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Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001)
Surface Science 82 (1979) 511-516 0 North-Holland Publishing Company
EFFECTS OF ADSORBATES ON ELECTRON SPIN POLARIZATION IN LOW ENERGY ELECTRON DIFFRACTION FROM TUNGSTEN (001) * I. Disordered CO and O2 overlayers T.W. RIDDLE, A.H. MAHAN, F.B. DUNNING ** and G.K. WALTERS Department
of Physics, Rice University, Houston,
Texas 77001,
USA
Received 6 September 1978; manuscript received in final form 24 November 1978
Electron spin polarization and intensity profiles have been measured in low energy electron diffraction (LEED) for the (00) beam at 8 = 13” and @= 0” from W(OO1) as a function of surface exposure to CO and 02. Significant changes have been observed in the profiles upon exposure to both adsorbate gases, and the implications of these results are discussed.
1. Introduction The properties of crystal surfaces, both clean and with adsorbate overlayers, have been widely investigated by measurement of the periodicities and intensities of the discrete beams of scattered electrons produced in low energy electron diffraction (LEED). Recently, studies of the degree of electron spin polarization of LEED beams have also been undertaken in an attempt to obtain further insight into surface parameters and comparisons between theory and experiment seem promising [l-6]. Surfaces covered with adsorbate layers, on the other hand, are not well understood, with the precise location and orien)tation of the adsorbed species as yet unknown in many cases. However, because calculations for clean surfaces have shown that measurements of polarization in LEED may yield information concerning surface parameters not obtainable from intensity studies alone [4,5], measurements of the spin polarization of the (00) beam from a W(OO1) surface covered with different adsorbate overlayers have been undertaken, together with measurements of the intensity, to evaluate the potential of spin polarization measurements in characterizing adsorbate covered surfaces. In addition, an important practical reason for undertaking such measurements is to determine whether electron spin polar* Supported by the Office of Basic Energy Sciences, US Department Robert A. Welch Foundation. ** Alfred P. Sloan Fellow. 511
of Energy, and the
512
T. W. Riddle et al. /Effects
of adsorbates on electron spin polarization. I
ization can be used to provide a sensitive monitor of surface cleanliness, and to determine just how clean a surface need be in order to yield polarization values truly representative of a clean surface. The present paper thus reports the results of measurements of the intensity and polarization of the specular (00) beam from a W(OO1) surface at an angle of incidence 0 = 13” and azimuthal angle r$ = 0” as a function of surface exposure to CO and O2 up to 1 Langmuir (1 L = 10e6 Torr set). The polarizations associated with the (00) beam from a clean W(OO1) surface have been reported elsewhere [2]. The rise in background intensity between the primary LEED beams, the absence of any additional LEED spots, and the fact that adsorption occurs on a relatively cool surface indicate that the polarizations and intensities are characteristic of disordered adsorbate layers. The rather different behavior of a W(OO1) surface covered with an ordered CO overlayer, obtained by annealing the adsorbate layer, is discussed in the following paper.
