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States of no adsorbed on W(100), (110) and (111) studied by electron stimulated desorption
Applied Surface Science 33/34 (1988) 301-308 North-Holland, Amsterdam
301
S T A T E S OF N O A D S O R B E D O N W(100), (110) AND (111) STUDIED BY E L E C T R O N S T I M U L A T E D D E S O R P T I O N
Shigeru S U G A I , Hiroshi Y O S H I K A W A , H i r o f u m i M I K I , Toshihide K I O K A a n d Koji K A W A S A K I
Department of Physics, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan Received 23 August 1987; accepted for publication 28 October 1987
The adsorption states of 15NO on W(100), (110) and (111) surfaces at room temperature have been studied by electron stimulated desorption (ESD). The only ion ejected by electron bombardment (5/~A/cm2, 100 eV) is O + for all the surfaces exposed to 15NO. On (110) and (111) surfaces, the incident electron energy effect on the ion intensity, measured with a quadrupole mass spectrometer (QMS), is characterized by a broad peak with a maximum in the region of 120-150 eV and a gradual increase above 300 eV. The ion energy distribution, measured with the retarding field method, forms a single peak, and its shape does not depend on the electron energy. Various neutral ESD species such as NO, N, O, N 2 and 02 are detected only for the (110) surface. The NO, N 2 and 02 ESD intensities fall in the same manner by heating the sample and disappear at about 420 K. It is considered that a part of the surface species forms molecular NO and complexes such as N20 and NO z. The results for (100) and (111) surfaces give evidence that NO dissociates completely into N and O.
1. Introduction There have been some reports o n the c h e m i s o r p t i o n of nitric oxide on tungsten surfaces with various techniques such as TDS, F E M , AES, UPS, XPS a n d EELS [1-7]. Some workers reported that N O c h e m i s o r b e d dissociatively o n polycrystalline or single-crystal t u n g s t e n surfaces at room t e m p e r a t u r e [1-3]. Recently, the authors have studied the c h e m i s o r p t i o n of N O o n polycrystalline a n d single-crystal (100) a n d (110) t u n g s t e n surfaces u s i n g TDS, F E M , AES a n d U P S [4,5]. It has been shown that N O c h e m i s o r b e d dissociatively on W(100), a n d that there was a molecular state o n W(110) at higher coverages, at room temperature. There is n o study of the c h e m i s o r p t i o n of N O o n tungsten surfaces using ESD. I n this p a p e r we have investigated the a d s o r p t i o n state of N O o n W(100), (110) a n d (111) surfaces b y ESD. The ESD technique is useful for the study of a d s o r p t i o n states. 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
302
S. Sugai et al. / E S D xtudv of NO adsorbed on W(IO0), (110) and (111)
2, Experimental The experimental system was made of stainless steel (ULVAC: model 50) and was evacuated to ultrahigh vacuum by using a getter ion pump. The base pressure in this system was 2 × 10-m Torr. This system was equipped with AES, LEED, QMS and electron gun facilities. The geometric arrangement of the sample, QMS, electron gun and A E S / L E E D facilities is shown in fig. 1. All components were mounted in a horizontal plane and the sample could be rotated around a vertical axis. In ESD measurements, the sample surface was perpendicular to the axis of the mass spectrometer and the electron beam was incident at 35 ° with respect to the surface normal. There was a retarding field analyzer, consisting of three spherical wire gauzes, in front of the QMS. The electron gun supplied currents of 0 5 /~A at energies in the range of 10-500 eV. The ESD intensity was measured with the QMS, and was recorded as a function of the incident electron energy or the retarding voltage. The samples used here were 8 mm in diameter and 0.5 mm in thickness (Metal Crystals Ltd., UK, 99.999% purity). The sample was cleaned by repeated cycles of Ar ion sputtering and heating to 2100 K for several hours. The surfaces were checked for impurities by AES. The temperature was measured by a thermocouple, W - 5 % R e / W - 2 6 % R e , spot-welded on the side face of the sample. In the present experiments, research grade ~sNO gas (99.7% purity) was used to distinguish N~ from CO.
