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AES and XPS spectra of sulfur in sulfur compounds
Applications of Surface Science 7 (1981) 325—331 North-Holland Publishing Company
AES AND XPS SPECTRA OF SULFUR IN SULFUR COMPOUNDS D. LICHTMAN, J.H. CRAIG, Jr.
“,
V. SAILER and M. DRINKWINE *
Physics Department and Laboratory for Surface Studies, University of Wisconsin—Milwaukee, Milwaukee, Wisconsin 5320], USA Received 2 September 1980 Revised manuscript received 24 November 1980
A wide variety of sulfides were studied using AES and XPS. Spectra of the LMM Auger transition and the 2p level of sulfur in these sulfides are presented and discussed. The appearance of a doublet structure in the XPS spectrum of the S 2p level for many of the sulfides is particularly significant. It is proposed that this splitting, typically about 7 eV, results from the formation of a thin sulfate surface layer. The observation is also made that the sulfides of the fourth row elements seem especially susceptible to this sulfate formation. No definite correlation could be made between the appearance of this surface complex and changes in the sulfur Auger line shape.
1. Introduction A program of research has recently been initiated whichis designed to use modern surface analytical techniques to obtain detailed morphology and chemical information on individual micron-sized airborne particulates. Using high resolution Auger electron spectroscopy, it is now possible to examine the elemental composition of individual particles. However, in addition to elemental analysis, it would be most desirable to extract information relating to the chemical state in which species find themselves. Since sulfur and its chemical compounds are one of the main concerns in airborne particulates, detailed chemical information on sulfur is most important. In particular, identification of specific sulfur compounds in these particles would be extremely useful in the chemical characterization process. The approach which has been taken in these studies is to acquire detailed analytical data (both AES and XPS) on many metal sulfides, focussing on the primary sulfur LMM Auger (150.6 eV) and 2p XPS (164 eV) lines. Once the energies and line *
Present address: Department of Physics, University of Nebraska at Omaha, Omaha, Nebraska
68182, USA. ~ Present address: ITT, Electro-Optical Products Division, 7635 Plantation Road, Roanoke, Virginia 24019, USA.
0 378—5963/81/0000---0000/$ 02.50 © North-Holland
Publishing Company
326
D. Lichtman etal. /AES and XPS spectra of sulfur
shapes corresponding to the sulfur species have been measured for different compounds of sulfur, this data may then serve as a standard to help identify the presence of specific sulfur compounds in particulate matter of unknown composition. AES and XPS spectra of sulfur compounds covering a wide range of the periodic table have been compiled and are presented below. Although some previous work of this type has been done on several specific sulfur compounds [1—11,l3—l5], the data presented below represents the most exhaustive compilation of AES and XPS sulfide chemical data published to date. The primary purpose of this paper is to disseminate the information acquired on the many sulfides studied. Several interesting features of the spectra wifi be pointed out since the appearance of doublets in the XPS spectra and structure in the sulfur Auger peaks is very strongly dependent upon where the metal species associated with the sulfide happens to be located in the periodic table.
