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The role of O and Cl adsorbates on the secondary emission properties of tungsten
Surface Science 459 (2000) 14–22 www.elsevier.nl/locate/susc
The role of O and Cl adsorbates on the secondary emission properties of tungsten W.S. Vogan, S.G. Walton 1, R.L. Champion * Department of Physics, College of William and Mary, Williamsburg, VA 23187, USA Received 23 February 2000; accepted for publication 29 March 2000
Abstract Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na+ incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na+ impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Ion bombardment; Ion emission; Oxidation; Secondary electron emission; Secondary ion mass spectroscopy; Tungsten
1. Introduction In applications where the interaction of low energy ions with metallic surfaces is common, ioninduced secondary electron and ion emission is of fundamental importance. Perhaps the most common applications involve materials processing techniques where plasma-generated ions interact with surfaces exposed to the plasma. Secondary electrons and ions ejected from the surface may affect equilibrium plasma characteristics such as species and charge densities, particle energy distributions, and the sheath thickness. The role of * Corresponding author. Fax: +1-757-221-3540. E-mail address: [email protected] (R.L. Champion) 1 Present address: Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375, USA.
adsorbates in producing these secondary electrons and ions is of obvious importance, since most surfaces will have some form of (possibly timedependent) adsorbate coverage. We have recently reported on studies of low energy, ion-induced secondary electron and negative ion emission from aluminum [1,2], molybdenum [3], and stainless steel surfaces [4] upon which resides a known coverage of oxygen. In those experiments, positive sodium ions, with energies up to 500 eV, were used to initiate the secondary emission for an oxygen coverage ranging from none to a few monolayers. It was shown that the presence of adsorbed oxygen is required for all but trivial levels of secondary electron and anion emission. In the case of 250 eV Na+ impacting stainless steel and Al substrates, for example, secondary electron emission was found to increase by more
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than an order of magnitude if the metallic substrate had acquired approximately 1 ML of adsorbed oxygen. Absolute yields and kinetic energy distributions of both the secondary electrons and negative ions were measured, and time-of-flight secondary ion mass spectroscopy ( TOF-SIMS ) measurements were used to identify the secondary anions. For these oxygen-exposed surfaces, the dominant anion was found to be O−, comprising more than 90% of all secondary negative ions. The choice of low energy, positive sodium ions to initiate secondary emission is unique in that it precludes two well-known mechanisms for secondary electron emission [5,6 ]: the so-called potential and kinetic emission. Potential emission is eliminated since the ionization potential of sodium is not greater than twice the work function of most metals (a necessary condition for Auger neutralization, leading to potential emission [7]) and, for impacting energies less than 500 eV, the probability of kinetic emission (from momentum transfer) is small [8–10]. In addition, it was shown that the increased secondary electron emission is not due simply to an adsorbate-altered work function [2,11]. In order to explain the observed secondary electron emission, a mechanism was proposed that could provide for both the observed increase in secondary emission and the electron and anion kinetic energy distributions [2–4]. The mechanism invoked a collision-induced, electronic excitation of a surface state, the decay of which leads to either electron emission or negative ion emission. This process is similar to that in models describing electron- and photon-induced ion emission, i.e. desorption induced by electronic transitions (DIET ) (see for example Ref. [12] and previous volumes), such as the well-known Menzel–Gomer– Redhead (MGR) theory of electron-stimulated ion desorption [13,14]. Photoemission studies for the O/Al system [15] have been shown to be compatible with the basic tenets of this simple description of collision-induced, secondary emission. The mechanism is described in detail elsewhere [2–4], and a general description of the model suffices to outline how it may be related to ion-induced emission from an adsorbate-covered tungsten surface. Let us assume that oxygen and chlorine dissoci-
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Fig. 1. A potential diagram used to represent the interaction of Cl− with a tungsten surface. The widths, D(z), in the lower portion of the figure represent the decay rates for electron emission of ( WCl−)1 into the vacuum or back into the metal.
atively chemisorb onto the tungsten surface, residing on the surface as anions. This results in the formation of a surface state resembling WX−, where X denotes the adsorbate; Fig. 1 depicts a potential diagram for the case of chlorine. If an impacting ion electronically excites WX− to an antibonding state ( WX−)1, the negative ion, X−, may exit the surface intact or decay by electron emission into the vacuum or alternatively back into the metal. The probability that the negative ion survives to the vacuum is derived from the fundamental rate equation: dP (t)=−D [z(t)]P (t)dt (1) ion total ion where P (t) is the ion survival probability as a ion function of time, D [z(t)] is the total decay rate tot as a function of distance from the surface z, and is the sum of the exponential decays to the vacuum and to the metal, i.e. D [z(t)]=D (z)+ tot vacuum D (z), each of which is also shown in Fig. 1. metal The aim of this work is to extend these types
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of measurements and model analyses to a tungsten substrate upon which resides a known coverage of oxygen or chlorine adsorbate.
2. Experimental method The experimental apparatus has been described in detail elsewhere [1–4]. Briefly, the experiments are conducted in an ultra-high vacuum chamber with a base pressure <5×10−10 Torr. The tungsten sample is a 3.2×40 mm2 polycrystalline ribbon. The incident Na+ beam is provided by an ion gun aligned at 60° with respect to the surface normal. All negatively charged products are focused and collected along the surface normal by a series of electrostatic lenses. About 75% of the negative products are collected on the lens closest to the surface ( lens one), while the remainder are focused down the lens stack for further analysis. In the second lens is a small electromagnet that may be used to separate the electrons from the negative ions without appreciably affecting the path of the latter. The third lens consists of two half cylinders that are electrically isolated from each other and may be biased to collect the negative products. In this way, the electron and anion yields may be determined independently. The absolute yields are simply the ratio of the total electron or anion current to that of the incident Na+ ions. The third lens may also be biased to inject the negative products into a spherical electrostatic energy analyzer ( EEA), used to measure the anion or electron kinetic energy distributions. By pulsing the primary Na+ beam, the analyzer may also be used in a fixed energy mode for TOF-SIMS measurements. The tungsten surface was cleaned by consecutive cycles of sputtering and annealing. Sputter-cleaning was effected by rastering a 3 keV Ar+ beam across the surface, while annealing was achieved by resistively heating the surface to temperatures in excess of 1000 K. The cycles were repeated until TOF-SIMS measurements confirmed that no adsorbate was present on the surface. Oxygen exposure is achieved by admitting high purity oxygen (99.9%) into the chamber via a precision leak valve while monitoring the oxygen
partial pressure with a residual gas analyzer. The exposure is reported in terms of Langmuir (L), where 1 L=10−6 Torr s. Chlorine exposure is achieved by admitting methyl chloride (99.9% purity) into the chamber while monitoring the partial pressure of CH Cl+. Since the presence of 3 alkali metal has been shown to alter the surface work function and effect secondary emission [9,16,17], great care was taken to limit the Na+ dose during the experiments. The Na+ beam is on the order of a few nA and is incident on the surface for, at most, a few minutes, resulting in the accumulation of only small fractions (~10−3) of a monolayer. TOF-SIMS measurements confirmed minimal Na+ accumulation.