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3. Results and discussion
3.1. Detection of ESD species The existence of ESD ions and neutrals from the tungsten surfaces covered fully with nitric oxide at room temperature was examined with the QMS. The measurement for ions was executed with a constant electron energy of 100 eV and without the ionization electron beam of the QMS. On all of the tungsten surfaces, a high intensity fluence of oxygen ions ejected by electron b o m b a r d ment was detected. The desorbed O + yield had a m a x i m u m in the direction of the surface normal. No other ions were detected with our measurement system. The ESD neutral intensity was measured from the difference between the QMS yields corresponding to the electron beam being switched on and off. For the measurement of ESD neutrals, it is necessary to exclude the signals from the ionization of residual gases, desorbed particles from the walls in the system and ESD ions from the sample. F r o m tests with argon gas leaked into the vacuum chamber, it was verified that residual gases did not make any contribution to the yield change between the electron beam being switched on and off, even if the partial pressure increased to several 10 -8 Torr. Furthermore, other contributions to the yield change were satisfactorily settled by holding the potential of the cathode of the electron gun at 3 V, and by applying an appropriate retarding voltage. The detected ions and neutrals in this work are shown in table 1. It is considered that the reaction of the ESD gives rise to a quantum mechanical energy transfer from the incident electron to the adsorbed particle as seen in the M G R model [8,9] or the K F model [10]. The reaction of the ESD should complete quickly, compared with the period of thermal vibration of surface species. It is almost impossible for the excited particle to combine with its neighbors in the ESD process, but it is possible for it to dissociate into fragments. N O desorbed from W ( l l 0 ) is detected in this work. Therefore, this indicates the existence of molecular N O on the W ( l l 0 ) surface. The absence of neutral N O for (100) and (111) surfaces suggests that N O molecules on the surfaces have been completely dissociated into atomic N and O. On the other Table 1 Detected ESD particles from tungsten surfaces at room temperature Surface
Ion
Neutral
W(100) W(ll0) W(lll)
O+ O+ O+
lsNO, 15N 2, 02, 15N, O -
Examined for all ions below 47 amu, and for neutrals of 15N, O, 15N2, 15NO, 0 2 , 15N20 and 15NO2
304
S. Sugai et al. / ESD study of N O adsorbed on W( I O0), (110) and (111)
hand, it has been found [11,12] that 0 2 adsorbs dissociatively and that N 2 is hardly able to adsorb on the tungsten surface. The existence of desorbed N2 and 0 2 gives evidence that s o m e a m o u n t of molecular N O adsorbed on the (110) surface forms c o m p l e x e s with the dissociated N and O, such as N z O and N O 2.
3.2. Electron energy dependence The ESD intensity dependence on the incident electron energy was measured for the tungsten surfaces exposed fully to ]sNO gas. Experimental results for the O + yield from the (110) and (111) surfaces are shown in fig. 2. Correction for the work function difference between the cathode of the electron gun and the sample has not been made. Each of the curves in fig. 2 is characterized by a broad peak with a maximum in the energy region of 120 to 150 eV and by an increase above 300 eV. The ratio of the intensities in the two energy regions changes with the arrangement of sample position. Results of the threshold measurements are added in fig. 2. From the assumption that the work functions of the clean surfaces and the cathode are all equal to 4.5 eV, and that the increase in the work function with adsorption of NO gas is 1.8 eV, we obtain the minimum electron energy for the desorption of oxygen ions, Eth, as follows. If the incident electron remains in
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the vacuum at the final stage of the ESD reaction, Eth(110 ) = 20.2 eV and Eth(111) = 19.0 eV, and if it falls to the Fermi level of the metal, Eth(ll0 ) = 26.5 eV and Eth(lll ) = 25.3 eV. The dependence of desorption intensities for neutrals from the (110) surface on the incident energy is illustrated in fig. 3. These energy dependences are all similar to the case of the oxygen ion, however the energies at the maximum intensity in the low energy region are rather lower than that of the ion. The discrepancy in the peak positions of the oxygen atom and ion means that the desorption of oxygen is not due to the Auger reneutralization of the ion escaping from the surface. It should be interpreted that the surface neutral is directly excited to an anti-bonding state [13], or the desorption is connected with the "core-hole Auger decay" [10]. 3.3. Kinetic energy of E S D 0 +
Kinetic energy distributions of the oxygen ions desorbed by the electron beam (100 eV and 400 eV, 1 t~A) were measured with the retarding field
S. Sugai et a L / ES D study of N O adsorbed on W( I O0), (110) and (l l l]
306
method at room temperature. Noticeable differences between the results using two different electron energies is not found. The most probable ion kinetic energies for (110) and (111) are 3.2 and 4.6 eV with a full-width-at-half-maximum of 2.6 and 3.0 eV, respectively. Although a slight fluctuation of the peak position is found among some measurements under the various gas adsorption conditions on the retarding mesh, the different of the work function between the mesh and the sample is disregarded. Each of the energy distribution curves has a single peak. Therefore the ESD of oxygen ions seems to be based on only one excitation mechanism.
