- Email: [email protected]
The dissociative electronic desorption of carbon monoxide from tungsten
SURFACE
THE
SCIENCE
11 (1968) 61-81 0 North-Holland
DISSOCIATIVE
ELECTRONIC
MONOXIDE
Publishing
DESORPTION
Co., Amsterdam
OF CARBON
FROM TUNGSTEN *
J. W. COBURN** PhysicaI
Electronics
Laboratory,
Received 8 November
University of Minnesota, U.S.A.
1967; revised manuscript
Minneapolis,
Minnesota
5.5454,
received 22 January 1968
A tungsten surface covered with adsorbed CO molecules has been subjected to low energy electron bombardment. A current of 0+ ions originating at the bombarded surface was observed but no C+ or CO+ ions were seen. The experimental structure permitted the observation of either the O+ surface ion current or the gas-phase CO+ ion current as the tungsten surface temperature was increased at a controlled rate. These thermal desorption spectra did not align with each other on the temperature axis in the expected way, and in order to explain this observation it is suggested that the electronic desorption efficiency is not a constant for CO molecules adsorbed in a specific type of site. The dependence of the 0+ current on the electron energy, the electron current density, and the CO pressure has been determined. An approximate energy distribution of the 0+ ions has been measured and values for the energy threshold, the minimum kinetic energy of the desorbed 0+ ions, and the average efficiency for the production of an 0+ ion were also obtained. An electron-bombardment-induced 0+ current which increases with increasing surface temperature has been observed for surface temperatures between 1000 and 1300°K.
1. Introduction The desorption of adsorbed species from surfaces by low energy electron impact has been studied recently by several workers. Moorel), Redhead 2-4), Lichtman and co-workerss-9), Sandstromls), and Yatesrl) have studied the process by observing the ionic desorption products. Menzel and Gomerr2mr4) and Ermichl5) have used the techniques of field emission microscopy to observe changes in the surface coverage caused by electron bombardment. Petermannls) has used a partial pressure analyzer to observe electronbombardment-induced pressure changes in a vacuum system. Most of this work has been carried out with one of the gases, carbon monoxide, oxygen, or hydrogen on molybdenum, tungsten or .nickel surfaces. The type of * Work supported by the Avionics Laboratory, Air Force Systems Command, United States Air Force. ** Present address: Department of Physics, Simon Fraser University, Burnaby 2, British Columbia, Canada. 61
62
J. W.
COBURN
measurements that have been made and the results that have been obtained vary considerably. The motivation for the work to be described in this paper was the disturbing influence introduced by the electronic desorption process on a study of the dissociation of MgO thin films by electron bombardment. ft was found that the surface ion current caused by electronic desorption of adsorbed CO, 0, and Mg obscured that caused by the dissociatron process.
2. Experimental
system
The experimental arrangement used in this work is shown in fig. 1. The electrons from the tungsten filament are directed onto the target by a retarding electric field between Plates 1 and 2. The ionic desorption products are drawn from the target surface through a series of grids and an electrostatic focusing and deflection system into a quadrupole mass spectrometerr’). The output of the mass spectrometer was directed onto the first dynode of a lo-stage electron multiplier with a measured gain of about 2 x 106. Grids 1 and 2 were kept at, or slightly above, the potential of the target when the surface ion current was being observed. The potential barrier presented by these grids prevents ions formed in the gas phase from reaching the mass spectrometer. The ambient gas can be mass-analyzed by setting the potential of Grids 1 and 2 below the target potential (but above the potential of Plate
Fig.
I.
Schematic
diagram
of experimental
system
DISSOCIATIVE
1) thus permitting Plate
1 to reach
ions formed
ELECTRONIC
DESORPTION
OF co
in part of the volume
the mass spectrometer.
The ability
between
63
the target and
to observe
either
the
surface ion current or the gas-phase ion current, as the target temperature is increased at a controlled rate, has been very useful in this study. Grids 1 and 2 have also been used to record retarding potential plots of the surface ion current. The ion energy distributions which are obtained by numerical differentiation of these plots are, however, only approximations to the true energy distributions because of the unfavorable geometry in the ion collection system. The tungsten target was a ribbon 0.0076 mm thick, 6.3 mm wide and 25 mm long. Only the center 9.5 mm of the ribbon was subjected to electron bombardment. It was heated to about 2100°K for 24 hours in 10d6 Torr of oxygen to remove carbon impuritiesls) before the measurements were initiated. During the measurements, the target was periodically heated in oxygen to remove the carbon which remained on the surface after the dissociative electronic desorption of CO. Laue back-reflection photographs, taken of the target after all of the electronic desorption work was completed, revealed that the area of the ribbon subjected to electron bombardment consisted of two crystallites each with a (411) orientation. The crystallites were rotated approximately 135 ’ with respect to each other (in the plane of the ribbon). The vacuum system consisted of a 12.5 cm diameter high vacuum bell jar and accessories. The bell jar was made of 7052 glass sealed to Kovar which was in turn inert-gas welded to a 304 stainless steel flange. The baseplate and accessories were also constructed of 304 stainless steel. The background pressure in the system after a bake at 250°C was between 10m9 and lo-” Torr. A 5 liter per second titanium ion pump provided the pumping for the system and was operated continuously during the electronic desorption measurements. A Granville-Phillips Type C high vacuum valve was used to admit CO to the system at a constant rate. 3. Results and discussion 3.1. ELECTRONIC DESORPTIONEFFICIENCY The ion liberated by the electron bombardment of a CO covered surface is the O+ ion. The fact that no C+ or CO+ surface ions were is in agreement with the results of other workerslS3’9’10). Initially a surface ion current was observed and only by prolonged heating of sten ribbon was this current eliminated. The magnitude of the bombardment-induced Of surface ion current was not noticeably by bombardment of the tungsten ribbon with a current density
tungsten observed large F+ the tungelectronaffected of about
.I. W.