3. Results and discussion Combined secondary electron and negative ion yields for Na+ on a polycrystalline tungsten substrate are shown in Fig. 2 as functions of adsorbate exposure and energy of the impinging sodium ions. The exposure (in Langmuir [L]) refers to oxygen (O ) and methyl chloride (CH Cl ). In both circum2 3 stances, it is clear that the presence of an adsorbate significantly enhances secondary emission, although in completely different ways. For exposure to oxygen, the total yield increases as sodium impact energy increases, whereas for CH Cl expo3 sure, the total yield exhibits a maximum at an impact energy of about 150 eV, decreasing to level off at the highest impact energies. The yields increase with adsorbate coverage and appear to saturate for exposures in the range of 5–10 L. However, secondary emission for O/W initially decreases for small exposure. This decrease may be due to an increase in the work function associated with a small oxygen exposure: Daniels and Gomer [18] report Dw#+1.2 eV after exposing W(100) to 1 L of O . 2 It has been reported that exposing polycrystalline W to 5 L of oxygen corresponds to a coverage of approximately 1 ML [19–21], with the formation of the second monolayer occurring for exposures on the order of 100 L [20]. In the present experiments, the tungsten surface is subjected to repeated annealing and thermal cycling, a process
W.S. Vogan et al. / Surface Science 459 (2000) 14–22
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Fig. 2. The total (anion plus electron) emission probabilities for Na+ impacting a tungsten substrate exposed to (a) oxygen or (b) methyl chloride. The absolute probabilities are expressed as a per cent and are displayed as a function of Na+ impact energy and adsorbate exposure.
which has been found to induce reconstruction such that the surface becomes multifaceted, exhibiting several low-order planes [22–24]. Although the extent and nature of the surface reconstruction are reported to be highly dependent on the temperature and oxygen coverage, several studies have indicated that W(100) facets dominate polycrystalline surfaces prepared as in the present experiments [23]. For single crystal studies of W(100), it has been observed that exposures as small as 2–3 L result in the formation of 1 ML [25–27]. For the purposes of what follows, we shall assume that monolayer coverage occurs for oxygen exposure of about 6 L in the present experiments. For tungsten exposed to methyl chloride, Zhou et al. [28] argue that CH Cl molecules bind to the surface 3 via the halide, and that about 16% of the molecules at molecular saturation coverage (~1.5 molecular monolayer coverage) undergo decomposition in which the alkyl group dissociates from the molecule, leaving Cl firmly attached to the tungsten substrate. The remaining CH Cl molecules readily 3 desorb intact from the surface at room temperature, whereas Cl desorbs only at very elevated temperatures. Hence one would expect Cl monolayer coverage to occur for an exposure of about 1.5/0.16#10 L CH Cl. The present experimental 3
results suggest that this is indeed the case, as saturation in secondary emission clearly occurs for CH Cl exposures in the neighborhood of 10 L. 3 Moreover, TOF-SIMS data show that Cl− is the sole sputtered anion for W at 300 K exposed to CH Cl. 3 The secondary electron and negative ion yields are shown separately in Fig. 3 as functions of Na+ impact energy for monolayer oxygen and chlorine coverages. In the case of an oxygen adsorbate, secondary electron emission increases markedly with impact energy, exceeding that of secondary anions for impact energies greater than 300 eV. On the other hand, secondary electron emission for the Cl adsorbate tends to remain small and, as the impact energy increases, fairly constant. The anion yield reaches a maximum at about 150 eV impact energy for both adsorbates, but the yield for chlorine decreases for energies greater than 150 eV, whereas the yield for oxygen tends to remain fairly constant for these higher impact energies. The uncertainties depicted as error bars in Fig. 3 are determined by repeating the experiments three or four times. The experiments are repeated by cleaning the surface (as discussed in Section 2) and re-exposing it to 6 L (10 L) of O (CH Cl ). 2 3
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Fig. 3. Electron and anion emission probabilities are shown separately as a function of Na+ impact energy for about 1 ML of (a) oxygen and (b) chlorine adsorbate. Secondary electron yields are given by the open circles; anion yields are indicated by the solid squares.
The identity of the secondary ions sputtered from the W surface has been determined by TOFSIMS analysis. Fig. 4a shows the TOF-SIMS spectrum for 250 eV Na+ impacting W with an adsorbed monolayer of oxygen. The spectrum is dominated by two peaks: O−, and a broad peak centered roughly around 220 amu. The centroid of the higher mass peak corresponds to WO− , but 2 resolution at the higher masses is such that the observed peak could also have contributions from other tungsten oxides such WO−, WO−, and 3 WO−, all of which have been seen in previous 4 SIMS measurements [29,30]; for purposes of subsequent discussion, the peak is denoted as WO− . x The relative contributions of heavy ( WO− ) and x light (O−) products for different regions of the kinetic energy spectrum can be studied by adjusting the EEA to pass ions of the specified emission energy. WO− is found to dominate O− emission x
Fig. 4. Time-of-flight spectra for secondary negative ions emitted from (a) O/W and (b) Cl/W for about 1 ML adsorbate and an impact energy of 250 eV. The inset in (a) illustrates the ratio [ WO− ]/[O−] as a function of oxygen exposure for several x impact energies.