3. 4. Temperature programmed E S D measurement The changes of ESD intensity owing to thermal desorption were measured with the step heating technique. The sample exposed fully to the N O gas at room temperature was heated by the rear electron beam heater which was put in the shielding case as shown in fig. 1. The sample was then kept at each temperature step for - 5 s. The ESD intensity measurement was executed at near room temperature, The results for oxygen ions are indicated in fig. 4. For all of the surfaces, ion intensities increase in the initial stage of temperature evaluation. Rawlings et al. [1] reported in their work with NO covered W ( I I 0 ) that the AES intensity decreases for O(KLE), and increases for N(KLL) with increasing temperature in the region of about 300 to 800 K. These observations suggest that the N, O and W atoms rearrange themselves on the surface or in the
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Fig. 5. Variation of intensity of neutrals with temperature from NO-coveredW(110). topmost several layers, and that the rearrangement must raise the efficiency of oxygen ion ejection by electron impact. The decrease of intensity in the temperature range from about 600 to 700 K may be due to the diffusion of oxygen atoms into the bulk. The tendency described above seems to continue up to about 950 K. The O ÷ yield curves for (110) and (111) surfaces have shoulders around 800 K. For the (100) surface data, the existence of such a shoulder is not clear, but Pelach et al. [2] reported a distinct shoulder in the XPS intensity curve after step heating. Bauer and Engel [14] indicated from their work on W(110) exposed to 02 that annealing at temperatures above 800 K produced some kind of tungsten oxide layer. The O ÷ yield in the region from about 700 to 1400 K is considered to be due to contributions from atomic oxygen and tungsten oxides. Results of neutral desorption measurements for W ( l l 0 ) are shown in fig. 5. The intensities of NO, N 2 and 02 desorption are instantly reduced in the same manner by heating the sample and they disappear at about 420 K. The temperature dependences of the intensities for N and O are quite similar to the cases of the molecules in the initial stage of heating. After that, they maintain weak yields up to 850 and 1050 K respectively. The intensity behavior of the N O is due to its own thermal desorption and to dissociation of N O into N and O. The extinction temperature of N O yield is in good agreement with the peak temperature of N O desorption obtained by thermal desorption mass spec-
308
s. Sugai et aL / E S D stud), of N O adsorbed on W(IO0), (110) and (111)
t r o s c o p y [4]. C o n v e r s e l y , the e x t i n c t i o n t e m p e r a t u r e o f 0 2 y i e l d is e x t r e m e l y low, in c o m p a r i s o n w i t h its o r d i n a r y d e s o r p t i o n t e m p e r a t u r e o n t u n g s t e n surface. T h e b e h a v i o r o f N 2 a n d 0 2 in E S D m e a s u r e m e n t s s u p p o r t s the p r e v i o u s a r g u m e n t s t h a t c o m p l e x e s s u c h as N 2 0 a n d N O 2 are f o r m i n g . O n the o t h e r h a n d , it s e e m s t h a t e a c h a t o m of n i t r o g e n a n d o x y g e n is e j e c t e d t h r o u g h t w o k i n d s of p r o c e s s e s . O n e o f t h e m c o n t a i n s the p r o d u c t i o n o f the f r a g m e n t a t o m f r o m the e x c i t e d N O , a n d t h e o t h e r is d u e to the e x c i t a t i o n of the a d s o r b e d a t o m . T h e a d s o r p t i o n states o f N a n d O p r o d u c e d b y h e a t i n g o n the (110) s u r f a c e m u s t b e d i f f e r e n t f r o m t h a t of a t o m s a d s o r b e d d i s s o c i a t i v e l y b e f o r e the h e a t i n g , b e c a u s e n o d e s o r b e d n e u t r a l is d e t e c t a b l e o n t h e (100) a n d (111) surfaces.
