 2 H 4 adsorption. No graphite (sp 2 hybridization) structures were found. At T = 700-850 K chemisorbed (with no C - C , but P t - C bonds only) carbon layer grows. At higher temperatures additional accumulation of C atoms occurs and
[1] K. Baron, D.W. Blakely, G.A. Somorjai, Surf. Sci. 41 (1974) 45. [2] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger, G. Comsa, Appl. Phys. A 53 (1991) 414. [3] A.Y. Tontegode, Prog. Surf. Sci. 38 (1991) 201. [4] A.I. Boronin, V.I. Bukhtiyarov, R. Kvon, V.V. Chesnokov, R.A. Buyanov, Surf. Sci. 258 (1991) 289. [5] A.I. Boronin, V.I. Bukhtiyarov, R. Kvon, V.V. Chesnokov, R.A. Buyanov, Rep. Sib. Br. USSR Acad. Sci. 2 (1990) 79, in Russian. [6] D.N. Belton, S.J. Shmieg, Surf. Sci. 233 (1990) 131. [7] S.K. Shaikhutdinov, D.I. Kochubey, J. Struct. Chem. 35 (1993) 956. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. [9] T.L. Barr, M. Yin, J. Vac. Sci. Technol. A 10 (1992) 2788. [10] Y. Mizokava, T. Miyasato, S. Nakamura, K.M. Geib, C.W. Wilmsen, J. Vac. Sci. Technol. A 5 (1987) 2809.
appleo surface ELSEVIER
science
Applied Surface Science 120 (1997) 239-242
XPS and STM study of carbon deposits at the surface of platinum (110) R.I. Kvon *, A.I. Boronin, Sh.K. Shaikhutdinov, R.A. Buyanov Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia
Received 19 May 1997; accepted 21 June 1997
Abstract The deposition of carbon due to high temperature ethylene decomposition at Pt(110) was studied by XPS and STM techniques. In the temperature range 700-1400 K no graphite species were observed. Instead, two carbon states were distinguished by XPS. At temperatures 700-850 K chemisorbed carbon layer is formed with BE(Cls)= 284.2 eV, this carbon state reacting readily with both oxygen and hydrogen. At T > 850 K carbon layer with BE(Cls) = 284.6-284.9 eV is formed. Further study showed this carbon species to be stable up to 1150 K and to be inert towards both hydrogen and oxygen up to 1000 K. This state was attributed to diamond-like carbon (DLC). STM study of DLC on Pt(110) revealed the patched pseudo-C-(3 × 1) structure. This reconstruction is believed to account for the DLC formation at platinum (110) surface. © 1997 Elsevier Science B.V. PACS: 82.80.Pv; 81.05.Tp; 61.16.Ch Keywords: XPS; Carbon; Platinum
1. Introduction The nowadays petrochemical industry exploits alumina supported Pt catalysts with the high activity and selectivity. But the serious problem still to be solved is the catalyst degradation due to the process of coke formation because of side hydrocarbons decomposition reactions. Such carbonaceous deposits are usually considered to have graphite nature. However this question is still not clear since it is quite difficult to study the supported catalysts by the modem physical methods of surface analysis. As for the
* Corresponding author. Tel.: + 7-3832-352269; fax: + 7-3832355756; e-mail: [email protected].
investigation of carbon on massive Pt metal samples there is plenty of works ([1-4] and references therein). However the nature of carbon on Pt has not been understood in details. In our previous papers [4,5], ethylene adsorption at P t ( l l 0 ) and its decomposition up to elementary carbon were studied, and 3-dimensional graphite islands were supposed to form at elevated temperatures. But our recent investigation of carbon at Pt(100) surface has not confirmed this proposal: neither graphite layers nor 3D structures were found. It should be noted that Belton and Shmieg have shown the formation of thin layers of diamond at the surface of polycrystaUine platinum with aid of activated C H a / H 2 mixture [6]. This result also stimu-
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)0025 1-1
240
R.I. Kvon et al. // Applied Surface Science 120 (1997) 239-242
lated us to investigate thoroughly the C / P t ( l l 0 ) system where carbon is produced by simple catalytic way of ethylene decomposition. We have assumed that not only sp 2 (graphite) but also sp 3 (diamondlike) carbon species could be formed at platinum surface. Some results confirming this assumption are presented in this paper.