2. Experimental considerations All experiments were conducted in a commercial UHF system equipped with a high resolution single pass Varian CMA (10 kV, 1—2 p beam size integral electron gun) and a Physical Electronics XPS module, the heart of which is a double pass CMA. The sulfides studied were typical high purity powders. The powders were removed from their sealed containers (as received from the supplier) on the day they were to be put into the system. They underwent no special aging or other processing. The data therefore is representative of the samples under the conditions described. The powder samples were then pressed between two thin layers of indium foil. Separation of the two indium layers resulted in samples which were easily mounted in the UHV system for analysis. In acquiring all Auger data, a standardized procedure was developed and followed which minimizes the possibility of instrumental distortion of the Auger spectra. Before each new sample was analyzed, the 2 kV elastic peak was monitored and the sample position adjusted to place it precisely at the focal point of the CMA. Initially a full scan was acquired to verify composition of the sample. To measure the sulfur Auger peak, the CMA was scanned between the limits 120—170 eV, bracketing the 150.6 eV sulfur peak. All spectra were measured using a modulation voltage of 1 V p—p to ensure that line shape distortion would not be introduced by overmodulation. The voltage values indicated for the Auger peaks are not corrected or charge referenced. The intent here is to provide values which are to be compared to each other for the various peaks. The width of the Auger peaks is such that absolute energy values do not seem to be really significant. Initially, it was feared that significant beam induced damage as well as substantial charging effects would occur in many of the sulfur compounds. Many experiments were performed to test the effect of the bombarding electron beam. Specifically, data were taken on virgin
D. Lichtman et al. /AES and XPS spectra of sulfur
327
regions and compared to data taken at various bombardment times. It was found that nearly all the sulfides studied were very stable under electron bombardment, exhibiting very reproducible Auger spectra as long as the input power densities were kept sufficiently low to prevent thermal effects. Sample charging also proved not to be a significant problem with the sulfides. However, it should be pointed out that several sulfites and sulfates studied, exhibited large beam induced effects and were very unstable during electron bombardment. Most sulfide spectra measured were obtained using a beam energy of 2 kV and beam current of 2 pA. The )(PS spectra were acquired in a manner similar to the AES procedure. The photon beam used was the Kcs line from a magnesium anode. Initially a full scan was obtained to verify that all appropriate peaks were present. Subsequent spectra were then acquired over a narrower energy range to permit location of the sulfur 2p peak and to determine if a singlet or doublet was present. In all cases, the peak positions indicated on the XPS spectra below were charge referenced to the carbon 284.6 eV line associated with hydrocarbon impurity present on the surface.
3. Results and discussion The results of AES and XPS studies on a wide variety of sulfides are presented in figs. la—f. The derivative Auger spectra shown focus on the sulfur LMM transition. This line is shown in detail in order to demonstrate shape and structure changes which occur as the sulfur atom is subjected to different chemical environments. The accompanying XPS spectrum for each of the sulfides shown focuses on the sulfur 2p level. The appearance of doublets and the magnitude of the splitting in many of the S(2p) sulfide XPS spectra is particularly interesting. A careful examination of full scan XPS and AES spectra of each sulfide reveals the following features: (1) Every sulfide exhibiting an S(2p) doublet also shows evidence of the presence of oxygen; (2) Those sulfides which exhibited only a singlet S(2p) state showed little or no detectable oxygen present; (3) There is not a strong correlation between the existence of structure in the Auger spectra and doublet appearance in the S(2p) line. For example, sulfides of Co, Ni and Cu reveal a multiplet structure in both AES and XPS. However, Ag2S and CdS exhibit structure in the sulfur Auger peak, but the S(2p) line is a singlet; (4) Only sulfides of Mo and Hg revealed neither structure in Auger nor XPS sulfur spectra and showed no significant oxygen present. ZnS displayed a singlet S(2p) peak and a barely resolvable shoulder in the Auger spectra and also showed no significant oxygen present. It is clear that the presence of oxygen has a profound effect on the sulfide XPS spectra, whereas little if any correlation is found between Auger line structure and oxygen presence. The critical ingredient involved in attempting to understand the origin of the doublet structures is the binding energies associated with each of the two lines. Every sulfide exhibiting a doublet has the higher energy peak located in
328
D. Lichtpnan et al.
/ AES and
XPS spectra of sulfur
r
_
~
~v (a)
(c) Fig. 1. (a) XPS and AES spectra of K
~
~
(b)
(d)
2S, CaS, TiS2 and V2S3. For each sulfide in figs. la—f the upper curve is the XPS spectrum of the S2p peak scanned from 200—150 eV. All peak positions have been charge referenced to the C is line at 284.6 eV. The lower curve is the sulfur LNIM Auger line shown from 132.5—170 éV. All indicated peak positions are in eV; (b) XPS and AES spectra of Cr2S3, FeS, CoS and NiS; (c) XPS and AES spectra of CuS, Cu2S, ZnS and ZrS2 (d) XPS and AES spectra of NbS2, MoS2, Ag2 S and CdS.