at all impact and emission energies, with ratio R=[ WO− ]/[O−] ranging over 3
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AlO−. The interaction of oxygen with a tungsten substrate is not as simple, however, as O− is not observed to be the dominant secondary anion. Bauer et al. [24] make the argument that oxygen is chemisorbed on top of the W(100) surface; as the coverage increases, the stoichiometry of the adsorbate–substrate changes such that multiple oxygen atoms are bound to a single W atom. This, in turn, results in a reduced surface–bulk WMW binding energy. Thermal desorption spectra for polycrystalline W and W(100) [20,21,24,31,32] support this argument, showing that tungsten oxides desorb at a lower temperature than atomic oxygen and that the relative intensity of tungsten oxides increases with increasing oxygen coverage. The present observations clearly show that WO− x emission dominates that of O−, an observation compatible with the desorption experiments discussed above. The simplest view for the oxygen– tungsten interaction, then, involves the consideration of two distinct interaction patterns, with the formation of WO− at the lowest exposures and the additional formation of WMWO− at higher x exposures. A precise characterization of oxygen adsorption onto the tungsten surface is made difficult by the coexistence of several different adsorption structures, the identity and extent of which are highly dependent on coverage, temperature and annealing history, particularly for low coverage [24,27,33]. For example, Bauer et al. [24] report the coexistence of p(2×1), p(4×1), and c(8×2) structures for less than half monolayer oxygen coverage of W(100) at 300 K. (In fact, the rapid changes in adsorption site patterns and surface work function for low coverage [18,24,33] may be the cause of the rather dramatic variation in yield for low coverage, as shown in Fig. 2.) In the present experiments, it is almost certain that the source of the secondary O− for low coverage is a one-coordinate oxide rather than a precursor oxide of higher coordination, since the latter obviously requires many near-neighbor adatoms [24,27,30]. Moreover, the observation that the O− yield actually decreases as the coverage increases (see inset to Fig. 4) suggests that O− continues to arise from a one-coordinate oxide even for large coverages of oxygen. The WMWO− complex from above, then, could be x
considered as WO− bound to the substrate W, x where the WMW bond has been weakened as discussed above. In the context of these assumptions, collisional excitation of WO− is presumably responsible for O− emission, while the excitation of the W O− formation would result in the emis2 x sion of WO− . x Thermal desorption spectra for chlorine adsorbed on various tungsten surfaces show Cl desorption alone; no WCl products were observed x in those studies [34–36 ]. Unlike oxygen, chlorine does not induce reconstruction of the tungsten surface [37], suggesting that chlorine adsorption does not weaken the substrate–bulk WMW bond to the extent that oxygen adsorption does. Hence, the assumption that the WCl− surface state is favored is also compatible with the present observations, which show that Cl− is the only anion emission product of consequence. Once the appropriate surface states are proposed for each adsorbate, we may calculate the probability that the emitted ions will survive into the vacuum. The ion survival probability, P (z), ion may be evaluated after a change of variable as:
CP
D
(z∞)dz∞ zD total (2) ion v(z∞) zi where z is the initial starting point of the ion on i the excited state repulsive curve and v(z) is the velocity of the ion as it exits from the surface. The probability that the ion survives intact into the vacuum is P (2). For simplicity we assume that ion the ion exits the surface along the normal direction. The decay width for electron emission to the vacuum, D (z), effectively terminates at z , vacuum c where the potential of ( WX−)1 falls below that of WX. Hence Eq. (2) may be written: (z)=exp−
P
CP
zc D (z∞)dz∞ vacuum v(z∞) zi (z∞)dz∞ zD metal + . (3) v(z∞) zi Fig. 1 illustrates that an increase in the electron affinity ( EA) can, in general, be correlated with a decrease in z and a concomitant increase in the c ion survival probability. The probability of P
ion
(z)=exp−
P
D
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electron ejection into the vacuum, i.e. P (z)=[1−P (z)] electron ion
C
D (z) vacuum D (z) total
D
(4)
will decrease as z is lowered. Accordingly, higher c ion yields and lower electron yields can be anticipated for sputtering of adsorbates with higher electron affinities. These trends are clearly seen in the results of Fig. 3: for the Cl/W system the ratio of the secondary anion yields (Y ) to the electron A yields (Y ) is large compared with that for the e O/W system. For example, at an impact energy of 150 eV, {Y /Y }#8 for Cl/W whereas A e {Y /Y }#1/4 for O/W at the same impact energy. A e The electron affinity of Cl− (3.6 eV ) is substantially larger than that of WO− (2 eV [38]) and 2 slightly larger than the value reported for WO− 3 (3.3 eV [39]). The normalized anion and secondary electron energy distributions resulting from the impact of 150 eV Na+ with a W surface are shown in Fig. 5a for monolayer oxygen coverage. The secondary electron distribution is nearly symmetric with the most probable emission energy at approximately 2.0 eV and a full-width at half-maximum of about 2.0 eV. The electron kinetic energy distributions for other exposures and Na+ impact energies (not shown) are found to be very similar to that shown in Fig. 5. The most probable kinetic energy for the anion distributions, approximately 0.75 eV, is substantially less than that for the electrons and the spectra exhibit a tail at higher energies. These distributions are also independent of impact energies and exposures. Fig. 5b shows the normalized anion and secondary electron kinetic energy distributions resulting from the impact of 150 eV Na+ with a W surface with monolayer chlorine coverage. The distributions are very similar to those for oxygen, with a roughly symmetric distribution for electrons and a distribution with a high energy tail for ions. The peak occurs around 2 eV for the electron distribution and at lower energy for the negative ion distribution; the most probable energies for electron and ion distributions differ by about 1 eV, as with oxygen. Distributions taken for other impact energies and other exposures ranging from
Fig. 5. The kinetic energy spectra for electrons (open circles) and anions (solid squares) are shown for (a) O/W and (b) Cl/W. The adsorbate coverage in each case is 1 ML and the Na+ impact energy is 150 eV. The solid and dashed curves are the ion and electron spectra calculated from Eqs. (5) and (6).
0 to 10 L methyl chloride (not shown) display the same trends. By using the ion survival probability Eq. (2), kinetic energy distributions S(E ) for ions and secondary electrons may be determined as a function of the emission energy, E: S (E)=P (z)P (z)dE (z) (5) ion ex ion ion S (E )=P (z)P (z)dE (z) (6) elec ex elec elec where P (z), the assumed excitation probability, ex is a broad Gaussian function used to map the initial distribution of states onto the repulsive curve ( WX−)1. The quantity dE (z) is the energy ion above vacuum level for the ( WX−)1 curve, and dE (z) is the energy difference between the elec ( WX−)1 and WX potentials (see Fig. 1). Several features of these potentials can be adjusted to bring the calculation into agreement with the observed kinetic energy distributions, as shown by the theoretical curves in Fig. 5. The potentials used
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to model and to generate S(E ) for the Cl/W system are those displayed in Fig. 1. The adsorption and gas phase bond strengths for WCl are taken from previous work [40], and the electron decay widths D (z) and D (z) are taken to be similar to vacuum metal the widths used to describe the O/Al system [1,2,41]. Since the EA of Cl− is larger than that of O−, the widths for electron emission are taken to be smaller for Cl− than those for O−. Parameters for the calculations of S(E) for O/W are slightly different from those used to describe Cl/W. Ota et al. [42] have shown that the nearestneighbor OMW distance for a 0.5 ML O/W system corresponds closely to the OMW bond length in WO . Hence we take the exiting anion mass to be 2 WO−. The crossing distance, z , is increased to 2 c 6.5a , reflecting the fact that the EA of WO− is o 2 less than that of Cl−. Widths describing the decay of WO− are taken to be 60% (D ) and 80% 2 vacuum (D ) of those used to describe the decay of C metal l−.