References [1] [21 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
K.J. Rawlings, S.D. Foulias and B.J. Hopkins, Surface Sci. 108 (1981) 49. E. Pelach, R.E. Viturro and M. Folman, Surface Sci. 161 (1985) 553. R.I. Masel, E. Umbach, J.C. Fuggle and D. Menzel, Surface Sci. 79 (1979) 26. H. Miki, H. Inomata, K. Kato, T. Kioka and K. Kawasaki, Surface Sci. 141 (1984) 473. T. Kioka, A. Kawana, H. Miki, S. Sugai and K. Kawasaki, Surface Sci. 182 (1987) 28. J.T. Yates, Jr. and T.E. Madey, J. Chem. Phys. 45 (1966) 1623. A.K. Bhanacharya, J.Q. Broughton and D.L. Perry, Surface Sci. 78 (1978) L689. D. Menzel and R. Gomer, J. Chem. Phys, 41 (1964) 3311. P.A. Redhead, Can. J. Phys. 42 (1964) 886. M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964. D.A. King, T.E. Madey and J.T. Yates, Jr., J. Chem. Phys. 55 (1971) 3236. P.W. Tamm and L.D. Schmidt, Surface Sci. 26 (1971) 286. M. Nishijima and F.M. Propst, Phys. Rev. B 2 (1970) 2368. E. Bauer and T. Engel, Surface Sci. 71 (1978) 695.
301
S T A T E S OF N O A D S O R B E D O N W(100), (110) AND (111) STUDIED BY E L E C T R O N S T I M U L A T E D D E S O R P T I O N
Shigeru S U G A I , Hiroshi Y O S H I K A W A , H i r o f u m i M I K I , Toshihide K I O K A a n d Koji K A W A S A K I
Department of Physics, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan Received 23 August 1987; accepted for publication 28 October 1987
The adsorption states of 15NO on W(100), (110) and (111) surfaces at room temperature have been studied by electron stimulated desorption (ESD). The only ion ejected by electron bombardment (5/~A/cm2, 100 eV) is O + for all the surfaces exposed to 15NO. On (110) and (111) surfaces, the incident electron energy effect on the ion intensity, measured with a quadrupole mass spectrometer (QMS), is characterized by a broad peak with a maximum in the region of 120-150 eV and a gradual increase above 300 eV. The ion energy distribution, measured with the retarding field method, forms a single peak, and its shape does not depend on the electron energy. Various neutral ESD species such as NO, N, O, N 2 and 02 are detected only for the (110) surface. The NO, N 2 and 02 ESD intensities fall in the same manner by heating the sample and disappear at about 420 K. It is considered that a part of the surface species forms molecular NO and complexes such as N20 and NO z. The results for (100) and (111) surfaces give evidence that NO dissociates completely into N and O.