2. Experimental The experimental apparatus has been discussed elsewhere [2] and will not be described in detail here. Briefly, LEED is performed in a chamber with a base pressure of -1.5 X lo-” Torr using a conventional commercial LEED optics assembly and electron gun. The LEED optics and tungsten crystal are mounted on manipulators which permit any one of several LEED beams to be directed through a slit cut in the phosphor LEED screen and into the fured entrance aperture of an accelerating column. The electrons are accelerated to 100 keV as they pass through this column and enter a second vacuum chamber where their polarization is determined by use of the usual Mott scattering technique [7]. The tungsten surface was cleaned prior to each exposure to the gas of interest by resistive heating to 1200°C for 3 min in lo- ’ Torr of oxygen, followed by repeated flashings in high vacuum to 24OO’C. Auger electron spectroscopy showed that no contaminants, including carbon, remained on the surface following this treatment. Further evidence that the surface is indeed clean is provided by the fact that intensity profiles measured from this clean surface have been shown to agree well with those of other workers as is discussed in in detail in ref. [2]. The polarization as a function of time, and hence surface exposure, was measured as an initially clean surface was exposed to a constant pressure of CO or 02. Immediately following the final cleaning flash, the adsorbate gas was admitted into the vacuum chamber using a UHV leak valve. Pressures of 3 X 10m9 and 4 X 10e9 Torr for CO and 0s respectively were typically employed, corresponding to exposure rates of 0.18 L min-’ for CO and 0.24 L min-’ for OZ. The gas pressure was measured by a nude ion gauge located near the tungsten crystal, and was maintained at a constant value throughout a given series of measurements [8]. The polarization of the (00) beam was measured over successive 15 set intervals until a
T. W. Riddle et al. /Effects of adsorbates on electron spin polarization. I
513
total gas exposure of 1 L was reached. The changes in polarization resulting from exposure to CO and Oa were found to be very reproducible and the results in this paper represent the averages of data obtained from repeated measurements. The overall statistical uncertainty in the data is -3%. In addition, there may be an offset _ f 3% in the polarization profiles which may arise from systematic errors in the of < determination of the instrumental asymmetry. The intensity profiles, on the other hand, were measured by use of a spot photometer. For these measurements the crystal was first exposed to the desired amount of gas, after which the gas inlet leak valve was closed and the remaining gas in the chamber pumped out, and a complete intensity versus energy scan was recorded. The form of the intensity profiles obtained by use of the spot photometer agreed well with those inferred from independent measurements of the current recorded at a Faraday cup located in the Mott scattering chamber, which also provides a measure of the intensity of the beam. In order to test for any effects in the measurements resulting from either the changing crystal temperature or background gas deposition, polarization determinations were undertaken both at different exposure rates, ranging from 0.24 to 0.43 L min-', and with different time delays between flashing the crystal and admitting the adsorbate gas. These studies confirmed that neither temperature nor background gas significantly influenced the measurements reported here. This is to be expected in view of the low background gas pressure, and the fact that, since the majority of the crystal cooling occurs during the time interval required after a flash to admit the adsorbate gas and stabilize its pressure, changes in temperature during data acquisition are relatively small in magnitude and are expected to result in only small polarization changes, even near large polarization features [9].
3. Results and discussion Polarization and intensity profiles versus energy as a function of adsorbate exposure to CO and Oa are shown in figs. 1 and 2 for electron energies between 26 and 141 eV. The polarization profiles in these figures were drawn through data points taken at 5 eV intervals, except in the regions of the sharp features located near 30 eV and 75 eV where data were recorded at intervals of 1 eV or smaller. The intensity curves represent a continuous scan, obtained with a spot photometer, as described in the preceding section. It is evident from figs. 1 and 2 that both polarization and intensity are significantly influenced by surface exposure to CO or Os, and that the resultant changes are of the same general character. In particular, energy regions are observed in which either the polarization or the intensity, but generally not both, is especially sensitive to surface exposure. For example, the polarization at 75 eV decreases in magnitude from about -75% to -0% with little attendant change in beam intensity, upon exposure to only -0.5 L of either CO or Oa, while the intensity at -57 eV increases by a factor of -8 for the same 0.5 L exposure without pro-
514
T. W. Riddle et al. /Effects
of adsorbates on electron spin polarization. I
Energy
(eV
)
Fig. 1. Electron spin polarization and intensity of the (00) beam at 0 = 13” and @= 0” as a function of incident electron energy for selected exposures of the clean W(OO1) surface to CO.