64
COBURN
100 PA/cm2 of 100 eV electrons for a few minutes. During preliminary measurements on other systems (CO and Cl, on Pd and 0, on Si) the surface ion current was decreased severely by electron bombardment. The decrease in these cases is attributed to reduction of the surface coverage caused by the bombardment. An analysis of this transient behavior provided a convenient estimate of the total cross section for electronic desorption: [a(O, on Si)=6 x 10-r’ cm2,0(COonPd)==lx10-‘7cm2,0(C120nPd)= 1.6 x lo- I7 cm2, (electron energy = 100 eV, surface temperature =350X)]. However, even though the gas pressure (with the exception of C1,) and the electron current density were approximately the same in ail this electronic desorption work, no transient behavior of the 0” current was seen for the CO on W system. Thus, in order to estimate the efficiency of the process, it was necessary to measure the collection efficiency of the ion collection system. A sodium zeolite ion sourcel9), placed in the system at the target location, was used for the calibration. The uncertainties caused by changes in the multiplier gain, the focusing voltages, and the mass spectrometer transmission were eliminated by using the first collector (fig. 11, with a large retarding voltage on Grid 4, for the calibration. The measured transmission from the ion source to the first collector was 0.2f0.05. The ratio of the surface ion current measured at the first collector to the incident electron current varied from 0.4 x lo-’ to 1.1 x 10e7. The lower values were recorded shortiy after a thermal cleaning of the target (but after adequate CO exposures to result in monolayer coverage of the surface), whereas the higher values were obtained when the target had been exposed to CO for hours or even days. Therefore, the measured electronic desorption efficiency for the desorption of O+ ions is 2 x lo-? ions per electron from a clean tungsten surface which has been subjected to an exposure of about 10e6 Tort--min of CO. The results obtained using the electron multiplier output instead of the first collector were consistent with the above figures but contained a larger uncertainty. The above efficiency includes desorption from c1and /? sitesso) and was determined using 100 eV electrons and a tungsten target temperature of about 340°K. 3.2. co’
AND o+
THERMAL DESORPTION SPECTRA
The ability to record the surface Of current as the target temperature is increased at a controlled rate permits the determination of the relative contribution of the c1 and P-bonded CO molecules to the O+ current. A typical ‘Q thermal desorption spectrum” is shown in fig. 2. The initial decrease in the Of current is attributed to the thermal desorption of cc-bonded CO molecules and the second step in the curve is believed to be caused by thermal desorption of &-bonded CO molecules. The contribution
DISSOCIATIVE
J,
a 125
MC.WMWS/d
‘i 2.7
I IO-’
ELECTRONIC
DESORPTION
65
OF co
,..’ ,,_...J
l -
P
7ORR
,.....”
OF CO _, .,,,,
. . . ...“’
a= dt - 7*K/SEC
0
I 400
I
I
I
I
500
600
700
800
ADSORBENT
Fig. 2.
I SO0
I 1000
SURFACE TMFERATLRE
O+ and CO+ thermal desorption
1 1 II00 1200 TCK-NONLINEAR)
spectra for CO on W.
of the former to the O+ current is usually about three times that of the latter. A typical “CO+ thermal desorption spectrum” is also shown in fig. 2. It consists of a plot of the CO pressure immediately in front of the target surface as the surface is heated, whereas the O+ thermal desorption spectrum is a plot of the O+ surface ion current under identical conditions. The target is bombarded during the recording of both spectra. The effect of the bombardment on the thermal desorption spectra is negligrble for the amount of bombardment required to record the spectra. Before these spectra were recorded, the ribbon was heated to about 2100°K for a few seconds and then exposed to a pressure of about lo-’ Torr of CO for 10 minutes. The horizontal axis of the XY recorder was driven by a voltage proportional to the angular displacement of the motor-driven autotransformer used to heat the W ribbon. Therefore, the temperature scale of fig. 2 is nonlinear. The time taken to record the spectra was varied but was typically between 2 and 3 minutes. The small, broad a peak is difficult to see clearly in this spectrum, The nonlinear temperature scale makes it appear broader than it would appear on a linear scale. There are several features of these thermal desorption spectra which require discussion. First of all, the failure of the O+ current to reach a zero value at the high temperature end is discussed in section 3.3. Secondly, the CO+ retrace curve shown at the right side of the spectrum is the CO+ current as the surface is cooled. The maximum temperature reached in the thermal desorption spectra is about 1400°K (as opposed to 2100°K for thermal cleaning) and the retrace curve is recorded after approximately one
66
J. w. COBUKN
!‘1‘i;++ ;: ,;
2V,
\ ,
_---__--.---\ : it:
I: 100 VOLTS :
Je r SO MiCR~~~~* \ ’ -
P p 1.8 X id7 dT E q S*K/SEC
TORR (CO AND Ot
MIXTURE1
:
I \
,I’
‘.l_ _, .____.--
.j I 400
0
I
L
500
1
I
I
SOo7w8cam AWORMiNT
Fig. 3.
I
loo0
SURFACE TEWERATURE
--r--y
‘\
----___ __
I
A 1200
I100 (°K-NGtdLINEAR)
for;CO and 02 on W.
O+ and CO+ thermal de~orption~s~tra -T_-
4
:: :
\
I
I
163076
--Y
5r---
-I
2-
\
Ja p 120 MICROAMP~CM’ P
B 2.1
II
IO-?
