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Chemical and physical adsorption of oxygen on the (110) plane of tungsten
SURFACE
SCIENCE 25 (1971) 633-642 0 North-Holland
CHEMICAL
AND PHYSICAL
ADSORPTION
(110) PLANE
B. J. HOPKINS, Surface Physics,
OF OXYGEN
ON THE
OF TUNGSTEN
C. B. WILLIAMS
The University
Publishing Co.
of Southampton,
and P. C. WILMER* Southampton
SO9 5NH. England
Received 14 September 1970 Low energy electron diffraction and the Zisman modification of the Kelvin contact potential difference method have been used to follow the structural and surface potential changes during the adsorption of oxygen on the (110) surface of tungsten at temperatures of 300 and 80 K. Two chemisorbed states are formed at 300 K. The first of these exhibits a relationship between surface coverage, 0, and incident flux of gas atoms, closer to linear than a square law and a sticking coefficient of approximately 0.2. The second state, which begins to form at a coverage of half a monolayer, requires thermal activation and has a much lower sticking coefficient. At low temperatures (80 K) the first chemisorbed state is again formed and the work function versus incident flux is identical to that at 300 K up to B = 0.5. At this coverage, unlike the 300 K state, the oxygen is not ordered. Beyond 0 = 0.5 an electropositive state forms. It is readily desorbed by a 108 eV LEED beam or by heating. The characteristic energy of desorption has been measured to be 0.08 eV. Since this low binding energy and large positive surface dipole are similar to the corresponding values obtained for inert gas adsorption on tungsten single crystals, this state is considered to be one of physisorption.
1. Introduction The LEED patterns observed on adsorption of oxygen on (110) tungsten at temperatures of 300 K and above have been well establishedITs) but their interpretation has notl-4). The associated work function changes have also been measureds’5.s”). At lower temperatures, however, measurements are restricted to those of Bell et al.s) who used a field emission technique. The object of the present work is to report some interesting new features of the low temperature (80 K) adsorption which have been observed using LEED and work function measurements. 2. Experimental The work
function
measurements
were performed
* Present address: Berkeley Nuclear Laboratories, Berkeley, Glouscetershire, England. 633
in both
the LEED
Central Electricity Generating
Board,
634
B. XHOPKINS,
C.B.
WILLIAMS
Fig. l(a)
Fig. l(b)
AND
P.C.
WILMER
ADSORPTION OF OXYGEN ON (110)
PLANE OF TUNGSTEN
635
Fig. l(c)
Fig. l(d) Fig. 1. LEED patterns at 1OSeV For oxygen adsarbed on the W(110) surface for: (a) Clean surface; (b) 0.5 monolayers at 300 K; (c) 0.5 monolayers at 80 K; (d) heavily exposed surface at 80 K. The two anomalous “spots” common to all the photographs are due to field emission from the grids.
636
B. 5. HOPKINS,
C. 8. WlLLIAMS
AND
P. C. WILMER
apparatus and a mercury pumped glass chamber which has already been describeds); the LEED observations were made in a standard Varian two grid system. In the latter the crystal was mounted in a fixed position on a tungsten rod which passed into the chamber through a re-entrant glass side arm. Liquid nitrogen surrounding the emergent end of the rod caused conduction cooling of the crystal. The temperature of the latter was followed using a high temperature W/W 26% Rh thermocouple (calibrated to 80 XI) spot welded close to the crystal. A reference electrode of aged and cleaned polycrystalline