nounced changes in polarization. Indeed, an intensity minimum is observed at -57 eV for the clean surface, whereas that energy is close to an intensity maximum following the same 0.5 L surface exposure. The fact that there exist energy regions where either the polarization or intensity is extremely sensitive to surface exposure suggests that measurements of these quantities in such regions might be used to monitor surface cleanliness. Polarization is perhaps a more useful measurement in this respect because it is absolute, independent of incident beam current, collection efficiencies, etc., factors which will cause the measured intensity to vary in magnitude. Indeed, in this laboratory we have found that measurement of the polarization at 76 eV, which corresponds to the peak of the large negative feature, provides a sensitive and convenient day to day monitor and check on surface cleanliness. In addition, the marked sensitivity of the polarization to surface contamination indicates that considerable care must be exercise when making polarization measurements on a clean surface to insure that the surface is indeed clean. The present data for both CO and Oa also serve to emphasize again the complementary nature of measurements of the electron spin polarization and intensity features previously noted in studies of LEED from clean surfaces: It is evident from
T. W.Riddle et al. /Effects of adsorbates on electron spin polarization. I
-8clean 0.24 L 0.48 L 0.72 L I.2 L
30
50
70 Energy
90
110
515
* 5
6x
-
$
k p
-4.z
130
(eV)
Fig. Z.-Electron spin polarization and intensity of the (00) beam at 0 = 13” and 4~= 0” as a function of incident electron energy for selected exposures of the clean W(OO1) surface to 02.
the present work that not only do electron spin polarization maxima generally occur at electron energies corresponding to regions of intensity minima, but also, as noted previously, that at energies where neither the polarization or intensity varies markedly with surface exposure, the other typically remains relatively constant. While the changes in polarization and intensities observed as a result of exposure to CO and O2 are of the same general character, detailed differences in the behavior are apparent. Perhaps the most noticeable example occurs near 70 eV where exposure to CO is found to result in a large, positive polarization feature which is not as evident in the case of exposure to OZ. The appearance of this positive polarization feature at low CO exposures again points to the need for strict surface cleanliness when undertaking measurements pertaining to a clean surface, since CO is normally a major constituent of the background gas. Further, it serves to demonstrate that the polarization does not always decay monotonically to zero with increasing gas exposure. A similar behavior of the pola~zation has been observed in this laboratory in certain energy ranges at other angles of incidence. Detailed differences also exist in the intensity behavior near, for example, 75 eV where the intensity is increased following exposure to O2 but not to CO. Neither does increasing surface exposure necessarily result in a monotonic change in the intensity observed at a pa~i~ular energy. This is readily apparent from an inspection of the data on either side of the intensity feature centered at 55 eV.
516
T. W. Riddle et al. /Effects of adsorbates on electron spin polarization. I
In conclusion, the significant and distinctive effects due to adsorbed gases not only emphasize the need for strict surface cleanliness if results characteristic of a clean surface are to be obtained, but also offer hope that further insight into the properties of adsorbate covered surfaces may ultimately be obtained by a comparison with theoretical calculations incorporating spin.
[l] M.R. O’NeiJl, M. Kalisvaart, F.B. Dunning and G.K. Walters, Phys. Rev. Letters 34 (1975) 1167. [2] M. Kahsvaart, M.R. O’Neill, T.W. Riddle, F.B. Dunning and G.K. Walters, Phys. Rev. B17 (1978) 1570. [3] R.L. Calvert, G.J. Russell and D. Haneman, Phys. Rev. Letters 39 (1977) 1226. [4] P.J. Jennings and R.O. Jones, Surface Sci. 20 (1970) 18; 26 (1971) 509; 27 (1971) 221; 71 (1978) 101; P.J. Jennings and B.K. Sim, Surface Sci. 33 (1972) 1. [5] R. Feder, Phys. Status Solidi B46 (1971) K31; 49 (1972) 699; 56 (1973) K43; 58 (1973) K137; 62 (1974) 135; Surface Sci. 51 (1975) 297; 63 (1977) 283; Phys. Rev. Letters 36 (1976) 598. [6] N. Miiller, D. Wolf and R. Feder, Inst. Phys. Conf. Ser. No. 41 (1978) 281; R. Feder, N. Mtiller and D. Wolf, 2. Physik B28 (1977) 265. [7] J. Van Klinken, Nucl. Phys. 75 (1966) 161. [S] The ion gauge was not absolutely calibrated and the pressures were determined using the gauge sensitivities quoted by the manufacturer. [ 91 T.W. Riddle, A.H. Mahan, F.B. Dunning and G.K. Walters, J. Vacuum Sci. Technol. 15 (1978) 1686.