D. Lichtinan et al.
(e)
/ AES and
XPS spectra of sulfur
329
(f
fig. 1. (e) XPS and AES spectra of In
2S3, SnS, Sb2S3 and BaS; (f) XPS and AES spectra of
WS2, FIgS, PbS and BiS.
the 168—170 eV range and the low energy peak in the 161—163 eV region. The typical observed doublet splitting of about 7 eV is larger than is normally associated with changes in chemical bonding among the different sulfides. A more likely explanation is that the low energy peak is associated with the sulfur in a sulfide configuration. On the other hand, the high energy peak is undoubtedly related to the presence of a surface sulfate complex. That this explanation is reasonable sup2p binding energies in theisrange ported by eV previously published XPSfor results. 160—163 have been measured Na Sulfur 2S [4], PbS [5,13], SnS [5], FeS [6], MoS2 [7], WS2 [8], and NiS [8]. In the sulfate state, S(2p) binding energies have been observed, for example in Na2 SO4 [4], FeSO4 [4], and LiSO4 [12], to lie in the 168—1 69 eV range. Furthermore, in recent XPS studies by Manocha and Park [13] on PbS, and Brion [14] on several sulfide minerals, the appearance ofa doublet structure in the S(2p) state was observed upon exposure of the sulfide to air. In both cases, the high energy peak was assumed to be due to the formation of a sulfate surface layer. It is also interesting to note that for the case of sphalerite (ZnS) oxidation of the surface layer to ZnSO4 was a very slow process [14]. This observation is completely consistent with our data on ZnS powder which exhibited a singlet S(2p) peak and no evidence of oxygen in either AES or XPS spectra. Consequently, the evidence clearly points to the correspondence between the S(2p) splitting in
330
D. Lichtman eta!. /AES and XPS spectra of sulfur
our data and the presence of a sulfate component in the surface region. It is certainly reasonable to speculate that a thin sulfate layer could form during sample preparation prior to insertion into the UHV environment. The fact that the sulfide peak is of comparable size to the sulfate peak indicates that the sulfate layer cannot be either too complete or too thick (i.e., probably less than 10 A). One additional observation should be made regarding the formation of surface sulfates. It is quite apparent from the appearance of the S(2p) doublets that sulfides of the fourth row elements seem especially susceptible to formation of these surface sulfate complexes. Beyond copper, however, only SnS and PbS exhibited any substantial doublet which might indicate sulfate formation. This observation may be of considerable importance in analyzing the chemical composition of real materials. Assuming the existence of a thin sulfate layer on many of the sulfide surfaces, the question arises as to the effect this layer would have on the sulfur Auger peak. Unfortunately, it is not possible to ascertain unambiguously from the Auger data what, if any, contribution to the peak shape and position is made by surface sulfur atoms in a sulfate complex. One should also consider the fact that the Auger electrons are of lower energy than the XPS electrons and, therefore, have a somewhat shorter mean free path. It is not clear how this difference would explain any of the differences observed in our data, since the sulfate layer, where it exists, does not obscure the sulfide material and, therefore, cannot be too thick. As indicated above, several sulfides show at least some structure in the Auger sulfur peak. However, only the sulfides of copper and silver exhibit pronounced structure. In fact these sulfides reveal a complex triplet structure. It is unlikely that this complex line shape is related to the sulfate formation discussed above. The Ag2S sample showed no significant levels of oxygen present and a singlet S(2p) peak. However, the two copper sulfides clearly had a sulfate surface layer as evidenced by the doublet appearance. Yet, all three sulfides show similar splittings between the multiplet peaks although the peak height ratios are substantially different in Ag2 S. It is thus quite likely that the complex line shape observed for these sulfides is a function of the electronic structure of the metal atoms and is not related to the oxidation state of the surface sulfur.
4. Summary AES and XPS spectra of sulfur in a wide variety of sulfides has been presented. The striking appearance of doublets in the S(2p) peak for many of the fourth row metal suluIdes is attributed to the presence of a thin layer of sulfate on the sample surface. Several sulfides exhibited structure in the sulfur Auger peak, but this is thought to be unrelated to the sulfate formation especially in the cases of CuS, ~i2S and Ag2S. It is anticipated that compilation of data of this type will provide a standard which can be used to help in the determination of chemical composition of atmospheric particulates, among other applications.