tungsten sheet, ultimately saturated with oxygen, was mounted on a special bellows and permitted the work function of the crystal to be followed using the Zisman modification of the Kelvin technique. Cleaning of the crystal was by extensive heat treatment in an oxygen atmosphere followed by high temperature flashing. The resulting well-defined LEED spot pattern is shown in fig. la. Oxygen was admitted to the glass chamber through a silver diffusion tube and to the LEED system via a variable leak valve. The gas purity was comparable with that of previous work in this laboratory 5). 3. Results The LEED observations at 300 K were essentially the same as those already established. The work function versus exposure curve is shown in fig. 2 and exhibits two clear and distinct regions. At 80 K the curve was exactly the same over the first stage of the adsorption as that at 300 K but then a work function decrease took place with increasing oxygen exposure. The only change in LEED pattern throughout the latter process was an increasing diffuseness of the spots which are characteristic of the clean (110) surface up to 6’=0.5. Fig. Id shows this effect at saturation. In experiments to study this low temperature state more closefy the effects of heating and of electron bombardment were examined. When the surface was covered to point A (fig. 2) at 80 K and the pattern of fig. Ic observed, the surface was warmed to 300 K. The p(2 x 1) pattern typical of this exposure at 300 K then appeared (fig. 1b). In the region A to B of the work function curves at 80 K there was no change in the LEED pattern at all. This was suspected to be caused by oxygen desorption with the LEED beam. The Kelvin technique provides an ideal way of checking this since it does not disturb the surface in any way. The surface was therefore exposed at 80 K to point B (fig. 2) using the work function as indicator. The LEED beam (108 eV) was switched onto the surface and defocused for a few minutes while the true LEED pattern was as shown in fig. lc and Id. The LEED beam was then removed and the work function determined. This had now increased and moved towards A of fig. 2. Repeating this procedure gradually
ADSORPTION
OF OXYGEN
ON (110)
PLANE
OF TUNGSTEN
637
increased the work function until, when magnets external to the vacuum were used to fully scan the crystal, the value at A was obtained. No further change was then observed. During this process no pressure change was observed on either the gauge or the quadrupole mass spectrometer. 6.1
I
,,---O5.9-
2
z = 2
a-*-e*
5.5.
/
1
cf
I
-*\ ‘\
,/*
5*3-
‘\
,CV* _-
y,
‘1
,.
‘1
5.1-
B
o 300K
“r 49-
.
4.71
‘d8OK
0,
molecules
Change in work function
0.6
\ -
I lOI
lOId
10”
_
‘\
8OK
I lOI
Fig. 2.
K
0’
A
-57-
‘0 .-
300
/-
10”
incident
/
10IP
cm?
with oxygen exposure W(ll0) plane.
I
1o’8
at 300 K and SOK for the
, .@
0.5
-
‘3‘
.
/
-
i?s go;Q
.
/
6
r
u 0.3
.
0’ ._ t t
/
/
0,2-
l
a /
tz g
0.1
-
i /’ .I*
@O-
0
100
temperature
Fig. 3.
#
I
200
300
(OK)
Change in work function with temperature for oxygen previously adsorbed the W (110) surface at 80 K.
on
638
B. J. HOPKINS,
C. B. WILLIAMS
AND P. C. WILMER
Finally, starting at point B on the work function versus exposure curve the crystal was allowed to warm up to 300 K and the work function followed continually. The result is shown in fig. 3 and in fig. 4 is an Arrhenius plot which gives for the desorption
energy a value of 0.08 eV.
-24
,
I
I
50
4*0
\*.
4
6.0
I/T x 103”K-’ Fig. 4.
An Arrhenius-type
plot for the W(110) surface derived from the data in fig. 3.