EFFECTS OF ADSORBATES ON ELECTRON SPIN POLARIZATION IN LOW ENERGY ELECTRON DIFFRACTION FROM TUNGSTEN (001) * I. Disordered CO and O2 overlayers T.W. RIDDLE, A.H. MAHAN, F.B. DUNNING ** and G.K. WALTERS Department
of Physics, Rice University, Houston,
Texas 77001,
USA
Received 6 September 1978; manuscript received in final form 24 November 1978
Electron spin polarization and intensity profiles have been measured in low energy electron diffraction (LEED) for the (00) beam at 8 = 13” and @= 0” from W(OO1) as a function of surface exposure to CO and 02. Significant changes have been observed in the profiles upon exposure to both adsorbate gases, and the implications of these results are discussed.
1. Introduction The properties of crystal surfaces, both clean and with adsorbate overlayers, have been widely investigated by measurement of the periodicities and intensities of the discrete beams of scattered electrons produced in low energy electron diffraction (LEED). Recently, studies of the degree of electron spin polarization of LEED beams have also been undertaken in an attempt to obtain further insight into surface parameters and comparisons between theory and experiment seem promising [l-6]. Surfaces covered with adsorbate layers, on the other hand, are not well understood, with the precise location and orien)tation of the adsorbed species as yet unknown in many cases. However, because calculations for clean surfaces have shown that measurements of polarization in LEED may yield information concerning surface parameters not obtainable from intensity studies alone [4,5], measurements of the spin polarization of the (00) beam from a W(OO1) surface covered with different adsorbate overlayers have been undertaken, together with measurements of the intensity, to evaluate the potential of spin polarization measurements in characterizing adsorbate covered surfaces. In addition, an important practical reason for undertaking such measurements is to determine whether electron spin polar* Supported by the Office of Basic Energy Sciences, US Department Robert A. Welch Foundation. ** Alfred P. Sloan Fellow. 511
of Energy, and the
512
T. W. Riddle et al. /Effects
of adsorbates on electron spin polarization. I
ization can be used to provide a sensitive monitor of surface cleanliness, and to determine just how clean a surface need be in order to yield polarization values truly representative of a clean surface. The present paper thus reports the results of measurements of the intensity and polarization of the specular (00) beam from a W(OO1) surface at an angle of incidence 0 = 13” and azimuthal angle r$ = 0” as a function of surface exposure to CO and O2 up to 1 Langmuir (1 L = 10e6 Torr set). The polarizations associated with the (00) beam from a clean W(OO1) surface have been reported elsewhere [2]. The rise in background intensity between the primary LEED beams, the absence of any additional LEED spots, and the fact that adsorption occurs on a relatively cool surface indicate that the polarizations and intensities are characteristic of disordered adsorbate layers. The rather different behavior of a W(OO1) surface covered with an ordered CO overlayer, obtained by annealing the adsorbate layer, is discussed in the following paper.