4. Conclusions The absolute probabilities for generating secondary electrons and negative ions have been measured as a function of impact energy and adsorbate coverage for 50–500 eV Na+ impacting a polycrystalline tungsten surface exposed to oxygen and methyl chloride. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined with TOFSIMS analysis. Exposure of the tungsten surface to methyl chloride results in adsorption of the halide; accumulation of a chlorine monolayer occurs at around 10 L methyl chloride exposure. The presence of adsorbate is observed to significantly enhance secondary emission. The total yields for both adsorbates increase with adsorbate coverage, but the dependence on Na+ impact energy differs significantly. The identity of the emitted anions reflects the different interactions that oxygen and chlorine undergo with the tungsten surface. Tungsten oxides are the dominant secondary anions for the O/W system, whereas only chloride is emitted from the Cl/W system.
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Although the present studies cannot adequately address issues of surface morphology associated with either the oxygen or chlorine adsorbate, it appears that the Cl/W surface does not undergo reconstruction to the extent known to occur for O/W, suggesting that chlorine adsorption does not significantly weaken the substrate–bulk WMW bond. Differences between electron affinities of the sputtered anions lead to differences in anion survival probabilities, as modeled by the proposed excitation mechanism and observed in the ratio of anion/electron yields. The model for the excitation mechanism also permits calculations of kinetic energy distributions. By adjusting potential parameters to account for differing electron affinities and binding energies, these distributions are found to replicate, to a reasonable approximation, the observed electron and anion kinetic energy distributions for both oxygen and chlorine adsorbed tungsten.
Acknowledgement This work was supported in part by the US Department of Energy, Office of Science, Division of Chemical Sciences.
References [1] J.C. Tucek, S.G. Walton, R.L. Champion, Phys. Rev. B 53 (1996) 14 127. [2] J.C. Tucek, R.L. Champion, Surf. Sci. 382 (1997) 137. [3] J.C. Tucek, S.G. Walton, R.L. Champion, Surf. Sci. 410 (1998) 258. [4] S.G. Walton, R.L. Champion, Y. Wang, J. Appl. Phys. 84 (1998) 1706. [5] G. Lakits, F. Aumayr, M. Heim, H. Winter, Phys. Rev. A 42 (1990) 5780. [6 ] H. Winter, F. Aumayr, G. Lakits, Nucl. Instrum. Meth. B 58 (1991) 301. [7] A. Bonanno, A. Amoddeo, A. Oliva, Nucl. Instrum. Meth. B 46 (1990) 456. [8] E.V. Alonso, M.A. Alurralde, R.A. Baragiola, Surf. Sci. 166 (1986) L155. [9] H. Brenten, H. Mu¨ller, V. Kempter, Surf. Sci. 271 (1992) 103. [10] H. Mu¨ller, R. Hausmann, H. Brenten, V. Kempter, Surf. Sci. 291 (1993) 78.
22
W.S. Vogan et al. / Surface Science 459 (2000) 14–22
[11] J. Ferro´n, E.V. Alonso, R.A. Baragiola, A. Oliva-Florio, Surf. Sci. 120 (1982) 427. [12] A.R. Burns, E.B. Stechel, D.R. Jennison (Eds.), Desorption Induced by Electronic Transitions (DIET V ), Springer, Berlin, 1992. [13] D. Menzel, R. Gomer, J. Chem. Phys. 41 (1964) 3311. [14] P.A. Redhead, Can. J. Phys. 42 (1964) 886. [15] S.G. Walton, B.L. Peko, R.L. Champion, Phys. Rev. B 58 (1998) 15 430. [16 ] M. Bernheim, F. Le Bourse, Nucl. Instrum. Meth. B 27 (1987) 94. [17] D.H. Baker, L.D. Doverspike, R.L. Champion, Phys. Rev. A 46 (1992) 296. [18] E.A. Daniels, R. Gomer, Surf. Sci. 397 (1998) 209. [19] D. Hasselkamp, A. Scharmann, N. Stiller, Nucl. Instrum. Meth. 168 (1980) 579. [20] D.A. King, T.E. Madey, J.T. Yates, J. Chem. Phys. 55 (1971) 3236. [21] R.H. Prince, G.R. Floyd, Surf. Sci. 74 (1978) 342. [22] W. Jelend, D. Menzel, Surf. Sci. 40 (1973) 295. [23] J. Lepage, A. Mezin, M. Nivoit, Surf. Sci. 161 (1985) 255. [24] E. Bauer, H. Poppa, Y. Viswanath, Surf. Sci. 58 (1976) 517. [25] E. Bauer, T. Engel, Surf. Sci. 71 (1978) 695. [26 ] H.M. Kramer, E. Bauer, Surf. Sci. 93 (1980) 407. [27] P. Alnot, D.J. Auerbach, J. Behm, C.R. Brundle, A. Viescas, Surf. Sci. 213 (1989) 1.
[28] X.-L. Zhou, C. Yoon, J.M. White, Surf. Sci. 206 (1988) 379. [29] A. Benninghoven, E. Loebach, C. Plog, N. Treitz, Surf. Sci. 39 (1973) 397. [30] M.L. Yu, Surf. Sci. 71 (1978) 121. [31] D.A. King, T.E. Madey, J.T. Yates, J. Chem. Phys. 55 (1971) 3247. [32] J.M. Lo´pez-Sancho, J.L. De Segovia, Surf. Sci. 30 (1972) 419. [33] M. Tanaka, K. Ota, T. Tsutsumi, S. Usami, Surf. Sci. 357/ 358 (1996) 255. [34] K.J. Rawlings, S.D. Foulias, G.G. Price, B.J. Hopkins, Surf. Sci. 118 (1982) 47. [35] F. Bonczek, T. Engel, E. Bauer, Surf. Sci. 97 (1980) 595. [36 ] G.G. Price, K.J. Rawlings, B.J. Hopkins, Surf. Sci. 85 (1979) 379. [37] K. Faulian, E. Bauer, Surf. Sci. 70 (1978) 271. [38] G.E. Davico, R.L. Schwartz, T.M. Ramond, W.C. Lineberger, J. Phys. Chem. A 103 (1999) 6167. [39] C.W. Walter, C.F. Hertzler, P. Devynck, G.P. Smith, J.R. Peterson, J. Chem. Phys. 95 (1991) 824. [40] R.H. Prince, R.M. Lambert, Chem. Phys. Lett. 67 (1979) 388. [41] B. Bahrim, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 316 (1994) 189. [42] K. Ota, M. Tanaka, S. Usami, Surf. Sci. 402–404 (1998) 813.