4. Discussion The work function versus exposure curves of fig. 2 show clearly that the adsorption of oxygen on the (110) face of tungsten takes place in two quite distinct phases. The initial phase, formed at 300 K, has been examined in some detail by Tracy and Blakely2) using the same techniques as those of the present work. The model which they proposed was one of initial molecular adsorption followed by migration to growing islands of chemisorbed oxygen. The remarkable sharpness of the p(2 x 1) diffraction features from their first appearance can be accounted for on the basis of such agrowth process. Aside from the diffraction patterns “kinetic data” was obtained by Tracy and Blakely from the work function versus exposure curves in support of the model. They consider that, provided the Kelvin technique is used, the surface potential associated with island formation is a linear function of coverage. However, the same can also be true for isolated dipoles if only low coverages are considered since depolarisation forces are then very small. The exact coincidence between the 80 and 300 K work function curves of fig. 2 could be an illustration of this point. The diffuseness of the LEED pattern at 80 K suggests that isolated dipoles are formed. Since the work function at Point A of fig. 2 is the same at 300 K as that at 80 K this suggests isolated dipoles at 300 K also, but now in an ordered form. Given that the depolarisa-
ADSORPTION
OF OXYGEN
ON (110)
tion effects are not too large in the islands,
PLANE
639
OF TUNGSTEN
it is still probably
not possible
to
use surface potential data in this way to establish the growth mechanism. Fitting the two separately measured curves together could probably not be done to better than 10 mV in the first stage of the adsorption. Depolarisation effects could easily be of this order. By making the assumption that for coverage up to 0.2 monolayer the work function change is directly proportional to the coverage, Tracy and Blakeley show the functional dependence of the coverage 8 on exposure I to be (30~1~(the actual slope of their log-log plot is 2.3) in agreement with a model in which the island growth is surface diffusion limited. The functional relation we determine is 1.3 (fig. 5). The error bars shown are those that would be due to individual measurements (+ 10 mV on each). In practice the points were obtained from a continuous chart recording and the errors are probably appreciably less than indicated. Early results obtained by one of us5) were not sufficiently accurate for this analysis and also, as pointed out by Tracy and Blakely and confirmed by Williamss), contained an inaccuracy at
10.
no.
Fig. 5.
of
molecules
incident
/
cm2
Plot to determine the functional relationship between the coverage, 8, obtained from the p(2 x 1) structure, and the exposure, 2. The result is 0 a 11.3.
640
B. J. HOPKINS,
C. B. WILLIAMS
AND
P. C. WILMER
8>0.5 which rendered the plateau of fig. 2 as a dip, The overall surface potential due to the first phase of adsorption giving rise to a p(2 x 1) structure was 0.42 eV in the present work compared with 0.7 eV as estimated from the maximum intensity of the p(2 x 1) LEED spots by Tracy and Blakely. The sticking coefficient in the region up to about 0.4 monolayer (work function 5.45 eV) was measured to be 0.25. This value compares well with that of 0.26 obtained by Kohrt and Gomerle). Another interesting feature of the two work function curves below the half monolayer is that they imply exactly the same sticking coelhcient at both 80 and 300 K. For temperatures of about 300 K Tracy and Blakely inferred a sticking coefficient that decreased with increasing temperature. The usual interpretation of the diffraction datal) indicates that between 8 =0.5 and 1.0 exactly the same structure is built up on the tungsten as between 13= 0 and OS. This appears to explain the observation in the present measurements that, at 300 K, the surface potential due to the first half monolayer is within 5% of that due to the second half. The good agreement also indicates that the depolarisation forces due to the two nearest neighbours in the half monolayer structure are not significantly different from those for four nearest neighbours at the full monolayer. The final value of the work function was 6.00 eV. The corresponding surface potential was 0.85 eV. These values differ considerably from those due to other workers. Zingerman and Ishchuk@) obtained a value of 6.15 eV for the work function at saturation and a total surface potentiaf of 1.28 V was measured by Tracy and Blakely e), From 0.5 to 0.75 monoIayers the work function-exposure plot in the present work, fig. 6, is linear but indicate a much lower sticking coefficient, 3 x 10v3. The functional dependence of coverage on exposure is now 1.5 compared with 1.3 in the first stage. At 80 K the second 300 K phase was not formed at all, in agreement with the suggestion of Zingerman and Ishchuk@) that thermal activation is necessary. Instead, a new electropositive phase was observed that has not been previously reported. The very rapid decrease in work function with increasing oxygen exposure beyond a coverage of 0.5 is very similar to physical adsorption of the inert gases onto tungsten crystals at 80 Kll). In view of this and the low measured heat of desorption, 0.08 eV, this phase is though to be a physisorbed molecular state forming on top of the chemisorbed half monolayer. As indicated in the previous section, the 108 eV electrons of the LEED beam were sufficient to desorb this state completely. The only other low temperature study of the oxygen-tungsten system was by field emission*) in which the electropositive phase was not observed. There it is possible that the electric field may have been sufficient to desorb the gas. Electron induced desorption is, of course, well known12).