3
s @‘K&EC
TORROF
CO
.
\ \
I-
\ \ \
I 400
3--
I 500
600
I 700
ABSORBENT
Fig. 4.
\
I ‘._ 800 900 SURFACE
OI and CO+ thermal desorption
I 1100
TEMPERATURE
I 1200 PK-MONLINEAR
SCALE)
spectra for CO on carburized W.
The retrace curve is believed to be an indication of the rate of adsorption into fiZ and p3 sites and may give information about the surface cleanliness. Note the shape of the retrace curves for figs. 3 and 4, which show thermal desorption spectra for a mixture of 70% CO and 30”//, 0, on W, and CO on carburized W respectively. The CO adsorption into p2 and p3 sites appears to be blocked in these examples, probably by 0, and C respectively. minute
at this temperature.
I 1000
DISSOCIATIVE
In addition
ELECTRONIC
DESORPTION
in fig. 2, note that the CO+ current
OF co
between
67
the c1and the pi
peaks is less than the steady state value at the left side of the trace. This observation is believed to be an indication of the electronic desorption of neutral CO molecules from c( sites. The steady state CO+ current is due to both the ambient pressure in the system, plus a fluxofmoleculeselectronically desorbed from the surface, whereas the CO+ current between the tl and the pi peaks is due only to the ambient pressure, since the a sites which are presumed to be the source of these electronically desorbed molecules have been thermally emptied. The CO+ and O+ thermal desorption spectra for a mixture of 70% CO and 30% 0, on tungsten is shown in fig. 3. Note that the contribution of the c1sites to the Of current has been eliminated and also that the CO+ current does not fall below the steady state value. The absence of an a contribution to the O+ current indicates that the number of a-bonded CO molecules has been decreased appreciably and as a result it might be expected that the number of neutral CO molecules electronically desorbed from a sites would be reduced. The fact that in fig. 3 the CO+ current does not fall below the steady state value supports the idea that neutral CO molecules are electronically desorbed from a sites. An order of magnitude estimate for the electronic desorption efficiency of 3 x low3 CO molecules per electron (for 100 eV electrons and a surface temperature of 340 “K) can be obtained from the contribution to the CO+ current attributed to electronically desorbed CO molecules. Another important aspect of the spectra shown in fig. 2 is their relative misalignment along the temperature axis. This observation is in contradiction to the work of Sandstrom and co-workersis) where the disappearance of the ion current coincided precisely with the onset of a-phase CO desorption. The only other known work in which both surface and ambient thermal desorption spectra are plotted so they might be compared, is that of Lichtman. In some cases the alignment of these spectra is goods) and in other cases there appears to be noticeable misalignment5,s). Intuitively, one would expect the peaks in the time derivative of the O+ spectrum to occur at the same temperature as the peaks in the CO+ spectrum. This expectation is based on the assumption that the O+ current due to a-bonded (or PI-bonded) CO molecules is proportional to the density of a-bonded (or fir-bonded) CO molecules. However, this expected alignment has not been observed. The CIpeak in the CO+ spectrum, although not well defined in fig. 2, occurs at a lower temperature than the largest negative slope of the 0’ spectrum in the CI region. However, the opposite situation is observed for the /3i sites. It has not been possible to explain the misalignment in terms of defects in the experimental technique. The conditions under which the O+ and CO+ spectra are recorded differ in only one respect. The O+ ions are electronically
68
1. W.
COBURN
desorbed from one side of the center 9.5 mm of the 2.5mm long W ribbon, whereas the CO molecules are thermally desorbed from both sides of the entire ribbon. That is, the temperature of the source of O+ ions is more uniform than the source temperature of the CO molecules. However, this difference is not believed to be significant for the following reasons: i) The thin (0.0076 mm) ribbon appeared to be isothermal over at least 75% of its surface area when it was observed with an optical pyrometer at temperatures above 1000°K. This may be indicative of the temperature distribution at 920”K, the temperature at which the & peak occurs in the CO” spectra. If temperature nonuniformities were influencing the position of the 01peak in the CO+ spectra, the actual misalignment of the O+ and CO+ spectra in the CIregion would be greater than the observed misalignment. ii) The sensitivity of the CO ionization facility is greatest for CO molecules that are thermally desorbed from the center of the front side of the target. iii) The activation energies for desorption calculated from the CO’ thermal desorption spectra are 1.2 eV and 2.4 eV for the Mand fil sites respectively. These values agree quite well with the results obtained from thermal desorption spectra recorded on polycrystalline wires under more ideal conditions. [&(a)=1.26 eV and &(&)=2.48 and 2.65 eV (ref. 20); Ed(g)= 1.0 eV and E,@,)=2.74 eV (ref. 21)-j. Some additional information which supports the conclusion that the observed misalignment of the 0’ and CO+ spectra is not a spurious effect is presented in figs. 3 and 4. Fig. 3 shows the CO+ and O+ spectra observed when the ambient gas was about 30% O2 and 70% CO. The spectra of fig. 4 are for pure CO on the W ribbon after the ribbon had been carburized by heating in propane. The relative alignment of the O+ and CO+ spectra is, therefore, influenced by the condition of the adsorbent surface and by the composition of the ambient gas and does not appear to be peculiar to the experimental apparatus. In fig. 3, dO*/dt has a maximum 22°K below the temperature at which the CO’ current has a maximum. For convenience, the temperature at which the M(or p,) contribution to the Oi current has decreased by a factor of two has been used as a basis for determining the misalignment. In fig. 3, this misalignment is 35X, whereas in spectra for pure CO on W the misalignment in the b1 region was about 80°K. An observation which is believed to be related to the misalignment is illustrated by figs. 5 and 6, These figures illustrate the dependence of the two types of spectra on the duration of the thermal cleaning. Note that the O+ spectra are influenced much more noticeably than the CO” spectra. If all the adsorbed CO molecules contributed equally to the O+ ion current, one would expect that perturbing factors such as the thermal cleaning procedures woutd cause equivalent changes in the O+ and CO+ spectra.