2. Experimental The experimental apparatus has been discussed elsewhere [2] and will not be described in detail here. Briefly, LEED is performed in a chamber with a base pressure of -1.5 X lo-” Torr using a conventional commercial LEED optics assembly and electron gun. The LEED optics and tungsten crystal are mounted on manipulators which permit any one of several LEED beams to be directed through a slit cut in the phosphor LEED screen and into the fured entrance aperture of an accelerating column. The electrons are accelerated to 100 keV as they pass through this column and enter a second vacuum chamber where their polarization is determined by use of the usual Mott scattering technique [7]. The tungsten surface was cleaned prior to each exposure to the gas of interest by resistive heating to 1200°C for 3 min in lo- ’ Torr of oxygen, followed by repeated flashings in high vacuum to 24OO’C. Auger electron spectroscopy showed that no contaminants, including carbon, remained on the surface following this treatment. Further evidence that the surface is indeed clean is provided by the fact that intensity profiles measured from this clean surface have been shown to agree well with those of other workers as is discussed in in detail in ref. [2]. The polarization as a function of time, and hence surface exposure, was measured as an initially clean surface was exposed to a constant pressure of CO or 02. Immediately following the final cleaning flash, the adsorbate gas was admitted into the vacuum chamber using a UHV leak valve. Pressures of 3 X 10m9 and 4 X 10e9 Torr for CO and 0s respectively were typically employed, corresponding to exposure rates of 0.18 L min-’ for CO and 0.24 L min-’ for OZ. The gas pressure was measured by a nude ion gauge located near the tungsten crystal, and was maintained at a constant value throughout a given series of measurements [8]. The polarization of the (00) beam was measured over successive 15 set intervals until a
T. W. Riddle et al. /Effects of adsorbates on electron spin polarization. I
513
total gas exposure of 1 L was reached. The changes in polarization resulting from exposure to CO and Oa were found to be very reproducible and the results in this paper represent the averages of data obtained from repeated measurements. The overall statistical uncertainty in the data is -3%. In addition, there may be an offset _ f 3% in the polarization profiles which may arise from systematic errors in the of < determination of the instrumental asymmetry. The intensity profiles, on the other hand, were measured by use of a spot photometer. For these measurements the crystal was first exposed to the desired amount of gas, after which the gas inlet leak valve was closed and the remaining gas in the chamber pumped out, and a complete intensity versus energy scan was recorded. The form of the intensity profiles obtained by use of the spot photometer agreed well with those inferred from independent measurements of the current recorded at a Faraday cup located in the Mott scattering chamber, which also provides a measure of the intensity of the beam. In order to test for any effects in the measurements resulting from either the changing crystal temperature or background gas deposition, polarization determinations were undertaken both at different exposure rates, ranging from 0.24 to 0.43 L min-', and with different time delays between flashing the crystal and admitting the adsorbate gas. These studies confirmed that neither temperature nor background gas significantly influenced the measurements reported here. This is to be expected in view of the low background gas pressure, and the fact that, since the majority of the crystal cooling occurs during the time interval required after a flash to admit the adsorbate gas and stabilize its pressure, changes in temperature during data acquisition are relatively small in magnitude and are expected to result in only small polarization changes, even near large polarization features [9].
3. Results and discussion Polarization and intensity profiles versus energy as a function of adsorbate exposure to CO and Oa are shown in figs. 1 and 2 for electron energies between 26 and 141 eV. The polarization profiles in these figures were drawn through data points taken at 5 eV intervals, except in the regions of the sharp features located near 30 eV and 75 eV where data were recorded at intervals of 1 eV or smaller. The intensity curves represent a continuous scan, obtained with a spot photometer, as described in the preceding section. It is evident from figs. 1 and 2 that both polarization and intensity are significantly influenced by surface exposure to CO or Os, and that the resultant changes are of the same general character. In particular, energy regions are observed in which either the polarization or the intensity, but generally not both, is especially sensitive to surface exposure. For example, the polarization at 75 eV decreases in magnitude from about -75% to -0% with little attendant change in beam intensity, upon exposure to only -0.5 L of either CO or Oa, while the intensity at -57 eV increases by a factor of -8 for the same 0.5 L exposure without pro-
514
T. W. Riddle et al. /Effects
of adsorbates on electron spin polarization. I
Energy
(eV
)
Fig. 1. Electron spin polarization and intensity of the (00) beam at 0 = 13” and @= 0” as a function of incident electron energy for selected exposures of the clean W(OO1) surface to CO.