The role of O and Cl adsorbates on the secondary emission properties of tungsten W.S. Vogan, S.G. Walton 1, R.L. Champion * Department of Physics, College of William and Mary, Williamsburg, VA 23187, USA Received 23 February 2000; accepted for publication 29 March 2000
Abstract Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na+ incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na+ impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Ion bombardment; Ion emission; Oxidation; Secondary electron emission; Secondary ion mass spectroscopy; Tungsten
1. Introduction In applications where the interaction of low energy ions with metallic surfaces is common, ioninduced secondary electron and ion emission is of fundamental importance. Perhaps the most common applications involve materials processing techniques where plasma-generated ions interact with surfaces exposed to the plasma. Secondary electrons and ions ejected from the surface may affect equilibrium plasma characteristics such as species and charge densities, particle energy distributions, and the sheath thickness. The role of * Corresponding author. Fax: +1-757-221-3540. E-mail address: [email protected] (R.L. Champion) 1 Present address: Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375, USA.
adsorbates in producing these secondary electrons and ions is of obvious importance, since most surfaces will have some form of (possibly timedependent) adsorbate coverage. We have recently reported on studies of low energy, ion-induced secondary electron and negative ion emission from aluminum [1,2], molybdenum [3], and stainless steel surfaces [4] upon which resides a known coverage of oxygen. In those experiments, positive sodium ions, with energies up to 500 eV, were used to initiate the secondary emission for an oxygen coverage ranging from none to a few monolayers. It was shown that the presence of adsorbed oxygen is required for all but trivial levels of secondary electron and anion emission. In the case of 250 eV Na+ impacting stainless steel and Al substrates, for example, secondary electron emission was found to increase by more
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W.S. Vogan et al. / Surface Science 459 (2000) 14–22
than an order of magnitude if the metallic substrate had acquired approximately 1 ML of adsorbed oxygen. Absolute yields and kinetic energy distributions of both the secondary electrons and negative ions were measured, and time-of-flight secondary ion mass spectroscopy ( TOF-SIMS ) measurements were used to identify the secondary anions. For these oxygen-exposed surfaces, the dominant anion was found to be O−, comprising more than 90% of all secondary negative ions. The choice of low energy, positive sodium ions to initiate secondary emission is unique in that it precludes two well-known mechanisms for secondary electron emission [5,6 ]: the so-called potential and kinetic emission. Potential emission is eliminated since the ionization potential of sodium is not greater than twice the work function of most metals (a necessary condition for Auger neutralization, leading to potential emission [7]) and, for impacting energies less than 500 eV, the probability of kinetic emission (from momentum transfer) is small [8–10]. In addition, it was shown that the increased secondary electron emission is not due simply to an adsorbate-altered work function [2,11]. In order to explain the observed secondary electron emission, a mechanism was proposed that could provide for both the observed increase in secondary emission and the electron and anion kinetic energy distributions [2–4]. The mechanism invoked a collision-induced, electronic excitation of a surface state, the decay of which leads to either electron emission or negative ion emission. This process is similar to that in models describing electron- and photon-induced ion emission, i.e. desorption induced by electronic transitions (DIET ) (see for example Ref. [12] and previous volumes), such as the well-known Menzel–Gomer– Redhead (MGR) theory of electron-stimulated ion desorption [13,14]. Photoemission studies for the O/Al system [15] have been shown to be compatible with the basic tenets of this simple description of collision-induced, secondary emission. The mechanism is described in detail elsewhere [2–4], and a general description of the model suffices to outline how it may be related to ion-induced emission from an adsorbate-covered tungsten surface. Let us assume that oxygen and chlorine dissoci-
15
Fig. 1. A potential diagram used to represent the interaction of Cl− with a tungsten surface. The widths, D(z), in the lower portion of the figure represent the decay rates for electron emission of ( WCl−)1 into the vacuum or back into the metal.
atively chemisorb onto the tungsten surface, residing on the surface as anions. This results in the formation of a surface state resembling WX−, where X denotes the adsorbate; Fig. 1 depicts a potential diagram for the case of chlorine. If an impacting ion electronically excites WX− to an antibonding state ( WX−)1, the negative ion, X−, may exit the surface intact or decay by electron emission into the vacuum or alternatively back into the metal. The probability that the negative ion survives to the vacuum is derived from the fundamental rate equation: dP (t)=−D [z(t)]P (t)dt (1) ion total ion where P (t) is the ion survival probability as a ion function of time, D [z(t)] is the total decay rate tot as a function of distance from the surface z, and is the sum of the exponential decays to the vacuum and to the metal, i.e. D [z(t)]=D (z)+ tot vacuum D (z), each of which is also shown in Fig. 1. metal The aim of this work is to extend these types
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of measurements and model analyses to a tungsten substrate upon which resides a known coverage of oxygen or chlorine adsorbate.
2. Experimental method The experimental apparatus has been described in detail elsewhere [1–4]. Briefly, the experiments are conducted in an ultra-high vacuum chamber with a base pressure <5×10−10 Torr. The tungsten sample is a 3.2×40 mm2 polycrystalline ribbon. The incident Na+ beam is provided by an ion gun aligned at 60° with respect to the surface normal. All negatively charged products are focused and collected along the surface normal by a series of electrostatic lenses. About 75% of the negative products are collected on the lens closest to the surface ( lens one), while the remainder are focused down the lens stack for further analysis. In the second lens is a small electromagnet that may be used to separate the electrons from the negative ions without appreciably affecting the path of the latter. The third lens consists of two half cylinders that are electrically isolated from each other and may be biased to collect the negative products. In this way, the electron and anion yields may be determined independently. The absolute yields are simply the ratio of the total electron or anion current to that of the incident Na+ ions. The third lens may also be biased to inject the negative products into a spherical electrostatic energy analyzer ( EEA), used to measure the anion or electron kinetic energy distributions. By pulsing the primary Na+ beam, the analyzer may also be used in a fixed energy mode for TOF-SIMS measurements. The tungsten surface was cleaned by consecutive cycles of sputtering and annealing. Sputter-cleaning was effected by rastering a 3 keV Ar+ beam across the surface, while annealing was achieved by resistively heating the surface to temperatures in excess of 1000 K. The cycles were repeated until TOF-SIMS measurements confirmed that no adsorbate was present on the surface. Oxygen exposure is achieved by admitting high purity oxygen (99.9%) into the chamber via a precision leak valve while monitoring the oxygen
partial pressure with a residual gas analyzer. The exposure is reported in terms of Langmuir (L), where 1 L=10−6 Torr s. Chlorine exposure is achieved by admitting methyl chloride (99.9% purity) into the chamber while monitoring the partial pressure of CH Cl+. Since the presence of 3 alkali metal has been shown to alter the surface work function and effect secondary emission [9,16,17], great care was taken to limit the Na+ dose during the experiments. The Na+ beam is on the order of a few nA and is incident on the surface for, at most, a few minutes, resulting in the accumulation of only small fractions (~10−3) of a monolayer. TOF-SIMS measurements confirmed minimal Na+ accumulation.