DISSOCiATlVE
ELECTRONIC
DESORPTION
69
DF co
163052 .Y
6
94 8 2lOCPK
&
FOR 5 SEC
i3 s
12 0 +‘8
CO EXPOSURE:
i .4 x
300
600
ADSORBENT
Fig. 5.
700
800
Effect of thermal cleaning treatment
t 5
t
IGWK
SW
SURFACE
t
Km0
TEMPERATURE
flO0
1200
T W- NONLINEAR
SCALE)
on the CO+ thermal desorption
/
/
I
PULSE
spectrum. 163052 I
f V*
= too VOLTS
J,
c 130
mlcROAMps/cM*
dT * __.. ._-_ z = 6’KISEC co EKlvSURE=
I
Of, 400
i
500
600
ADSORSENT
Fig, 6.
L
700
SWFACE
Effect of thermal cleaning treatment
8
I
800
900
I
/
I000
TEMPERATURE
I .43 x 10-6
It00 TPK-
J
t
mo
RONLIWEAR SCALEl
on the 0” thermal desorption
spectrum.
An attempt to explain the misalignment of the spectra, based on a ternperature dependence of the electronic desorption efficiency, was unsuccessful, However, it is possible to explain the observations qualitatively with a model which is based on the assumption that the electronic desorption efficiency for production of an 0 + ion (~‘1 is not the same for all CO moleeuIes that are adsorbed in a given type of site. Both the misalignment of
70
1. W.
the O+ and CO+ spectra, duration
of the thermal
COBUKN
and the greater sensitivity cleaning,
of the O+ spectra to the
suggest that not all of the cl-bonded
CO
molecules (nor all of the PI-bonded molecules) contribute equally to the O+ current. It has been suggested21) that there exists a distribution in desorption energies for CO adsorbed on W. In addition, the desorption energies for x-bonded CO on W can be calculated from the O+ thermal desorption spectra (figs. 2 or 6) using the first order Polanyi-Wigner equation: dQ/dt = - v0 exp (-
E,/kT)
The value of the activation energy for desorption (Ed) that is obtained varies from about 1.3 to 1.7 eV depending upon the point on the curve used for the calculation. This result suggests that the desorption energy of the a-bonded CO molecules which contribute to the O+ current is not constant. The reason for the variation in Ed (i.e. principally in the W-C bond) is believed to be interaction with adjacent adsorbed molecules, Rigbysr) has observed what is believed to be the result of an interaction of this type, using N, and CO on W. Evidence that such influences on the W-C bonding can extend to the C-O bond is provided by the infrared absorption work of Eischensss). The latter author found several values for the stretching frequency of the C-O bond, each presumably corresponding to a distinct state of adsorption of CO on Pd. Thus we may expect that molecule-molecule interactions which lead to a range of thermal desorption energies will also produce changes in the C-O bond which will be evident as a range of values observations which suggest such a of the quantity yl+. The experimental variation in yl+ will now be discussed further. The misalignment of the O+ and CO+ spectra indicate that only cc-bonded CO molecules with desorption energies at the high-energy end of the distribution of Ed(a) contribute appreciably to the O+ current. That is, the CO+ thermal desorption spectrum indicates that a substantial number of the a-bonded CO molecules have been thermally desorbed before any appreciable decrease in the O+ ion current is seen. The opposite situation is observed in the case of the fir-bonded CO molecules (fig. 2). Here apparently only the PI-bonded CO molecules with desorption energies at the lower end of the distribution of Ed@‘,) contribute to the O+ current. Figs. 5 and 6 can be used to support the ideas introduced above. The dotted spectra in the two figures were recorded under identical conditions, as were the two solid spectra. In these figures a change in the thermal cleaning time has caused significant changes in the O+ spectrum but only small changes in the CO+ spectrum. In fig. 6, increasing the duration of the thermal cleaning time from about 0.5 seconds to 5 seconds has caused a significant decrease in the CIcontribution to the O+ current and an increase in the /I1 contribution.