1. Introduction There have been some reports o n the c h e m i s o r p t i o n of nitric oxide on tungsten surfaces with various techniques such as TDS, F E M , AES, UPS, XPS a n d EELS [1-7]. Some workers reported that N O c h e m i s o r b e d dissociatively o n polycrystalline or single-crystal t u n g s t e n surfaces at room t e m p e r a t u r e [1-3]. Recently, the authors have studied the c h e m i s o r p t i o n of N O o n polycrystalline a n d single-crystal (100) a n d (110) t u n g s t e n surfaces u s i n g TDS, F E M , AES a n d U P S [4,5]. It has been shown that N O c h e m i s o r b e d dissociatively on W(100), a n d that there was a molecular state o n W(110) at higher coverages, at room temperature. There is n o study of the c h e m i s o r p t i o n of N O o n tungsten surfaces using ESD. I n this p a p e r we have investigated the a d s o r p t i o n state of N O o n W(100), (110) a n d (111) surfaces b y ESD. The ESD technique is useful for the study of a d s o r p t i o n states. 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
302
S. Sugai et al. / E S D xtudv of NO adsorbed on W(IO0), (110) and (111)
2, Experimental The experimental system was made of stainless steel (ULVAC: model 50) and was evacuated to ultrahigh vacuum by using a getter ion pump. The base pressure in this system was 2 × 10-m Torr. This system was equipped with AES, LEED, QMS and electron gun facilities. The geometric arrangement of the sample, QMS, electron gun and A E S / L E E D facilities is shown in fig. 1. All components were mounted in a horizontal plane and the sample could be rotated around a vertical axis. In ESD measurements, the sample surface was perpendicular to the axis of the mass spectrometer and the electron beam was incident at 35 ° with respect to the surface normal. There was a retarding field analyzer, consisting of three spherical wire gauzes, in front of the QMS. The electron gun supplied currents of 0 5 /~A at energies in the range of 10-500 eV. The ESD intensity was measured with the QMS, and was recorded as a function of the incident electron energy or the retarding voltage. The samples used here were 8 mm in diameter and 0.5 mm in thickness (Metal Crystals Ltd., UK, 99.999% purity). The sample was cleaned by repeated cycles of Ar ion sputtering and heating to 2100 K for several hours. The surfaces were checked for impurities by AES. The temperature was measured by a thermocouple, W - 5 % R e / W - 2 6 % R e , spot-welded on the side face of the sample. In the present experiments, research grade ~sNO gas (99.7% purity) was used to distinguish N~ from CO.
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S. Sugai et al. / ESD stud)' of NO adsorbed on W(100), (11 O) and (111)
303
3. Results and discussion
3.1. Detection of ESD species The existence of ESD ions and neutrals from the tungsten surfaces covered fully with nitric oxide at room temperature was examined with the QMS. The measurement for ions was executed with a constant electron energy of 100 eV and without the ionization electron beam of the QMS. On all of the tungsten surfaces, a high intensity fluence of oxygen ions ejected by electron b o m b a r d ment was detected. The desorbed O + yield had a m a x i m u m in the direction of the surface normal. No other ions were detected with our measurement system. The ESD neutral intensity was measured from the difference between the QMS yields corresponding to the electron beam being switched on and off. For the measurement of ESD neutrals, it is necessary to exclude the signals from the ionization of residual gases, desorbed particles from the walls in the system and ESD ions from the sample. F r o m tests with argon gas leaked into the vacuum chamber, it was verified that residual gases did not make any contribution to the yield change between the electron beam being switched on and off, even if the partial pressure increased to several 10 -8 Torr. Furthermore, other contributions to the yield change were satisfactorily settled by holding the potential of the cathode of the electron gun at 3 V, and by applying an appropriate retarding voltage. The detected ions and neutrals in this work are shown in table 1. It is considered that the reaction of the ESD gives rise to a quantum mechanical energy transfer from the incident electron to the adsorbed particle as seen in the M G R model [8,9] or the K F model [10]. The reaction of the ESD should complete quickly, compared with the period of thermal vibration of surface species. It is almost impossible for the excited particle to combine with its neighbors in the ESD process, but it is possible for it to dissociate into fragments. N O desorbed from W ( l l 0 ) is detected in this work. Therefore, this indicates the existence of molecular N O on the W ( l l 0 ) surface. The absence of neutral N O for (100) and (111) surfaces suggests that N O molecules on the surfaces have been completely dissociated into atomic N and O. On the other Table 1 Detected ESD particles from tungsten surfaces at room temperature Surface
Ion
Neutral
W(100) W(ll0) W(lll)
O+ O+ O+
lsNO, 15N 2, 02, 15N, O -
Examined for all ions below 47 amu, and for neutrals of 15N, O, 15N2, 15NO, 0 2 , 15N20 and 15NO2
304
S. Sugai et al. / ESD study of N O adsorbed on W( I O0), (110) and (111)
hand, it has been found [11,12] that 0 2 adsorbs dissociatively and that N 2 is hardly able to adsorb on the tungsten surface. The existence of desorbed N2 and 0 2 gives evidence that s o m e a m o u n t of molecular N O adsorbed on the (110) surface forms c o m p l e x e s with the dissociated N and O, such as N z O and N O 2.