ADSORPTION
OF OXYGEN
ON (110)
PLANE
641
OF TUNGSTEN
Tuckerrs) observed changes in carbon monoxide on platinum electron beam in LEED and suggested that it may be impossible physisorption by LEED. Steiger et a1.14) also saw no extra features temperature physisorption of oxygen on silver. This could be due
due to the to observe in the low to desorp-
0.5 -9 & go.4 :
-
uti 0.3
-
.
.
.
._ c g 0.2
-
ezo.75 I/
2 Y Eo.1
-
i ezo.5
3
./
. 0.0
J
0
Fig. 6.
I
v
no. of
5molecules
incidEn+ / cm?
x 1016
15
Change in work function with oxygen exposure at 300 K on the W(110) surface for coverages greater than 0.5 monolayers.
tion in the LEED beam suggest. New diffraction carbonis). It is possible comparatively low. The surface, appears to be an
rather than to the disordered structure which they features have been observed however for xenon on in this case that the collision cross section may be Kelvin technique, causing no perturbation to the ideal method for examining physisorbed layers. Acknowledgements
One of us, PCW, would like to thank the Science Research provision of a maintenance grant.
Council
References 1) 2) 3) 4) 5) 6) 7) 8)
L. H. Germer and J. W. May, Surface Sci. 4 (1966) 452. J. C. Tracy and J. M. Blakely, Surface Sci. 15 (1969) 257. J. W. May, Surface Sci. 18 (1969) 431. J. J. Carroll and A. J. Melmed, Surface Sci. 16 (1969) 251. B. J. Hopkins and K. R. Pender, Surface Sci. 5 (1966) 155. Ya. P. Zingerman and V. A. Ishchuk, Soviet Phys.-Solid State 8 (1966) 728. T. D. Madey and J. T. Yates, Jr., Nuovo Cimento 5 (1967) 482. A. E. Bell, L. W. Swanson and L. C. Crouser, Surface Sci. 10 (1968) 254.
for the
642 9) 10) 11) 12) 13) 14)
B.J. HOPKINS,
C. B. WILLIAMS
AND
P. C. WILMER
C. B. Williams, Ph.D. Thesis, University of Southampton (1969). C. Kohrt and R. Gomer, J. Chem. Phys. 48 (1968) 3337. B. J. Hopkins and C. B. Williams, unpublished results. Ya. P. Zingerman and V. A. Ishchuk, Soviet Phys.-Solid State 9 (1967) 623. C. W. Tucker, Jr., Surface Sci. 2 (1964) 516. R. F. Steiger, J. M. Morabito, Jr., G. A. Somorjai and R. H. Muller, Surface Sci. 14 (1969) 279. 15) J. J. Lander and J. Morrison, Surface Sci. 6 (1967) 1.
SCIENCE 25 (1971) 633-642 0 North-Holland
CHEMICAL
AND PHYSICAL
ADSORPTION
(110) PLANE
B. J. HOPKINS, Surface Physics,
OF OXYGEN
ON THE
OF TUNGSTEN
C. B. WILLIAMS
The University
Publishing Co.
of Southampton,
and P. C. WILMER* Southampton
SO9 5NH. England
Received 14 September 1970 Low energy electron diffraction and the Zisman modification of the Kelvin contact potential difference method have been used to follow the structural and surface potential changes during the adsorption of oxygen on the (110) surface of tungsten at temperatures of 300 and 80 K. Two chemisorbed states are formed at 300 K. The first of these exhibits a relationship between surface coverage, 0, and incident flux of gas atoms, closer to linear than a square law and a sticking coefficient of approximately 0.2. The second state, which begins to form at a coverage of half a monolayer, requires thermal activation and has a much lower sticking coefficient. At low temperatures (80 K) the first chemisorbed state is again formed and the work function versus incident flux is identical to that at 300 K up to B = 0.5. At this coverage, unlike the 300 K state, the oxygen is not ordered. Beyond 0 = 0.5 an electropositive state forms. It is readily desorbed by a 108 eV LEED beam or by heating. The characteristic energy of desorption has been measured to be 0.08 eV. Since this low binding energy and large positive surface dipole are similar to the corresponding values obtained for inert gas adsorption on tungsten single crystals, this state is considered to be one of physisorption.