DISSOCIATIVE
ELECTRONIC
D&SORPTION
OF co
71
However, in fig. 5, the same change in the cleaning time caused only a barely perceptible decrease in the high temperature end of the CIpeak of the CO+ spectrum and a more pronounced increase in the low temperature end of the fil peak, whereas, the height of the fll peak was unaffected. If all Pi-bonded CO molecules were contributing equally to the O+ current, it would be expected that a factor of two increase in the B1 contribution to the O+ current would require a factor of two increase in the fi, peak in the CO+ spectrum. A similar argument can be applied to the E-bonded molecules. Since this expectation is contrary to the observations, figs. 5 and 6 lend some support to the concept of a variation in the electronic desorption efficiency among CO molecules adsorbed in a specific site. It is somewhat reassuring that the thermal desorption of a-bonded molecules terminates at about the same temperature in both the 0’” and the CO” spectra. Also the thermal desorption of PI-bonded molecules begins at about the same point in both types of spectra. The fact that the duration of the thermal cleaning influences the results is disturbing. The temperature at which the surface is cleaned, provided that it is greater than about 2OOO”K, does not influence the results noticeably. If the cleaning time is reproduced exactly the reproducibility of the O+ thermal desorption spectra is quite good. The cause of the dependence of the O+ spectra on the cleaning time (fig. 6) is believed to be CO adsorption that occurs at elevated W ribbon temperatures. Ideally the ribbon should be cleaned in vacuum and then be allowed to cool to the ambient temperature before any adsorption occurs. However, in this work the ambient pressure is about lo-’ Torr of CO and is influenced by the duration of the thermal cleaning. If the thermal cleaning is brief, the large surfaces nearby, such as Plate 1, remain below the minimum temperature required desorption. However, for longer thermal cleaning times these become hot enough for thermal desorption to occur and a large the CO pressure is seen. In addition, the increased temperature
for thermal surfaces do increase in of the sur-
roundings caused by prolonged thermal cleaning of the ribbon decreases the cooling rate of the W surface. Both the higher pressure and the smaller cooling rate lead to more CO adsorption at elevated temperatures. An extreme example of this situation will now be discussed. 3.3. Of CURRENT AT HIGH SURFACE TEMPERATURES It has been observed that an appreciable 0’ surface ion current is detected even when the W ribbon temperature is greater than the temperature required for the thermal desorption of CO molecules from pi sites. If the W ribbon is heated to temperatures in the range of 1000 to 1300°K for about 10 minutes in about toe7 Torr of CO, the high temperature 0’ current caused by
J. W.
72
COBURN
electron bombardment reaches a steady state value which is comparable in magnitude to the steady state O+ current observed when the ribbon is at the ambient temperature of about 340°K after a thermal cleaning and a CO exposure of about low6 Torr-min. The following are the characteristics of this steady state high temperature 0’ current: i) It decreases until it is no longer observable when the W ribbon temperature is greater than about 1500°K. ii) It has a dependence on the ribbon temperature which can be characterized by an Arrhenius plot with an activation energy in the range of 0.4 to 0.8 eV over a limited temperature range (fig. 7). The reason for the spread in activation energy is not known. iii) It is independent of CO pressure. iv) It is independent of the magnitude or direction of dT/dt, the rate of
IO9-
\
0
B-
0’ \
?-
=
C,
EXP(-E,/kT)
WHERE
0
L= C,=
0.65 eV CONSTANI’
\
6-
5-
4-
3-
V,
=
100
VOLTS
J,
?
140
MICROAMPS/CM’
P
s
2
IO-‘TORR
$+
z -5’K/SEC
x
OFCO
2-
Ia;O°K
,
13yOeK
,
12q0°K,
;I~““”
ISZC
7.5
8.5
8.0 IO?T
Fig. 7.
Temperature
dependence
9.0
,
1
9.5
(OK-‘)
of the O+ current
at high tungsten
surface
temperatures.
DISSOCIATIVE
ELECTRONIC
DESORPTION
OF co
73
change of the ribbon temperature provided the temperature has not exceeded about 1400 “K. v) If the ribbon is cooled to the ambient temperature after the steady state high temperature O+ current has been established, the resulting 0’ current is much lower (a factor of 5 or 10) than the O+ current observed after a normal thermal cleaning and CO exposure. Additional exposure to CO does not appreciably increase this small 0” current. Fig. 8 shows an Of spectrum taken after this treatment, and the relatively small contribution of the o! and &-bonded CO molecules to the Oi‘ current can be seen. The high temperature O+ current is believed to be caused by electronic desorption of CO molecuIes which are adsorbed in sites with large desorption energies [p2 or p3 sitesae)]. Heating the W ribbon in CO at temperatures of the order of 1000°K apparently results in more CO adsorption of this type and fewer molecules adsorbed in the a and pi sites than would be the case if all the adsorption occurred at the ambient temperature of 340°K. At 340°K it is believed that some of the p2 and /I3 sites which are available for CO adsorption can be eliminated by CO adsorption into a ,8i site. It has been suggested 21) that the differences among the /I sites lies in the spacing of the W atoms to which the bridged P-bond is formed. Therefore, if a particular W surface atom has adjacent W atoms at distances suitable for both p1 and fi3 bonds, it is not difficult to see how the occupation of the pi site can eliminate the /I3 site when only one bond per surface W atom is allowedzl). Thus, when adsorption takes place at a surface temperature above the thermal desorption temperature of the pi sites, more of the /I3 sites will be occupied. The a sites are though to be a result of adsorption onto a W atom whose nearest neighbors are all involved with b-bonded CO molecules20). Since CO molecules are mobile on the W surface at temperatures above about 7OO”Kes), it is not surprising that heating the surface at 1000 “K would result in a more closely packed P-adsorbed array of CO molecules, thus reducing the number of CI sites. The temperature dependence of the high temperature Of current does not appear to be due to coverage changes. It is believed to be a result of thermal excitation of the adsorbed CO molecule into higher vibrational energy states from which electronic desorption is more probable. 3.4. THEENERGYTHRESHOLD
AND THEENERGY
DEPENDENCE OF THE 0"
CURRENT
The 0’ current has been observed for electron energies up to 550 eV and the results are shown in fig. 9. In order to focus the electron beam onto the target at different electron energies, the potential difference between Plates 1 and 2 (fig. 1) must be changed. Unfortunately this results in an ion collection
74
J. W.
COBURN
6
s-
ve -
100
VOLTS
J, E 160 MICROAhWWf P = 6.6 x lO*TORR
4-
% -
OF CO
5 IZ*K/SEC
HEATlffi
SEVERAJ_ HOURS AT
134O.K
3-
600
700
600
SO0
1000
ADSORBENT SURFACE TEMPERATURE
Fig. 8.