D. Lichtman eta!. /AES and XPS spectra of sulfur
331
Acknowledgement The authors wish to acknowledge the support of the Electric Power Research Institute during the course of this work. Also, we wish to acknowledge the aid of Dr. Peter Sherwood in providing valuable insight into the interpretation of some of the data.
References [1] E.N. Sickafus and F. Steinisser, J. Vacuum Sci. Technol. 10 (1973) 43. [2] M.K. Bernett, J.S. Murday and N.H. Turner, J. Electron Spectry. Relat. Phenom. 12 (1977) 375. [3] W. Losch and A.J. Monhemius, Surface Sci. 60 (1976) 196. [4] B.J. Lindberg, K. Hamrin, G. Johansson, U. Gelius, A. Fahlmann, C. Nordberg and K. Siegbahn, Phys. Scr. 1(1970) 286. [5] R.B. Shalvoy, GB. Fisher and P.J. Stiles, Phys. Rev. B15 (1977) 1680. [6] H. Binder, Z. Naturforsch. B28 5 (1973) 256. [7] T.A. Patterson, J.C. Carver, D.E. Leyden and D.M. Hercules, J. Phys. Chem. 80 (1976) 1702. [8] K.T. Ng and D.M. Hercules, 1. Phys. Chem. 80 (1976) 2095. [9] N.H. Turner, iS. Murday and D.E. Ramaker, J. Vacuum Sci. Technol. 17 (1980) 214. [10] W. Losch, J. Vacuum Sci. Technol. 16 (1979) 865. [11] H. Windawi and J.R. Katzer, J. Vacuum Sd. Technol. 16(1979)497. [12] R.O. Ansell, T. Dickinson, A.F. Povey and P.M.A. Sherwood, J. Electroanal. Chem. 98
(1979) 79. [13] A.S. Manocha and R.L. Park, Appl. Surface Sci. 1(1977)129. [141 D. Brion, Appl. Surface Sci. 5 (1980) 133. [15] N.H. Turner, J.S. Murday and D.E. Ramaker, Anal. Chem. 52 (1980) 84.
AES AND XPS SPECTRA OF SULFUR IN SULFUR COMPOUNDS D. LICHTMAN, J.H. CRAIG, Jr.
“,
V. SAILER and M. DRINKWINE *
Physics Department and Laboratory for Surface Studies, University of Wisconsin—Milwaukee, Milwaukee, Wisconsin 5320], USA Received 2 September 1980 Revised manuscript received 24 November 1980
A wide variety of sulfides were studied using AES and XPS. Spectra of the LMM Auger transition and the 2p level of sulfur in these sulfides are presented and discussed. The appearance of a doublet structure in the XPS spectrum of the S 2p level for many of the sulfides is particularly significant. It is proposed that this splitting, typically about 7 eV, results from the formation of a thin sulfate surface layer. The observation is also made that the sulfides of the fourth row elements seem especially susceptible to this sulfate formation. No definite correlation could be made between the appearance of this surface complex and changes in the sulfur Auger line shape.
1. Introduction A program of research has recently been initiated whichis designed to use modern surface analytical techniques to obtain detailed morphology and chemical information on individual micron-sized airborne particulates. Using high resolution Auger electron spectroscopy, it is now possible to examine the elemental composition of individual particles. However, in addition to elemental analysis, it would be most desirable to extract information relating to the chemical state in which species find themselves. Since sulfur and its chemical compounds are one of the main concerns in airborne particulates, detailed chemical information on sulfur is most important. In particular, identification of specific sulfur compounds in these particles would be extremely useful in the chemical characterization process. The approach which has been taken in these studies is to acquire detailed analytical data (both AES and XPS) on many metal sulfides, focussing on the primary sulfur LMM Auger (150.6 eV) and 2p XPS (164 eV) lines. Once the energies and line *
Present address: Department of Physics, University of Nebraska at Omaha, Omaha, Nebraska
68182, USA. ~ Present address: ITT, Electro-Optical Products Division, 7635 Plantation Road, Roanoke, Virginia 24019, USA.