1. Introduction The LEED patterns observed on adsorption of oxygen on (110) tungsten at temperatures of 300 K and above have been well establishedITs) but their interpretation has notl-4). The associated work function changes have also been measureds’5.s”). At lower temperatures, however, measurements are restricted to those of Bell et al.s) who used a field emission technique. The object of the present work is to report some interesting new features of the low temperature (80 K) adsorption which have been observed using LEED and work function measurements. 2. Experimental The work
function
measurements
were performed
* Present address: Berkeley Nuclear Laboratories, Berkeley, Glouscetershire, England. 633
in both
the LEED
Central Electricity Generating
Board,
634
B. XHOPKINS,
C.B.
WILLIAMS
Fig. l(a)
Fig. l(b)
AND
P.C.
WILMER
ADSORPTION OF OXYGEN ON (110)
PLANE OF TUNGSTEN
635
Fig. l(c)
Fig. l(d) Fig. 1. LEED patterns at 1OSeV For oxygen adsarbed on the W(110) surface for: (a) Clean surface; (b) 0.5 monolayers at 300 K; (c) 0.5 monolayers at 80 K; (d) heavily exposed surface at 80 K. The two anomalous “spots” common to all the photographs are due to field emission from the grids.
636
B. 5. HOPKINS,
C. 8. WlLLIAMS
AND
P. C. WILMER
apparatus and a mercury pumped glass chamber which has already been describeds); the LEED observations were made in a standard Varian two grid system. In the latter the crystal was mounted in a fixed position on a tungsten rod which passed into the chamber through a re-entrant glass side arm. Liquid nitrogen surrounding the emergent end of the rod caused conduction cooling of the crystal. The temperature of the latter was followed using a high temperature W/W 26% Rh thermocouple (calibrated to 80 XI) spot welded close to the crystal. A reference electrode of aged and cleaned polycrystalline tungsten sheet, ultimately saturated with oxygen, was mounted on a special bellows and permitted the work function of the crystal to be followed using the Zisman modification of the Kelvin technique. Cleaning of the crystal was by extensive heat treatment in an oxygen atmosphere followed by high temperature flashing. The resulting well-defined LEED spot pattern is shown in fig. la. Oxygen was admitted to the glass chamber through a silver diffusion tube and to the LEED system via a variable leak valve. The gas purity was comparable with that of previous work in this laboratory 5). 3. Results The LEED observations at 300 K were essentially the same as those already established. The work function versus exposure curve is shown in fig. 2 and exhibits two clear and distinct regions. At 80 K the curve was exactly the same over the first stage of the adsorption as that at 300 K but then a work function decrease took place with increasing oxygen exposure. The only change in LEED pattern throughout the latter process was an increasing diffuseness of the spots which are characteristic of the clean (110) surface up to 6’=0.5. Fig. Id shows this effect at saturation. In experiments to study this low temperature state more closefy the effects of heating and of electron bombardment were examined. When the surface was covered to point A (fig. 2) at 80 K and the pattern of fig. Ic observed, the surface was warmed to 300 K. The p(2 x 1) pattern typical of this exposure at 300 K then appeared (fig. 1b). In the region A to B of the work function curves at 80 K there was no change in the LEED pattern at all. This was suspected to be caused by oxygen desorption with the LEED beam. The Kelvin technique provides an ideal way of checking this since it does not disturb the surface in any way. The surface was therefore exposed at 80 K to point B (fig. 2) using the work function as indicator. The LEED beam (108 eV) was switched onto the surface and defocused for a few minutes while the true LEED pattern was as shown in fig. lc and Id. The LEED beam was then removed and the work function determined. This had now increased and moved towards A of fig. 2. Repeating this procedure gradually
ADSORPTION
OF OXYGEN
ON (110)
PLANE
OF TUNGSTEN
637
increased the work function until, when magnets external to the vacuum were used to fully scan the crystal, the value at A was obtained. No further change was then observed. During this process no pressure change was observed on either the gauge or the quadrupole mass spectrometer. 6.1
I
,,---O5.9-
2
z = 2
a-*-e*
5.5.