O+ thermal desorption
1100
TPK-NONLINEAR
1200 SCALE)
spectrum after heating the W surface in CO.
efficiency which varies with electron energy. The resulting distortion of energy dependence curves is shown in fig. 9 by the comparison of the measured CO+ current from the ambient with a published CO+ ionization efficiency curve24). The principal effect is seen to be a progressive lowering of the apparent yield at higher energies. In contrast to the q+ versus V, curve shown on fig. 9 (maximum at about 320 V), most published datal-sT14) sh ow maxima between 100 and 150 V. Recently however, Redheadd) has published a curve showing a maximum at an electron
energy greater than 280 eV. The reason for these discrepancies
is
not known. The behavior of the O+ current in the region of the energy threshold is illustrated in fig. 10. Curve A of this figure is a plot of O+ versus V, whereas Curve B is a plot of (O+)+ versus V, .The energy threshold ek’, can be determined quite easily from Curve B. The reason for the cubic dependence of O+ on the excess energy is not clear and may not be significant. It is interesting to plot V,, the threshold voltage for the formation of an O+ ion, versus V,, the retarding potential applied to Grids 1 and 2. The results that have been obtained are shown in fig. 11. Plots of this type have been used often in studies of dissociative ionization processes25) in the ambient and RedheadzsJ) has also made such plots in his studies of the electronic desorption of O2 from MO and CO from W. For a dissociative ionization event of the type XY+e-+X++Y+2e
DISSOCIATIVE
ELECTRONIC
DESORPTION
15
OF co
163070 163092 I
I
I
I
I
160
140
120
4c
o+ 1
2(
co+:
P 6 9x
IO-*TOFIR
2
*
J
J,
P=
1x10-7
5 f
Je * 35
TORR
100
The dependence
OF
CO
MICRa4MPS/CM’
200 Ve
Fig. 9.
OF CO
MIC!?OAMPS/CM2
1
I
I
300
400
500
I
I
0
I5
(VOLTS)
of the Of and CO+ currents
on the electron
energy.
the threshold energy depends on the kinetic energy of the dissociation products X+ and Y. The total kinetic energy can be expressed in terms of I&(X+), the kinetic energy of the ionized fragment, by invoking momentum conservation. The threshold energy eV, can then be expressed in terms of E,(X+) and eV,,, the threshold energy when the kinetic energy of the fragments is zero. eV, = elf& +
m(X)+-4Y) l&(X+) -~~-~ [
m(Y)
. 1
Fig. 11 is a plot of eV, versus Ek(Ot) where E,(O+)=eV, (Vr=retarding potential applied to Grids 1 and 2). If Ek(Of) has a minimum value, the threshoId energy ek’, will be independent of eV, for eV~<&(O+),,i,. Thus,
76
J. W.
P z 2 x lO-7 20
= J,=
36
TORR
COBURN
OF CO
MICROAMPS/CM2
ELECTRON
Fig. 10.
The dependence
the following 4 ii) iii)
ENERGY
(~‘4)
of the 0+ current on the electron energy in the region of the energy threshold.
information
can be obtained
from fig. 11:
V,, = 16.2 V, E, (O’),i,
= 4.7 eV,
nz(O+)+m(ConW)_l m(ConW)
’
Therefore the mass of the 0’ ion is much smaller than the mass of the other dissociation product. The latter probably includes the tungsten lattice as well as the carbon atom. The sensitivity of the threshold measurements was limited by photoemission from the first dynode of the electron multiplier which was caused by the electron-bombardment induced-soft X-rays. A deflection type of mass spectrometer would eliminate this problem. Fig. 12 is a plot of the energy distribution of the 0’ ions obtained by differentiating an O+ retarding potential curve for three values of the electron energy. The fact that a substantial number of the ions appear to have energies less than the 4.7 eV minimum implied by fig. 11, is believed to be a result of the unfavourable geometry of the system. The geometry about the ribbon is planar instead of spherical, and in addition, there is considerable collimation of the ion beam (by the mass spectrometer apertures) beyond the retarding grids. Any electrostatic focusing effects which occur in the vicinity of these grids will be dependent on the ion energy and will result in a larger fraction
DISSOCIATIVE
ELECTRONIC
DESORPTION
OF co
77
6+:?. _.Mm. 2 4>’
,’ /’ /Y’
: 4
./’
2,/%LOfE
8
= I
.’ i E 3
I/’ o.--E; p:
L -2 -
P E 2 I IO-’ TOM
P. ‘D; #i .:
-4
OF CO
20 6 Je = 36 MICROAMFS/CMr Q I 17
’ I:’
1 IS
1 16
I 20
THRESHOLD
Fig. 11.
The retarding
I 21 VOLTAGE
22 V+
voltage V, versus Vt, the threshold of an Of ion.
I 24
I 23
_
MLTS)
voltage for the production
163046 5
P 5 E
1.6 I
I
I
I
IO -7
I
TOAR OF CO
4-
3
o.5
Fig. 12.
’ I
I 2
I 3
B.
v,
C.
Ve -265
= 98 VOLTS.