0 378—5963/81/0000---0000/$ 02.50 © North-Holland
Publishing Company
326
D. Lichtman etal. /AES and XPS spectra of sulfur
shapes corresponding to the sulfur species have been measured for different compounds of sulfur, this data may then serve as a standard to help identify the presence of specific sulfur compounds in particulate matter of unknown composition. AES and XPS spectra of sulfur compounds covering a wide range of the periodic table have been compiled and are presented below. Although some previous work of this type has been done on several specific sulfur compounds [1—11,l3—l5], the data presented below represents the most exhaustive compilation of AES and XPS sulfide chemical data published to date. The primary purpose of this paper is to disseminate the information acquired on the many sulfides studied. Several interesting features of the spectra wifi be pointed out since the appearance of doublets in the XPS spectra and structure in the sulfur Auger peaks is very strongly dependent upon where the metal species associated with the sulfide happens to be located in the periodic table.
2. Experimental considerations All experiments were conducted in a commercial UHF system equipped with a high resolution single pass Varian CMA (10 kV, 1—2 p beam size integral electron gun) and a Physical Electronics XPS module, the heart of which is a double pass CMA. The sulfides studied were typical high purity powders. The powders were removed from their sealed containers (as received from the supplier) on the day they were to be put into the system. They underwent no special aging or other processing. The data therefore is representative of the samples under the conditions described. The powder samples were then pressed between two thin layers of indium foil. Separation of the two indium layers resulted in samples which were easily mounted in the UHV system for analysis. In acquiring all Auger data, a standardized procedure was developed and followed which minimizes the possibility of instrumental distortion of the Auger spectra. Before each new sample was analyzed, the 2 kV elastic peak was monitored and the sample position adjusted to place it precisely at the focal point of the CMA. Initially a full scan was acquired to verify composition of the sample. To measure the sulfur Auger peak, the CMA was scanned between the limits 120—170 eV, bracketing the 150.6 eV sulfur peak. All spectra were measured using a modulation voltage of 1 V p—p to ensure that line shape distortion would not be introduced by overmodulation. The voltage values indicated for the Auger peaks are not corrected or charge referenced. The intent here is to provide values which are to be compared to each other for the various peaks. The width of the Auger peaks is such that absolute energy values do not seem to be really significant. Initially, it was feared that significant beam induced damage as well as substantial charging effects would occur in many of the sulfur compounds. Many experiments were performed to test the effect of the bombarding electron beam. Specifically, data were taken on virgin
D. Lichtman et al. /AES and XPS spectra of sulfur
327
regions and compared to data taken at various bombardment times. It was found that nearly all the sulfides studied were very stable under electron bombardment, exhibiting very reproducible Auger spectra as long as the input power densities were kept sufficiently low to prevent thermal effects. Sample charging also proved not to be a significant problem with the sulfides. However, it should be pointed out that several sulfites and sulfates studied, exhibited large beam induced effects and were very unstable during electron bombardment. Most sulfide spectra measured were obtained using a beam energy of 2 kV and beam current of 2 pA. The )(PS spectra were acquired in a manner similar to the AES procedure. The photon beam used was the Kcs line from a magnesium anode. Initially a full scan was obtained to verify that all appropriate peaks were present. Subsequent spectra were then acquired over a narrower energy range to permit location of the sulfur 2p peak and to determine if a singlet or doublet was present. In all cases, the peak positions indicated on the XPS spectra below were charge referenced to the carbon 284.6 eV line associated with hydrocarbon impurity present on the surface.