3.2. Electron energy dependence The ESD intensity dependence on the incident electron energy was measured for the tungsten surfaces exposed fully to ]sNO gas. Experimental results for the O + yield from the (110) and (111) surfaces are shown in fig. 2. Correction for the work function difference between the cathode of the electron gun and the sample has not been made. Each of the curves in fig. 2 is characterized by a broad peak with a maximum in the energy region of 120 to 150 eV and by an increase above 300 eV. The ratio of the intensities in the two energy regions changes with the arrangement of sample position. Results of the threshold measurements are added in fig. 2. From the assumption that the work functions of the clean surfaces and the cathode are all equal to 4.5 eV, and that the increase in the work function with adsorption of NO gas is 1.8 eV, we obtain the minimum electron energy for the desorption of oxygen ions, Eth, as follows. If the incident electron remains in
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the vacuum at the final stage of the ESD reaction, Eth(110 ) = 20.2 eV and Eth(111) = 19.0 eV, and if it falls to the Fermi level of the metal, Eth(ll0 ) = 26.5 eV and Eth(lll ) = 25.3 eV. The dependence of desorption intensities for neutrals from the (110) surface on the incident energy is illustrated in fig. 3. These energy dependences are all similar to the case of the oxygen ion, however the energies at the maximum intensity in the low energy region are rather lower than that of the ion. The discrepancy in the peak positions of the oxygen atom and ion means that the desorption of oxygen is not due to the Auger reneutralization of the ion escaping from the surface. It should be interpreted that the surface neutral is directly excited to an anti-bonding state [13], or the desorption is connected with the "core-hole Auger decay" [10]. 3.3. Kinetic energy of E S D 0 +
Kinetic energy distributions of the oxygen ions desorbed by the electron beam (100 eV and 400 eV, 1 t~A) were measured with the retarding field
S. Sugai et a L / ES D study of N O adsorbed on W( I O0), (110) and (l l l]
306
method at room temperature. Noticeable differences between the results using two different electron energies is not found. The most probable ion kinetic energies for (110) and (111) are 3.2 and 4.6 eV with a full-width-at-half-maximum of 2.6 and 3.0 eV, respectively. Although a slight fluctuation of the peak position is found among some measurements under the various gas adsorption conditions on the retarding mesh, the different of the work function between the mesh and the sample is disregarded. Each of the energy distribution curves has a single peak. Therefore the ESD of oxygen ions seems to be based on only one excitation mechanism.
3. 4. Temperature programmed E S D measurement The changes of ESD intensity owing to thermal desorption were measured with the step heating technique. The sample exposed fully to the N O gas at room temperature was heated by the rear electron beam heater which was put in the shielding case as shown in fig. 1. The sample was then kept at each temperature step for - 5 s. The ESD intensity measurement was executed at near room temperature, The results for oxygen ions are indicated in fig. 4. For all of the surfaces, ion intensities increase in the initial stage of temperature evaluation. Rawlings et al. [1] reported in their work with NO covered W ( I I 0 ) that the AES intensity decreases for O(KLE), and increases for N(KLL) with increasing temperature in the region of about 300 to 800 K. These observations suggest that the N, O and W atoms rearrange themselves on the surface or in the
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E ~
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0
Z •
0.0
400
600
800 Temperature
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1000
1200
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Fig. 4. V a r i a t i o n of O ~ i n t e n s i t y with t e m p e r a t u r e .
....