~
1 i
i
284.2
i~
i
i
i
i
I
i
i i@
i2846 i,
shake up 6 5 4
2. Experimental details XPS experiments were carried out with photoelectron spectrometer VG ESCALAB HP. XPS spectra were recorded using AIK,, irradiation and calibrated against Au 4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.7 eV) lines. STM images were taken by home-made microscope as described elsewhere [7]. The calibration of the images was confirmed by atomic resolution of the Pt(110)-(1 x 1) surface. Both sides of polished Pt single crystal were cleaned by standard procedures until no impurities were detected by XPS and Pt(ll0) plane had clear (1 × 2) LEED pattern.
3. Results and discussion Temperature-programmed reaction shows that adlayer formed by ethylene adsorption at T > 670 K contains no hydrogen. Thus the temperature range of 700-1400 K was chosen to study hydrogen-free carbon layers. Fig. 1A presents C is spectra for saturated carbon adlayers at Pt(110) formed at different temperatures. Two peculiarities become obvious as the temperature rises. First, the XPS peak position shifts to higher BE values from 284.2 eV to 284.6 eV, and second, the peak intensity grows by factor of about 1.5, indicating the formation of denser carbon adlayer, but the latter one could hardly be graphite, since the binding energy of C ls for graphite is 284.0-284.4 eV [8]. To identify the state of carbon more precisely, C Is spectrum of the adlayer formed at 1000 K was compared with C ls spectra of perfect graphite and pure microcrystalline diamond samples. Fig. 1B demonstrates that graphite (C with sp 2 hybridization) is characterized by the main peak with BE = 284.0 eV and satellite peak at BE = 290-291 eV which is
3 ,4¢v~ 2 1
280
285
290
Binding Energy, eV
295
280
285
290
295
Binding Energy, eV
Fig. 1. (A) C ls spectra of carbon adlayers formed by ethylene adsorption at Pt(110) at: curve 1:750 K; curve 2:800 K; curve 3: 850 K; curve 4 : 9 5 0 K; curve 5 : 1 0 5 0 K; curve 6 : 1 1 5 0 K, (B) C ls spectra recorded from: curve 1: graphite; curve 2: carbon adlayer at Pt(110) formed at 950 K; curve 3: diamond.
related with 'shake-up' process in the course of C ls photoionization [9]. Since the diamond (sp 3) structure contains no 7r bonds, its C ls spectrum (Fig. 1B, curve 3) has not the satellite peak. Within the accuracy of measurements C ls peak of carbon adlayer on Pt(100) formed at T = 950 K does not demonstrate shake-up satellite. The value of BE C ls is substantially higher than that of graphite (see Fig. 1B, curve 2). The analysis of Auger spectra shows also that C-KLL structure of this carbon state at Pt(110) does not correspond to the graphite state of carbon [10]. On the contrary, it is clear from Fig. 2 that C KLL spectrum is quite similar to that of diamond sample rather than of graphite. Finally, our attempts to observe C-ring LEED patterns have failed. So one can conclude that at high temperatures ( T > 850 K) carbon adlayer at Pt(ll0) surface is very likely to form diamond-like carbon (DLC) species. The binding energy value for DLC is lower than that one measured for diamond probably because of the strong influence of the electron system of platinum. To characterize the properties of carbon adlayer mentioned above, we have used non-traditional application of XPS technique. To find out the tempera-
241
R.L Kvon et al. /Applied Surface Science 120 (1997) 239-242
1
240
260
280
300
Kinetic Energy, eV Fig. 2. C KLL spectra recorded from: curve 1: graphite; curve 2: carbon adlayer at Pt(110) formed at 950 K; curve 3: diamond. ture range where D L C / P t ( 1 1 0 ) is stable, slow heating up to 1400 K of the adlayer formed at 700 K was performed in the flow of ethylene (see Fig. 3A). Both C ls and P t 4 f peaks intensity were recorded in the same experiment run, so that C 1s / P t 4f intensity ratio was free from systematic experimental error. One can see that between 850 K and 1100 K carbon coverage increases by 40%, from T > 1150 K drops just to the starting value and from 1250 to 1400 K remains constant. Thus DLC state is stable in the temperature range of 850-1150 K (darker square in Fig. 3A). Fig. 3B illustrates some kinetic features of the formation of carbon adlayer at Pt(110) surface at 750 K and 1150 K. Curve 1 corresponds to chemisorbed carbon accumulation and curve 2 is for DLC formation. The initial parts of both curves (up to ~ 15 L of ethylene marked by asterisk in Fig. 3B) exactly coincide. Then, the rate of chemisorbed carbon formation retards substantially while the DLC growth is still quite rapid. It was also established that chemisorbed carbon adlayer is quite active in the reactions with both hydrogen and oxygen, while DLC is inert towards these gases. Fig. 3C shows that C Is spectrum obtained after 15 L ethylene exposure at 1150 K gives the BE value which is characteristic for diamond-like carbon (284.6 eV). Thus even at initial steps of ethylene adsorption the DLC growth occurs. On the basis of these data the nucleation mechanism of the DLC formation could be proposed.