3. Results and discussion Combined secondary electron and negative ion yields for Na+ on a polycrystalline tungsten substrate are shown in Fig. 2 as functions of adsorbate exposure and energy of the impinging sodium ions. The exposure (in Langmuir [L]) refers to oxygen (O ) and methyl chloride (CH Cl ). In both circum2 3 stances, it is clear that the presence of an adsorbate significantly enhances secondary emission, although in completely different ways. For exposure to oxygen, the total yield increases as sodium impact energy increases, whereas for CH Cl expo3 sure, the total yield exhibits a maximum at an impact energy of about 150 eV, decreasing to level off at the highest impact energies. The yields increase with adsorbate coverage and appear to saturate for exposures in the range of 5–10 L. However, secondary emission for O/W initially decreases for small exposure. This decrease may be due to an increase in the work function associated with a small oxygen exposure: Daniels and Gomer [18] report Dw#+1.2 eV after exposing W(100) to 1 L of O . 2 It has been reported that exposing polycrystalline W to 5 L of oxygen corresponds to a coverage of approximately 1 ML [19–21], with the formation of the second monolayer occurring for exposures on the order of 100 L [20]. In the present experiments, the tungsten surface is subjected to repeated annealing and thermal cycling, a process
W.S. Vogan et al. / Surface Science 459 (2000) 14–22
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Fig. 2. The total (anion plus electron) emission probabilities for Na+ impacting a tungsten substrate exposed to (a) oxygen or (b) methyl chloride. The absolute probabilities are expressed as a per cent and are displayed as a function of Na+ impact energy and adsorbate exposure.
which has been found to induce reconstruction such that the surface becomes multifaceted, exhibiting several low-order planes [22–24]. Although the extent and nature of the surface reconstruction are reported to be highly dependent on the temperature and oxygen coverage, several studies have indicated that W(100) facets dominate polycrystalline surfaces prepared as in the present experiments [23]. For single crystal studies of W(100), it has been observed that exposures as small as 2–3 L result in the formation of 1 ML [25–27]. For the purposes of what follows, we shall assume that monolayer coverage occurs for oxygen exposure of about 6 L in the present experiments. For tungsten exposed to methyl chloride, Zhou et al. [28] argue that CH Cl molecules bind to the surface 3 via the halide, and that about 16% of the molecules at molecular saturation coverage (~1.5 molecular monolayer coverage) undergo decomposition in which the alkyl group dissociates from the molecule, leaving Cl firmly attached to the tungsten substrate. The remaining CH Cl molecules readily 3 desorb intact from the surface at room temperature, whereas Cl desorbs only at very elevated temperatures. Hence one would expect Cl monolayer coverage to occur for an exposure of about 1.5/0.16#10 L CH Cl. The present experimental 3
results suggest that this is indeed the case, as saturation in secondary emission clearly occurs for CH Cl exposures in the neighborhood of 10 L. 3 Moreover, TOF-SIMS data show that Cl− is the sole sputtered anion for W at 300 K exposed to CH Cl. 3 The secondary electron and negative ion yields are shown separately in Fig. 3 as functions of Na+ impact energy for monolayer oxygen and chlorine coverages. In the case of an oxygen adsorbate, secondary electron emission increases markedly with impact energy, exceeding that of secondary anions for impact energies greater than 300 eV. On the other hand, secondary electron emission for the Cl adsorbate tends to remain small and, as the impact energy increases, fairly constant. The anion yield reaches a maximum at about 150 eV impact energy for both adsorbates, but the yield for chlorine decreases for energies greater than 150 eV, whereas the yield for oxygen tends to remain fairly constant for these higher impact energies. The uncertainties depicted as error bars in Fig. 3 are determined by repeating the experiments three or four times. The experiments are repeated by cleaning the surface (as discussed in Section 2) and re-exposing it to 6 L (10 L) of O (CH Cl ). 2 3
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Fig. 3. Electron and anion emission probabilities are shown separately as a function of Na+ impact energy for about 1 ML of (a) oxygen and (b) chlorine adsorbate. Secondary electron yields are given by the open circles; anion yields are indicated by the solid squares.
The identity of the secondary ions sputtered from the W surface has been determined by TOFSIMS analysis. Fig. 4a shows the TOF-SIMS spectrum for 250 eV Na+ impacting W with an adsorbed monolayer of oxygen. The spectrum is dominated by two peaks: O−, and a broad peak centered roughly around 220 amu. The centroid of the higher mass peak corresponds to WO− , but 2 resolution at the higher masses is such that the observed peak could also have contributions from other tungsten oxides such WO−, WO−, and 3 WO−, all of which have been seen in previous 4 SIMS measurements [29,30]; for purposes of subsequent discussion, the peak is denoted as WO− . x The relative contributions of heavy ( WO− ) and x light (O−) products for different regions of the kinetic energy spectrum can be studied by adjusting the EEA to pass ions of the specified emission energy. WO− is found to dominate O− emission x
Fig. 4. Time-of-flight spectra for secondary negative ions emitted from (a) O/W and (b) Cl/W for about 1 ML adsorbate and an impact energy of 250 eV. The inset in (a) illustrates the ratio [ WO− ]/[O−] as a function of oxygen exposure for several x impact energies.