4
VOLTS,
Je r
170 MICROAMPS/CM’
5
6
RETARDING
VOLTAGE
The approximate energy distribution
7 Vr
6
9
(VOLTS)
of the 0+ current at three electron energies.
of the low energy ions being removed from the ion beam by collimation. This effect would result in a spreading of the ion energy distribution curve to lower energies. However the data of fig. 3 1 should not be influenced by this geometrical difficulty since the threshold energy is determined for ions with energy eV,. If a fraction of these ions is removed from the ion beam by collimation beyond the retarding grids, the only effect is a reduction in
78 the sensitivity
.I. W. of the ion collection
CORURN
system. In order to check the accuracy
of
the retarding potential measurements, a retarding potential curve of the Naf and Kf ions that are emitted from a fresh tungsten ribbon2s) was measured. The energy distribution obtained in this way had a maximum at an accelerating potential of about 0.5 V instead of at a retarding potential of about 0.1 V. Redhead4) observes a most probable ion energy of 7 eV (compared with 6 eV in fig. 12) and a second peak at t eV which is believed to be due to electronically desorbed CO+ ions. No electronically desorbed CO+ ions were seen in the present work; the reason may be that the surface temperature of 340 “K is too high. Yatesll) also found two peaks in the ion energy distribution at 0.4 and 6 eV and assigned the low energy peak to ions desorbed from M sites and the high energy peak to ions desorbed from p, sites. This assignment disagrees with the present work in which ions originating at a sites were observed to have large kinetic energies. It is tempting to apply conservation of energy to the electronic desorption process as others3,4y9) have done. If there are no energy loss mechanisms in the interaction and ionization of the bound oxygen results in a free electron of zero kinetic energy, then el/,, = D(C0)
+ ev,(O+)
+ E,(CO)
- Ed(C),
where V,, = 16.2 V, D(C0) = dissociation energy of the CO molecule (11.1 eV) (ref. 26), Vi(O+) = ionization potential of the oxygen atom (13.6 V), E,(CO) = desorption energy for CO on W (1.2 eV) (ref. 20), E,(C) = desorption energy for C on W. The value of E,(C) that is obtained (9.7 eV), is not an unreasonable value in view of the tenacity with which carbon adheres to tungsten27). 3.5. MISCELLANEOUSOBSERVATIONS The magnitude of the O+ current was measured as a function of electron current density and of the CO pressure in the system. The Of current was proportional to the electron current and was almost independent of the CO pressure. A slight decrease in the O+ current was observed at high CO pressures (> 10e7 Torr). The desorption energy of cc-bonded CO molecules was determined from the transient response of the O+ current to a sudden increase in the W ribbon temperature. The O+ current decrease is approximately exponential with a time constant: l/z = v exp(If the ribbon
temperature
E,/kT).
is kept below the thermal
desorption
temperature
DISSOCIATIVE
of the /Ii-bonded can be obtained.
ELECTRONIC
CO molecules, values
79
OF co
a value of Ed for the a-bonded
The results obtained
agreed quite well with spectra (1.3-1.7 eV).
DESORPTION
with this approach
derived
from
molecules
(with v = 1013 set- ‘)
the O+ thermal
desorption
As was mentioned earlier, the Of current is not noticeably affected by electron bombardment in the amount required to record the desorption spectra. In this work apparently the CO pressure in the system is large enough (lo-’ Torr) to maintain nearly full coverage of the surface during the electron bombardment. However, if the ribbon is bombarded for times of the order of one hour, some effects are seen. The reproducibility of the bombardment effects is poor but the total ion current is decreased by about 20% by electron bombardment for one hour with a current density of about 130pA/cm2 of 1OOeV electrons. Even though the total Of current is decreased by electron bombardment, the p1 contribution to the O+ current is increased substantially (of the order of 50%) as indicated by subsequent O+ thermal desorption spectra. The CO+ spectra are only very slightly affected as might be expected, since only about 20% of the total ribbon surface is subjected to electron bombardment. The observation that the p1 contribution to the O+ current is increased by electron bombardment, whereas, the c( contribution is decreased, implies an a to p1 transition. This is probably a result of electronbombardment-induced reordering of the adsorbed CO molecules. 4. Summary The efficiency for the production of an O+ ion as a result of the dissociative electronic desorption of a CO molecule on a W ribbon determined m this study is about 2 x lo-’ ions per electron for 100 eV electrons at a surface temperature of about 340°K. About 75% of the ion yield is a result of desorption of the cc-bonded CO molecules and the remaining 25% is due to the /II-bonded CO molecules. This efficiency agrees well with Redhead’s value of 3 x lo-’ ions per electron for CO on MO 3). Lichtman has obtained a value of 1.3 x 10m4 ions per electrong) for CO on MO from LYsites only. It has not been possible to determine the density of adsorbed CO molecules in each of the adsorption sites and as a result it is difficult to estimate a cross section for the dissociative electronic desorption process. Redhead4) and Menzel and Gomerl4) obtain a total cross section for electronic desorption of CO from W of 3 x lo-‘* cm’. This figure includes the desorption of neutral atoms or molecules as well as the O+ ion. The 20.9 eV value for the energy threshold for the formation of an O+ ion agrees well with the 20 eV threshold observed by Lichtman for CO on Meg). However, the 16.2 eV threshold energy for the formation of an O+ ion
80
J. W.
with
zero
kinetic
energy
COBURN
is considerably
lower
than
Redhead’s
18.7 eV
value for CO on W “). The comparison of the W results with those for MO is questionable. In fact, it might be expected that the results for identical adsorbate-adsorbent systems will vary substantially as a result of variations in adsorbent crystallme structure and surface cleanliness. In this study the W ribbon was heated in oxygen to remove carbon impurities. This treatment caused severe changes in the electronic desorption characteristics and these changes were reversed by the intentional carburization of the W ribbon. The introduction of oxygen with the CO also resulted in severe changes in the behavior of the O+ current. The misalignment of the O+ and CO+ thermal desorption spectra that was observed in this study, although not reported by other workers, is not believed to be an experimental artifact. A tentative explanation has been presented, which is based on the assumption that the electronic desorption efficiency is not constant for CO molecules adsorbed in a specific type of site. Some additional observations support the proposal but the mechanisms involved are not understood in detail.