3. Results and discussion The results of AES and XPS studies on a wide variety of sulfides are presented in figs. la—f. The derivative Auger spectra shown focus on the sulfur LMM transition. This line is shown in detail in order to demonstrate shape and structure changes which occur as the sulfur atom is subjected to different chemical environments. The accompanying XPS spectrum for each of the sulfides shown focuses on the sulfur 2p level. The appearance of doublets and the magnitude of the splitting in many of the S(2p) sulfide XPS spectra is particularly interesting. A careful examination of full scan XPS and AES spectra of each sulfide reveals the following features: (1) Every sulfide exhibiting an S(2p) doublet also shows evidence of the presence of oxygen; (2) Those sulfides which exhibited only a singlet S(2p) state showed little or no detectable oxygen present; (3) There is not a strong correlation between the existence of structure in the Auger spectra and doublet appearance in the S(2p) line. For example, sulfides of Co, Ni and Cu reveal a multiplet structure in both AES and XPS. However, Ag2S and CdS exhibit structure in the sulfur Auger peak, but the S(2p) line is a singlet; (4) Only sulfides of Mo and Hg revealed neither structure in Auger nor XPS sulfur spectra and showed no significant oxygen present. ZnS displayed a singlet S(2p) peak and a barely resolvable shoulder in the Auger spectra and also showed no significant oxygen present. It is clear that the presence of oxygen has a profound effect on the sulfide XPS spectra, whereas little if any correlation is found between Auger line structure and oxygen presence. The critical ingredient involved in attempting to understand the origin of the doublet structures is the binding energies associated with each of the two lines. Every sulfide exhibiting a doublet has the higher energy peak located in
328
D. Lichtpnan et al.
/ AES and
XPS spectra of sulfur
r
_
~
~v (a)
(c) Fig. 1. (a) XPS and AES spectra of K
~
~
(b)
(d)
2S, CaS, TiS2 and V2S3. For each sulfide in figs. la—f the upper curve is the XPS spectrum of the S2p peak scanned from 200—150 eV. All peak positions have been charge referenced to the C is line at 284.6 eV. The lower curve is the sulfur LNIM Auger line shown from 132.5—170 éV. All indicated peak positions are in eV; (b) XPS and AES spectra of Cr2S3, FeS, CoS and NiS; (c) XPS and AES spectra of CuS, Cu2S, ZnS and ZrS2 (d) XPS and AES spectra of NbS2, MoS2, Ag2 S and CdS.
D. Lichtinan et al.
(e)
/ AES and
XPS spectra of sulfur
329
(f
fig. 1. (e) XPS and AES spectra of In
2S3, SnS, Sb2S3 and BaS; (f) XPS and AES spectra of
WS2, FIgS, PbS and BiS.
the 168—170 eV range and the low energy peak in the 161—163 eV region. The typical observed doublet splitting of about 7 eV is larger than is normally associated with changes in chemical bonding among the different sulfides. A more likely explanation is that the low energy peak is associated with the sulfur in a sulfide configuration. On the other hand, the high energy peak is undoubtedly related to the presence of a surface sulfate complex. That this explanation is reasonable sup2p binding energies in theisrange ported by eV previously published XPSfor results. 160—163 have been measured Na Sulfur 2S [4], PbS [5,13], SnS [5], FeS [6], MoS2 [7], WS2 [8], and NiS [8]. In the sulfate state, S(2p) binding energies have been observed, for example in Na2 SO4 [4], FeSO4 [4], and LiSO4 [12], to lie in the 168—1 69 eV range. Furthermore, in recent XPS studies by Manocha and Park [13] on PbS, and Brion [14] on several sulfide minerals, the appearance ofa doublet structure in the S(2p) state was observed upon exposure of the sulfide to air. In both cases, the high energy peak was assumed to be due to the formation of a sulfate surface layer. It is also interesting to note that for the case of sphalerite (ZnS) oxidation of the surface layer to ZnSO4 was a very slow process [14]. This observation is completely consistent with our data on ZnS powder which exhibited a singlet S(2p) peak and no evidence of oxygen in either AES or XPS spectra. Consequently, the evidence clearly points to the correspondence between the S(2p) splitting in
330
D. Lichtman eta!. /AES and XPS spectra of sulfur