at all impact and emission energies, with ratio R=[ WO− ]/[O−] ranging over 3
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AlO−. The interaction of oxygen with a tungsten substrate is not as simple, however, as O− is not observed to be the dominant secondary anion. Bauer et al. [24] make the argument that oxygen is chemisorbed on top of the W(100) surface; as the coverage increases, the stoichiometry of the adsorbate–substrate changes such that multiple oxygen atoms are bound to a single W atom. This, in turn, results in a reduced surface–bulk WMW binding energy. Thermal desorption spectra for polycrystalline W and W(100) [20,21,24,31,32] support this argument, showing that tungsten oxides desorb at a lower temperature than atomic oxygen and that the relative intensity of tungsten oxides increases with increasing oxygen coverage. The present observations clearly show that WO− x emission dominates that of O−, an observation compatible with the desorption experiments discussed above. The simplest view for the oxygen– tungsten interaction, then, involves the consideration of two distinct interaction patterns, with the formation of WO− at the lowest exposures and the additional formation of WMWO− at higher x exposures. A precise characterization of oxygen adsorption onto the tungsten surface is made difficult by the coexistence of several different adsorption structures, the identity and extent of which are highly dependent on coverage, temperature and annealing history, particularly for low coverage [24,27,33]. For example, Bauer et al. [24] report the coexistence of p(2×1), p(4×1), and c(8×2) structures for less than half monolayer oxygen coverage of W(100) at 300 K. (In fact, the rapid changes in adsorption site patterns and surface work function for low coverage [18,24,33] may be the cause of the rather dramatic variation in yield for low coverage, as shown in Fig. 2.) In the present experiments, it is almost certain that the source of the secondary O− for low coverage is a one-coordinate oxide rather than a precursor oxide of higher coordination, since the latter obviously requires many near-neighbor adatoms [24,27,30]. Moreover, the observation that the O− yield actually decreases as the coverage increases (see inset to Fig. 4) suggests that O− continues to arise from a one-coordinate oxide even for large coverages of oxygen. The WMWO− complex from above, then, could be x
considered as WO− bound to the substrate W, x where the WMW bond has been weakened as discussed above. In the context of these assumptions, collisional excitation of WO− is presumably responsible for O− emission, while the excitation of the W O− formation would result in the emis2 x sion of WO− . x Thermal desorption spectra for chlorine adsorbed on various tungsten surfaces show Cl desorption alone; no WCl products were observed x in those studies [34–36 ]. Unlike oxygen, chlorine does not induce reconstruction of the tungsten surface [37], suggesting that chlorine adsorption does not weaken the substrate–bulk WMW bond to the extent that oxygen adsorption does. Hence, the assumption that the WCl− surface state is favored is also compatible with the present observations, which show that Cl− is the only anion emission product of consequence. Once the appropriate surface states are proposed for each adsorbate, we may calculate the probability that the emitted ions will survive into the vacuum. The ion survival probability, P (z), ion may be evaluated after a change of variable as:
CP
D
(z∞)dz∞ zD total (2) ion v(z∞) zi where z is the initial starting point of the ion on i the excited state repulsive curve and v(z) is the velocity of the ion as it exits from the surface. The probability that the ion survives intact into the vacuum is P (2). For simplicity we assume that ion the ion exits the surface along the normal direction. The decay width for electron emission to the vacuum, D (z), effectively terminates at z , vacuum c where the potential of ( WX−)1 falls below that of WX. Hence Eq. (2) may be written: (z)=exp−
P
CP
zc D (z∞)dz∞ vacuum v(z∞) zi (z∞)dz∞ zD metal + . (3) v(z∞) zi Fig. 1 illustrates that an increase in the electron affinity ( EA) can, in general, be correlated with a decrease in z and a concomitant increase in the c ion survival probability. The probability of P
ion
(z)=exp−
P
D
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W.S. Vogan et al. / Surface Science 459 (2000) 14–22
electron ejection into the vacuum, i.e. P (z)=[1−P (z)] electron ion
C
D (z) vacuum D (z) total
D
(4)
will decrease as z is lowered. Accordingly, higher c ion yields and lower electron yields can be anticipated for sputtering of adsorbates with higher electron affinities. These trends are clearly seen in the results of Fig. 3: for the Cl/W system the ratio of the secondary anion yields (Y ) to the electron A yields (Y ) is large compared with that for the e O/W system. For example, at an impact energy of 150 eV, {Y /Y }#8 for Cl/W whereas A e {Y /Y }#1/4 for O/W at the same impact energy. A e The electron affinity of Cl− (3.6 eV ) is substantially larger than that of WO− (2 eV [38]) and 2 slightly larger than the value reported for WO− 3 (3.3 eV [39]). The normalized anion and secondary electron energy distributions resulting from the impact of 150 eV Na+ with a W surface are shown in Fig. 5a for monolayer oxygen coverage. The secondary electron distribution is nearly symmetric with the most probable emission energy at approximately 2.0 eV and a full-width at half-maximum of about 2.0 eV. The electron kinetic energy distributions for other exposures and Na+ impact energies (not shown) are found to be very similar to that shown in Fig. 5. The most probable kinetic energy for the anion distributions, approximately 0.75 eV, is substantially less than that for the electrons and the spectra exhibit a tail at higher energies. These distributions are also independent of impact energies and exposures. Fig. 5b shows the normalized anion and secondary electron kinetic energy distributions resulting from the impact of 150 eV Na+ with a W surface with monolayer chlorine coverage. The distributions are very similar to those for oxygen, with a roughly symmetric distribution for electrons and a distribution with a high energy tail for ions. The peak occurs around 2 eV for the electron distribution and at lower energy for the negative ion distribution; the most probable energies for electron and ion distributions differ by about 1 eV, as with oxygen. Distributions taken for other impact energies and other exposures ranging from
Fig. 5. The kinetic energy spectra for electrons (open circles) and anions (solid squares) are shown for (a) O/W and (b) Cl/W. The adsorbate coverage in each case is 1 ML and the Na+ impact energy is 150 eV. The solid and dashed curves are the ion and electron spectra calculated from Eqs. (5) and (6).
0 to 10 L methyl chloride (not shown) display the same trends. By using the ion survival probability Eq. (2), kinetic energy distributions S(E ) for ions and secondary electrons may be determined as a function of the emission energy, E: S (E)=P (z)P (z)dE (z) (5) ion ex ion ion S (E )=P (z)P (z)dE (z) (6) elec ex elec elec where P (z), the assumed excitation probability, ex is a broad Gaussian function used to map the initial distribution of states onto the repulsive curve ( WX−)1. The quantity dE (z) is the energy ion above vacuum level for the ( WX−)1 curve, and dE (z) is the energy difference between the elec ( WX−)1 and WX potentials (see Fig. 1). Several features of these potentials can be adjusted to bring the calculation into agreement with the observed kinetic energy distributions, as shown by the theoretical curves in Fig. 5. The potentials used
W.S. Vogan et al. / Surface Science 459 (2000) 14–22
to model and to generate S(E ) for the Cl/W system are those displayed in Fig. 1. The adsorption and gas phase bond strengths for WCl are taken from previous work [40], and the electron decay widths D (z) and D (z) are taken to be similar to vacuum metal the widths used to describe the O/Al system [1,2,41]. Since the EA of Cl− is larger than that of O−, the widths for electron emission are taken to be smaller for Cl− than those for O−. Parameters for the calculations of S(E) for O/W are slightly different from those used to describe Cl/W. Ota et al. [42] have shown that the nearestneighbor OMW distance for a 0.5 ML O/W system corresponds closely to the OMW bond length in WO . Hence we take the exiting anion mass to be 2 WO−. The crossing distance, z , is increased to 2 c 6.5a , reflecting the fact that the EA of WO− is o 2 less than that of Cl−. Widths describing the decay of WO− are taken to be 60% (D ) and 80% 2 vacuum (D ) of those used to describe the decay of C metal l−.