LEED study of both chemisorbed ( 7 0 0 - 8 5 0 K) and diamond-like (850-1150 K) carbon layers does not reveal any patterns, but strong background only. So one may conclude that both carbon species are formed by small islands with no long ordered structures. To obtain structural information STM technique was used in the other machine. Thus the main drawback of the procedure used was the sample transfer through the atmosphere. Since DLC is absolutely stable towards oxygen, STM images were taken only for DLC/Pt(110). Indeed, after pumping of STM machine Auger spectra were recorded and no O KLL peaks were observed. STM images of this sample showed step-like surface without any threedimensional islands. A large-scale high resolution image is presented in Fig. 4. It is interesting to see the 8.4 A modulation in (110) direction (especially in the upper-right part of image) which is almost equal to 8.34 A calculated for Pt(ll0)-(3 × 1) reconstruction. The (110) rows are slightly 'diffused' but also could be found if one looks at glancing angle in (110) direction. A close inspection of this structure showed that along the (110) rows the interatomic distances are much shorter than for Pt lattice which, in turn, obscured the whole structure. We attributed
o
"~
0.22
"~ ~u
0.21
0
1000 2000 Time, sec
0.20
0.19 750 1000 1250 Temperature, K
................ 280 285 290 BindingEnergy,eV
Fig. 3. Carbon formation at Pt(110): (A) carbon coverage during the programmed(/3 = 0.2 K/s) heating of the sample in 5 x 10-s mbar flow of ethylene; (B) kinetics of carbon deposition in the 5 × 10-s mbar flow of C2H 4 at: curve I: 750 K; curve 2:1150 K; *: c=15 L; (C) Cls spectra recorded at initial stage of carbon deposition: curve 1: at 750 K; curve 2: at 1150 K.
242
R.I. Kvon et al./ Applied Surface Science 120 (1997) 239-242
diamond-like c a r b o n ( s p 3 hybridization) forms. DLC is stable up to 1150 K and is inert both to hydrogen and to oxygen treatments. According to STM data the structure of the DLC adlayer is (3 × 1) with the extra rows of Pt atoms in the normal direction to original rows in well-established 'missing row' structure.
Acknowledgements
Fig. 4. STM image of the Pt(110) surface after ethylene treatment at 1150 K. Size 150 × 150 A. Tunnel parameters: bias = 30 mV, current = 2 hA.
This work was supported by Russian Foundation of Basic Research No. 95-03-96676.
References this structure to the pseudo-C-(3 X 1)-Pt(110) reconstruction. However, further STM and XPS investigations are needed.
4. Conclusions The state and the structure of carbon adlayers formed by ethylene decomposition at Pt(110) plane were studied by XPS, LEED and STM. The different carbon species were proved to form depending on the temperature of C 2 H 4 adsorption. No graphite (sp 2 hybridization) structures were found. At T = 700-850 K chemisorbed (with no C - C , but P t - C bonds only) carbon layer grows. At higher temperatures additional accumulation of C atoms occurs and
[1] K. Baron, D.W. Blakely, G.A. Somorjai, Surf. Sci. 41 (1974) 45. [2] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger, G. Comsa, Appl. Phys. A 53 (1991) 414. [3] A.Y. Tontegode, Prog. Surf. Sci. 38 (1991) 201. [4] A.I. Boronin, V.I. Bukhtiyarov, R. Kvon, V.V. Chesnokov, R.A. Buyanov, Surf. Sci. 258 (1991) 289. [5] A.I. Boronin, V.I. Bukhtiyarov, R. Kvon, V.V. Chesnokov, R.A. Buyanov, Rep. Sib. Br. USSR Acad. Sci. 2 (1990) 79, in Russian. [6] D.N. Belton, S.J. Shmieg, Surf. Sci. 233 (1990) 131. [7] S.K. Shaikhutdinov, D.I. Kochubey, J. Struct. Chem. 35 (1993) 956. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. [9] T.L. Barr, M. Yin, J. Vac. Sci. Technol. A 10 (1992) 2788. [10] Y. Mizokava, T. Miyasato, S. Nakamura, K.M. Geib, C.W. Wilmsen, J. Vac. Sci. Technol. A 5 (1987) 2809.