/
1
cf
I
-*\ ‘\
,/*
5*3-
‘\
,CV* _-
y,
‘1
,.
‘1
5.1-
B
o 300K
“r 49-
.
4.71
‘d8OK
0,
molecules
Change in work function
0.6
\ -
I lOI
lOId
10”
_
‘\
8OK
I lOI
Fig. 2.
K
0’
A
-57-
‘0 .-
300
/-
10”
incident
/
10IP
cm?
with oxygen exposure W(ll0) plane.
I
1o’8
at 300 K and SOK for the
, .@
0.5
-
‘3‘
.
/
-
i?s go;Q
.
/
6
r
u 0.3
.
0’ ._ t t
/
/
0,2-
l
a /
tz g
0.1
-
i /’ .I*
@O-
0
100
temperature
Fig. 3.
#
I
200
300
(OK)
Change in work function with temperature for oxygen previously adsorbed the W (110) surface at 80 K.
on
638
B. J. HOPKINS,
C. B. WILLIAMS
AND P. C. WILMER
Finally, starting at point B on the work function versus exposure curve the crystal was allowed to warm up to 300 K and the work function followed continually. The result is shown in fig. 3 and in fig. 4 is an Arrhenius plot which gives for the desorption
energy a value of 0.08 eV.
-24
,
I
I
50
4*0
\*.
4
6.0
I/T x 103”K-’ Fig. 4.
An Arrhenius-type
plot for the W(110) surface derived from the data in fig. 3.
4. Discussion The work function versus exposure curves of fig. 2 show clearly that the adsorption of oxygen on the (110) face of tungsten takes place in two quite distinct phases. The initial phase, formed at 300 K, has been examined in some detail by Tracy and Blakely2) using the same techniques as those of the present work. The model which they proposed was one of initial molecular adsorption followed by migration to growing islands of chemisorbed oxygen. The remarkable sharpness of the p(2 x 1) diffraction features from their first appearance can be accounted for on the basis of such agrowth process. Aside from the diffraction patterns “kinetic data” was obtained by Tracy and Blakely from the work function versus exposure curves in support of the model. They consider that, provided the Kelvin technique is used, the surface potential associated with island formation is a linear function of coverage. However, the same can also be true for isolated dipoles if only low coverages are considered since depolarisation forces are then very small. The exact coincidence between the 80 and 300 K work function curves of fig. 2 could be an illustration of this point. The diffuseness of the LEED pattern at 80 K suggests that isolated dipoles are formed. Since the work function at Point A of fig. 2 is the same at 300 K as that at 80 K this suggests isolated dipoles at 300 K also, but now in an ordered form. Given that the depolarisa-
ADSORPTION
OF OXYGEN
ON (110)
tion effects are not too large in the islands,
PLANE
639
OF TUNGSTEN
it is still probably
not possible
to
use surface potential data in this way to establish the growth mechanism. Fitting the two separately measured curves together could probably not be done to better than 10 mV in the first stage of the adsorption. Depolarisation effects could easily be of this order. By making the assumption that for coverage up to 0.2 monolayer the work function change is directly proportional to the coverage, Tracy and Blakeley show the functional dependence of the coverage 8 on exposure I to be (30~1~(the actual slope of their log-log plot is 2.3) in agreement with a model in which the island growth is surface diffusion limited. The functional relation we determine is 1.3 (fig. 5). The error bars shown are those that would be due to individual measurements (+ 10 mV on each). In practice the points were obtained from a continuous chart recording and the errors are probably appreciably less than indicated. Early results obtained by one of us5) were not sufficiently accurate for this analysis and also, as pointed out by Tracy and Blakely and confirmed by Williamss), contained an inaccuracy at
10.
no.