nounced changes in polarization. Indeed, an intensity minimum is observed at -57 eV for the clean surface, whereas that energy is close to an intensity maximum following the same 0.5 L surface exposure. The fact that there exist energy regions where either the polarization or intensity is extremely sensitive to surface exposure suggests that measurements of these quantities in such regions might be used to monitor surface cleanliness. Polarization is perhaps a more useful measurement in this respect because it is absolute, independent of incident beam current, collection efficiencies, etc., factors which will cause the measured intensity to vary in magnitude. Indeed, in this laboratory we have found that measurement of the polarization at 76 eV, which corresponds to the peak of the large negative feature, provides a sensitive and convenient day to day monitor and check on surface cleanliness. In addition, the marked sensitivity of the polarization to surface contamination indicates that considerable care must be exercise when making polarization measurements on a clean surface to insure that the surface is indeed clean. The present data for both CO and Oa also serve to emphasize again the complementary nature of measurements of the electron spin polarization and intensity features previously noted in studies of LEED from clean surfaces: It is evident from
T. W.Riddle et al. /Effects of adsorbates on electron spin polarization. I
-8clean 0.24 L 0.48 L 0.72 L I.2 L
30
50
70 Energy
90
110
515
* 5
6x
-
$
k p
-4.z
130
(eV)
Fig. Z.-Electron spin polarization and intensity of the (00) beam at 0 = 13” and 4~= 0” as a function of incident electron energy for selected exposures of the clean W(OO1) surface to 02.
the present work that not only do electron spin polarization maxima generally occur at electron energies corresponding to regions of intensity minima, but also, as noted previously, that at energies where neither the polarization or intensity varies markedly with surface exposure, the other typically remains relatively constant. While the changes in polarization and intensities observed as a result of exposure to CO and O2 are of the same general character, detailed differences in the behavior are apparent. Perhaps the most noticeable example occurs near 70 eV where exposure to CO is found to result in a large, positive polarization feature which is not as evident in the case of exposure to OZ. The appearance of this positive polarization feature at low CO exposures again points to the need for strict surface cleanliness when undertaking measurements pertaining to a clean surface, since CO is normally a major constituent of the background gas. Further, it serves to demonstrate that the polarization does not always decay monotonically to zero with increasing gas exposure. A similar behavior of the pola~zation has been observed in this laboratory in certain energy ranges at other angles of incidence. Detailed differences also exist in the intensity behavior near, for example, 75 eV where the intensity is increased following exposure to O2 but not to CO. Neither does increasing surface exposure necessarily result in a monotonic change in the intensity observed at a pa~i~ular energy. This is readily apparent from an inspection of the data on either side of the intensity feature centered at 55 eV.
516
T. W. Riddle et al. /Effects of adsorbates on electron spin polarization. I
In conclusion, the significant and distinctive effects due to adsorbed gases not only emphasize the need for strict surface cleanliness if results characteristic of a clean surface are to be obtained, but also offer hope that further insight into the properties of adsorbate covered surfaces may ultimately be obtained by a comparison with theoretical calculations incorporating spin.
[l] M.R. O’NeiJl, M. Kalisvaart, F.B. Dunning and G.K. Walters, Phys. Rev. Letters 34 (1975) 1167. [2] M. Kahsvaart, M.R. O’Neill, T.W. Riddle, F.B. Dunning and G.K. Walters, Phys. Rev. B17 (1978) 1570. [3] R.L. Calvert, G.J. Russell and D. Haneman, Phys. Rev. Letters 39 (1977) 1226. [4] P.J. Jennings and R.O. Jones, Surface Sci. 20 (1970) 18; 26 (1971) 509; 27 (1971) 221; 71 (1978) 101; P.J. Jennings and B.K. Sim, Surface Sci. 33 (1972) 1. [5] R. Feder, Phys. Status Solidi B46 (1971) K31; 49 (1972) 699; 56 (1973) K43; 58 (1973) K137; 62 (1974) 135; Surface Sci. 51 (1975) 297; 63 (1977) 283; Phys. Rev. Letters 36 (1976) 598. [6] N. Miiller, D. Wolf and R. Feder, Inst. Phys. Conf. Ser. No. 41 (1978) 281; R. Feder, N. Mtiller and D. Wolf, 2. Physik B28 (1977) 265. [7] J. Van Klinken, Nucl. Phys. 75 (1966) 161. [S] The ion gauge was not absolutely calibrated and the pressures were determined using the gauge sensitivities quoted by the manufacturer. [ 91 T.W. Riddle, A.H. Mahan, F.B. Dunning and G.K. Walters, J. Vacuum Sci. Technol. 15 (1978) 1686.