our data and the presence of a sulfate component in the surface region. It is certainly reasonable to speculate that a thin sulfate layer could form during sample preparation prior to insertion into the UHV environment. The fact that the sulfide peak is of comparable size to the sulfate peak indicates that the sulfate layer cannot be either too complete or too thick (i.e., probably less than 10 A). One additional observation should be made regarding the formation of surface sulfates. It is quite apparent from the appearance of the S(2p) doublets that sulfides of the fourth row elements seem especially susceptible to formation of these surface sulfate complexes. Beyond copper, however, only SnS and PbS exhibited any substantial doublet which might indicate sulfate formation. This observation may be of considerable importance in analyzing the chemical composition of real materials. Assuming the existence of a thin sulfate layer on many of the sulfide surfaces, the question arises as to the effect this layer would have on the sulfur Auger peak. Unfortunately, it is not possible to ascertain unambiguously from the Auger data what, if any, contribution to the peak shape and position is made by surface sulfur atoms in a sulfate complex. One should also consider the fact that the Auger electrons are of lower energy than the XPS electrons and, therefore, have a somewhat shorter mean free path. It is not clear how this difference would explain any of the differences observed in our data, since the sulfate layer, where it exists, does not obscure the sulfide material and, therefore, cannot be too thick. As indicated above, several sulfides show at least some structure in the Auger sulfur peak. However, only the sulfides of copper and silver exhibit pronounced structure. In fact these sulfides reveal a complex triplet structure. It is unlikely that this complex line shape is related to the sulfate formation discussed above. The Ag2S sample showed no significant levels of oxygen present and a singlet S(2p) peak. However, the two copper sulfides clearly had a sulfate surface layer as evidenced by the doublet appearance. Yet, all three sulfides show similar splittings between the multiplet peaks although the peak height ratios are substantially different in Ag2 S. It is thus quite likely that the complex line shape observed for these sulfides is a function of the electronic structure of the metal atoms and is not related to the oxidation state of the surface sulfur.
4. Summary AES and XPS spectra of sulfur in a wide variety of sulfides has been presented. The striking appearance of doublets in the S(2p) peak for many of the fourth row metal suluIdes is attributed to the presence of a thin layer of sulfate on the sample surface. Several sulfides exhibited structure in the sulfur Auger peak, but this is thought to be unrelated to the sulfate formation especially in the cases of CuS, ~i2S and Ag2S. It is anticipated that compilation of data of this type will provide a standard which can be used to help in the determination of chemical composition of atmospheric particulates, among other applications.
D. Lichtman eta!. /AES and XPS spectra of sulfur
331
Acknowledgement The authors wish to acknowledge the support of the Electric Power Research Institute during the course of this work. Also, we wish to acknowledge the aid of Dr. Peter Sherwood in providing valuable insight into the interpretation of some of the data.
References [1] E.N. Sickafus and F. Steinisser, J. Vacuum Sci. Technol. 10 (1973) 43. [2] M.K. Bernett, J.S. Murday and N.H. Turner, J. Electron Spectry. Relat. Phenom. 12 (1977) 375. [3] W. Losch and A.J. Monhemius, Surface Sci. 60 (1976) 196. [4] B.J. Lindberg, K. Hamrin, G. Johansson, U. Gelius, A. Fahlmann, C. Nordberg and K. Siegbahn, Phys. Scr. 1(1970) 286. [5] R.B. Shalvoy, GB. Fisher and P.J. Stiles, Phys. Rev. B15 (1977) 1680. [6] H. Binder, Z. Naturforsch. B28 5 (1973) 256. [7] T.A. Patterson, J.C. Carver, D.E. Leyden and D.M. Hercules, J. Phys. Chem. 80 (1976) 1702. [8] K.T. Ng and D.M. Hercules, 1. Phys. Chem. 80 (1976) 2095. [9] N.H. Turner, iS. Murday and D.E. Ramaker, J. Vacuum Sci. Technol. 17 (1980) 214. [10] W. Losch, J. Vacuum Sci. Technol. 16 (1979) 865. [11] H. Windawi and J.R. Katzer, J. Vacuum Sd. Technol. 16(1979)497. [12] R.O. Ansell, T. Dickinson, A.F. Povey and P.M.A. Sherwood, J. Electroanal. Chem. 98
(1979) 79. [13] A.S. Manocha and R.L. Park, Appl. Surface Sci. 1(1977)129. [141 D. Brion, Appl. Surface Sci. 5 (1980) 133. [15] N.H. Turner, J.S. Murday and D.E. Ramaker, Anal. Chem. 52 (1980) 84.