1400
&_ _ . . A _
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S. Sugai et aL / ESD study of NO adsorbed on W(IO0), (110) and (111)
3
307
(110)
N2
2 N(
]
( c
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,
,
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o
400
60O
80O--
1000
Temperature, K
Fig. 5. Variation of intensity of neutrals with temperature from NO-coveredW(110). topmost several layers, and that the rearrangement must raise the efficiency of oxygen ion ejection by electron impact. The decrease of intensity in the temperature range from about 600 to 700 K may be due to the diffusion of oxygen atoms into the bulk. The tendency described above seems to continue up to about 950 K. The O ÷ yield curves for (110) and (111) surfaces have shoulders around 800 K. For the (100) surface data, the existence of such a shoulder is not clear, but Pelach et al. [2] reported a distinct shoulder in the XPS intensity curve after step heating. Bauer and Engel [14] indicated from their work on W(110) exposed to 02 that annealing at temperatures above 800 K produced some kind of tungsten oxide layer. The O ÷ yield in the region from about 700 to 1400 K is considered to be due to contributions from atomic oxygen and tungsten oxides. Results of neutral desorption measurements for W ( l l 0 ) are shown in fig. 5. The intensities of NO, N 2 and 02 desorption are instantly reduced in the same manner by heating the sample and they disappear at about 420 K. The temperature dependences of the intensities for N and O are quite similar to the cases of the molecules in the initial stage of heating. After that, they maintain weak yields up to 850 and 1050 K respectively. The intensity behavior of the N O is due to its own thermal desorption and to dissociation of N O into N and O. The extinction temperature of N O yield is in good agreement with the peak temperature of N O desorption obtained by thermal desorption mass spec-
308
s. Sugai et aL / E S D stud), of N O adsorbed on W(IO0), (110) and (111)
t r o s c o p y [4]. C o n v e r s e l y , the e x t i n c t i o n t e m p e r a t u r e o f 0 2 y i e l d is e x t r e m e l y low, in c o m p a r i s o n w i t h its o r d i n a r y d e s o r p t i o n t e m p e r a t u r e o n t u n g s t e n surface. T h e b e h a v i o r o f N 2 a n d 0 2 in E S D m e a s u r e m e n t s s u p p o r t s the p r e v i o u s a r g u m e n t s t h a t c o m p l e x e s s u c h as N 2 0 a n d N O 2 are f o r m i n g . O n the o t h e r h a n d , it s e e m s t h a t e a c h a t o m of n i t r o g e n a n d o x y g e n is e j e c t e d t h r o u g h t w o k i n d s of p r o c e s s e s . O n e o f t h e m c o n t a i n s the p r o d u c t i o n o f the f r a g m e n t a t o m f r o m the e x c i t e d N O , a n d t h e o t h e r is d u e to the e x c i t a t i o n of the a d s o r b e d a t o m . T h e a d s o r p t i o n states o f N a n d O p r o d u c e d b y h e a t i n g o n the (110) s u r f a c e m u s t b e d i f f e r e n t f r o m t h a t of a t o m s a d s o r b e d d i s s o c i a t i v e l y b e f o r e the h e a t i n g , b e c a u s e n o d e s o r b e d n e u t r a l is d e t e c t a b l e o n t h e (100) a n d (111) surfaces.
References [1] [21 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
K.J. Rawlings, S.D. Foulias and B.J. Hopkins, Surface Sci. 108 (1981) 49. E. Pelach, R.E. Viturro and M. Folman, Surface Sci. 161 (1985) 553. R.I. Masel, E. Umbach, J.C. Fuggle and D. Menzel, Surface Sci. 79 (1979) 26. H. Miki, H. Inomata, K. Kato, T. Kioka and K. Kawasaki, Surface Sci. 141 (1984) 473. T. Kioka, A. Kawana, H. Miki, S. Sugai and K. Kawasaki, Surface Sci. 182 (1987) 28. J.T. Yates, Jr. and T.E. Madey, J. Chem. Phys. 45 (1966) 1623. A.K. Bhanacharya, J.Q. Broughton and D.L. Perry, Surface Sci. 78 (1978) L689. D. Menzel and R. Gomer, J. Chem. Phys, 41 (1964) 3311. P.A. Redhead, Can. J. Phys. 42 (1964) 886. M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964. D.A. King, T.E. Madey and J.T. Yates, Jr., J. Chem. Phys. 55 (1971) 3236. P.W. Tamm and L.D. Schmidt, Surface Sci. 26 (1971) 286. M. Nishijima and F.M. Propst, Phys. Rev. B 2 (1970) 2368. E. Bauer and T. Engel, Surface Sci. 71 (1978) 695.