Fig. 5.
of
molecules
incident
/
cm2
Plot to determine the functional relationship between the coverage, 8, obtained from the p(2 x 1) structure, and the exposure, 2. The result is 0 a 11.3.
640
B. J. HOPKINS,
C. B. WILLIAMS
AND
P. C. WILMER
8>0.5 which rendered the plateau of fig. 2 as a dip, The overall surface potential due to the first phase of adsorption giving rise to a p(2 x 1) structure was 0.42 eV in the present work compared with 0.7 eV as estimated from the maximum intensity of the p(2 x 1) LEED spots by Tracy and Blakely. The sticking coefficient in the region up to about 0.4 monolayer (work function 5.45 eV) was measured to be 0.25. This value compares well with that of 0.26 obtained by Kohrt and Gomerle). Another interesting feature of the two work function curves below the half monolayer is that they imply exactly the same sticking coelhcient at both 80 and 300 K. For temperatures of about 300 K Tracy and Blakely inferred a sticking coefficient that decreased with increasing temperature. The usual interpretation of the diffraction datal) indicates that between 8 =0.5 and 1.0 exactly the same structure is built up on the tungsten as between 13= 0 and OS. This appears to explain the observation in the present measurements that, at 300 K, the surface potential due to the first half monolayer is within 5% of that due to the second half. The good agreement also indicates that the depolarisation forces due to the two nearest neighbours in the half monolayer structure are not significantly different from those for four nearest neighbours at the full monolayer. The final value of the work function was 6.00 eV. The corresponding surface potential was 0.85 eV. These values differ considerably from those due to other workers. Zingerman and Ishchuk@) obtained a value of 6.15 eV for the work function at saturation and a total surface potentiaf of 1.28 V was measured by Tracy and Blakely e), From 0.5 to 0.75 monoIayers the work function-exposure plot in the present work, fig. 6, is linear but indicate a much lower sticking coefficient, 3 x 10v3. The functional dependence of coverage on exposure is now 1.5 compared with 1.3 in the first stage. At 80 K the second 300 K phase was not formed at all, in agreement with the suggestion of Zingerman and Ishchuk@) that thermal activation is necessary. Instead, a new electropositive phase was observed that has not been previously reported. The very rapid decrease in work function with increasing oxygen exposure beyond a coverage of 0.5 is very similar to physical adsorption of the inert gases onto tungsten crystals at 80 Kll). In view of this and the low measured heat of desorption, 0.08 eV, this phase is though to be a physisorbed molecular state forming on top of the chemisorbed half monolayer. As indicated in the previous section, the 108 eV electrons of the LEED beam were sufficient to desorb this state completely. The only other low temperature study of the oxygen-tungsten system was by field emission*) in which the electropositive phase was not observed. There it is possible that the electric field may have been sufficient to desorb the gas. Electron induced desorption is, of course, well known12).
ADSORPTION
OF OXYGEN
ON (110)
PLANE
641
OF TUNGSTEN
Tuckerrs) observed changes in carbon monoxide on platinum electron beam in LEED and suggested that it may be impossible physisorption by LEED. Steiger et a1.14) also saw no extra features temperature physisorption of oxygen on silver. This could be due
due to the to observe in the low to desorp-
0.5 -9 & go.4 :
-
uti 0.3
-
.
.
.
._ c g 0.2
-
ezo.75 I/
2 Y Eo.1
-
i ezo.5
3
./
. 0.0
J
0
Fig. 6.
I
v
no. of
5molecules
incidEn+ / cm?
x 1016
15
Change in work function with oxygen exposure at 300 K on the W(110) surface for coverages greater than 0.5 monolayers.
tion in the LEED beam suggest. New diffraction carbonis). It is possible comparatively low. The surface, appears to be an
rather than to the disordered structure which they features have been observed however for xenon on in this case that the collision cross section may be Kelvin technique, causing no perturbation to the ideal method for examining physisorbed layers. Acknowledgements
One of us, PCW, would like to thank the Science Research provision of a maintenance grant.
Council
References 1) 2) 3) 4) 5) 6) 7) 8)
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