4. Conclusions The absolute probabilities for generating secondary electrons and negative ions have been measured as a function of impact energy and adsorbate coverage for 50–500 eV Na+ impacting a polycrystalline tungsten surface exposed to oxygen and methyl chloride. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined with TOFSIMS analysis. Exposure of the tungsten surface to methyl chloride results in adsorption of the halide; accumulation of a chlorine monolayer occurs at around 10 L methyl chloride exposure. The presence of adsorbate is observed to significantly enhance secondary emission. The total yields for both adsorbates increase with adsorbate coverage, but the dependence on Na+ impact energy differs significantly. The identity of the emitted anions reflects the different interactions that oxygen and chlorine undergo with the tungsten surface. Tungsten oxides are the dominant secondary anions for the O/W system, whereas only chloride is emitted from the Cl/W system.
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Although the present studies cannot adequately address issues of surface morphology associated with either the oxygen or chlorine adsorbate, it appears that the Cl/W surface does not undergo reconstruction to the extent known to occur for O/W, suggesting that chlorine adsorption does not significantly weaken the substrate–bulk WMW bond. Differences between electron affinities of the sputtered anions lead to differences in anion survival probabilities, as modeled by the proposed excitation mechanism and observed in the ratio of anion/electron yields. The model for the excitation mechanism also permits calculations of kinetic energy distributions. By adjusting potential parameters to account for differing electron affinities and binding energies, these distributions are found to replicate, to a reasonable approximation, the observed electron and anion kinetic energy distributions for both oxygen and chlorine adsorbed tungsten.
Acknowledgement This work was supported in part by the US Department of Energy, Office of Science, Division of Chemical Sciences.
References [1] J.C. Tucek, S.G. Walton, R.L. Champion, Phys. Rev. B 53 (1996) 14 127. [2] J.C. Tucek, R.L. Champion, Surf. Sci. 382 (1997) 137. [3] J.C. Tucek, S.G. Walton, R.L. Champion, Surf. Sci. 410 (1998) 258. [4] S.G. Walton, R.L. Champion, Y. Wang, J. Appl. Phys. 84 (1998) 1706. [5] G. Lakits, F. Aumayr, M. Heim, H. Winter, Phys. Rev. A 42 (1990) 5780. [6 ] H. Winter, F. Aumayr, G. Lakits, Nucl. Instrum. Meth. B 58 (1991) 301. [7] A. Bonanno, A. Amoddeo, A. Oliva, Nucl. Instrum. Meth. B 46 (1990) 456. [8] E.V. Alonso, M.A. Alurralde, R.A. Baragiola, Surf. Sci. 166 (1986) L155. [9] H. Brenten, H. Mu¨ller, V. Kempter, Surf. Sci. 271 (1992) 103. [10] H. Mu¨ller, R. Hausmann, H. Brenten, V. Kempter, Surf. Sci. 291 (1993) 78.
22
W.S. Vogan et al. / Surface Science 459 (2000) 14–22
[11] J. Ferro´n, E.V. Alonso, R.A. Baragiola, A. Oliva-Florio, Surf. Sci. 120 (1982) 427. [12] A.R. Burns, E.B. Stechel, D.R. Jennison (Eds.), Desorption Induced by Electronic Transitions (DIET V ), Springer, Berlin, 1992. [13] D. Menzel, R. Gomer, J. Chem. Phys. 41 (1964) 3311. [14] P.A. Redhead, Can. J. Phys. 42 (1964) 886. [15] S.G. Walton, B.L. Peko, R.L. Champion, Phys. Rev. B 58 (1998) 15 430. [16 ] M. Bernheim, F. Le Bourse, Nucl. Instrum. Meth. B 27 (1987) 94. [17] D.H. Baker, L.D. Doverspike, R.L. Champion, Phys. Rev. A 46 (1992) 296. [18] E.A. Daniels, R. Gomer, Surf. Sci. 397 (1998) 209. [19] D. Hasselkamp, A. Scharmann, N. Stiller, Nucl. Instrum. Meth. 168 (1980) 579. [20] D.A. King, T.E. Madey, J.T. Yates, J. Chem. Phys. 55 (1971) 3236. [21] R.H. Prince, G.R. Floyd, Surf. Sci. 74 (1978) 342. [22] W. Jelend, D. Menzel, Surf. Sci. 40 (1973) 295. [23] J. Lepage, A. Mezin, M. Nivoit, Surf. Sci. 161 (1985) 255. [24] E. Bauer, H. Poppa, Y. Viswanath, Surf. Sci. 58 (1976) 517. [25] E. Bauer, T. Engel, Surf. Sci. 71 (1978) 695. [26 ] H.M. Kramer, E. Bauer, Surf. Sci. 93 (1980) 407. [27] P. Alnot, D.J. Auerbach, J. Behm, C.R. Brundle, A. Viescas, Surf. Sci. 213 (1989) 1.
[28] X.-L. Zhou, C. Yoon, J.M. White, Surf. Sci. 206 (1988) 379. [29] A. Benninghoven, E. Loebach, C. Plog, N. Treitz, Surf. Sci. 39 (1973) 397. [30] M.L. Yu, Surf. Sci. 71 (1978) 121. [31] D.A. King, T.E. Madey, J.T. Yates, J. Chem. Phys. 55 (1971) 3247. [32] J.M. Lo´pez-Sancho, J.L. De Segovia, Surf. Sci. 30 (1972) 419. [33] M. Tanaka, K. Ota, T. Tsutsumi, S. Usami, Surf. Sci. 357/ 358 (1996) 255. [34] K.J. Rawlings, S.D. Foulias, G.G. Price, B.J. Hopkins, Surf. Sci. 118 (1982) 47. [35] F. Bonczek, T. Engel, E. Bauer, Surf. Sci. 97 (1980) 595. [36 ] G.G. Price, K.J. Rawlings, B.J. Hopkins, Surf. Sci. 85 (1979) 379. [37] K. Faulian, E. Bauer, Surf. Sci. 70 (1978) 271. [38] G.E. Davico, R.L. Schwartz, T.M. Ramond, W.C. Lineberger, J. Phys. Chem. A 103 (1999) 6167. [39] C.W. Walter, C.F. Hertzler, P. Devynck, G.P. Smith, J.R. Peterson, J. Chem. Phys. 95 (1991) 824. [40] R.H. Prince, R.M. Lambert, Chem. Phys. Lett. 67 (1979) 388. [41] B. Bahrim, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 316 (1994) 189. [42] K. Ota, M. Tanaka, S. Usami, Surf. Sci. 402–404 (1998) 813.