Acknowledgement The author is indebted to Professor W.T. cussions throughout the course of this work.
Peria
for many
helpful
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
G.E. Moore, J. Appl. Phys. 32 (1961) 1241. P.A. Redhead, Can. J. Phys. 42 (1964) 886. P.A. Redhead, Appl. Phys. Letters 4 (1964) 166. P.A. Redhead, Suppl. Nuovo Cimento 5 (1967) 586. D. Lichtman, J. Vacuum Sci. Technol. 2 (1965) 70. D. Lichtman and T.R. Kirst, Phys. Letters 20 (1966) 7. D. Lichtman, T.R. Kirst and R.B. McQuistan, Phys. Letters 20 (1966) 129. D. Lichtman and T.R. Kirst, J. Vacuum Sci. Technol. 3 (1966) 224. D. Lichtman, R.B. McQuistan and T.R. Kirst, Surface Sci. 5 (1966) 120. D.R. Sandstrom, J.H. Leek and E.E. Donaldson, J. Appl. Phys. 38 (1967) 2851. J.T. Yates, T.E. Madey and J.K. Payn, Suppl. Nuovo Cimento 5 (1967) 558. R. Comer and D. Menzel, J. Chem. Phys. 40 (1964) 1164. D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3329. W. Ermich, Philips Res. Rept. 20 (1965) 94. L.A. Petermann, Suppl. Nuovo Cimento 1 (1963) 601. G.F. Sauter, R.A. Gerber and H.J. Oskam, Rev. Sci. Instr. 37 (1966) 572. J.A. Becker, E.J. Becker and R.G. Brandes, J. Appl. Phys. 32 (1961) 411. R.E. Weber and L.F. Cordes, Rev. Sci. Instr. 37 (1966) 112. P.A. Redhead, Trans. Faraday Sot. 57 (1961) 641. L.J. Rigby, Can. J. Phys. 42 (1964) 1256.
dis-
DISSOCIATIVE
22) 23) 24) 25) 26) 27) 28)
ELECTRONIC
DESORPTION
OF co
R.P. Eischens and W.A. Plisken, Advan. Catalysis 10 (1958) 1. R. Klein, J. Chem. Phys. 31 (1959) 1306. J.T. Tate and P.T. Smith, Phys. Rev. 39 (1932) 270. H.D. Hagstrum, Rev. Mod. Phys. 23 (1951) 185. M.A. Fineman and A.W. Petrocelli, J. Chem. Phys. 36 (1962) 25. N. J. Taylor, Surface Sci. 2 (1964) 544. D. Lichtman, J. Vacuum Sci. Technol. 2 (1965) 91.
81
THE
SCIENCE
11 (1968) 61-81 0 North-Holland
DISSOCIATIVE
ELECTRONIC
MONOXIDE
Publishing
DESORPTION
Co., Amsterdam
OF CARBON
FROM TUNGSTEN *
J. W. COBURN** PhysicaI
Electronics
Laboratory,
Received 8 November
University of Minnesota, U.S.A.
1967; revised manuscript
Minneapolis,
Minnesota
5.5454,
received 22 January 1968
A tungsten surface covered with adsorbed CO molecules has been subjected to low energy electron bombardment. A current of 0+ ions originating at the bombarded surface was observed but no C+ or CO+ ions were seen. The experimental structure permitted the observation of either the O+ surface ion current or the gas-phase CO+ ion current as the tungsten surface temperature was increased at a controlled rate. These thermal desorption spectra did not align with each other on the temperature axis in the expected way, and in order to explain this observation it is suggested that the electronic desorption efficiency is not a constant for CO molecules adsorbed in a specific type of site. The dependence of the 0+ current on the electron energy, the electron current density, and the CO pressure has been determined. An approximate energy distribution of the 0+ ions has been measured and values for the energy threshold, the minimum kinetic energy of the desorbed 0+ ions, and the average efficiency for the production of an 0+ ion were also obtained. An electron-bombardment-induced 0+ current which increases with increasing surface temperature has been observed for surface temperatures between 1000 and 1300°K.
1. Introduction The desorption of adsorbed species from surfaces by low energy electron impact has been studied recently by several workers. Moorel), Redhead 2-4), Lichtman and co-workerss-9), Sandstromls), and Yatesrl) have studied the process by observing the ionic desorption products. Menzel and Gomerr2mr4) and Ermichl5) have used the techniques of field emission microscopy to observe changes in the surface coverage caused by electron bombardment. Petermannls) has used a partial pressure analyzer to observe electronbombardment-induced pressure changes in a vacuum system. Most of this work has been carried out with one of the gases, carbon monoxide, oxygen, or hydrogen on molybdenum, tungsten or .nickel surfaces. The type of * Work supported by the Avionics Laboratory, Air Force Systems Command, United States Air Force. ** Present address: Department of Physics, Simon Fraser University, Burnaby 2, British Columbia, Canada. 61
62
J. W.
COBURN
measurements that have been made and the results that have been obtained vary considerably. The motivation for the work to be described in this paper was the disturbing influence introduced by the electronic desorption process on a study of the dissociation of MgO thin films by electron bombardment. ft was found that the surface ion current caused by electronic desorption of adsorbed CO, 0, and Mg obscured that caused by the dissociatron process.
2. Experimental
system
The experimental arrangement used in this work is shown in fig. 1. The electrons from the tungsten filament are directed onto the target by a retarding electric field between Plates 1 and 2. The ionic desorption products are drawn from the target surface through a series of grids and an electrostatic focusing and deflection system into a quadrupole mass spectrometerr’). The output of the mass spectrometer was directed onto the first dynode of a lo-stage electron multiplier with a measured gain of about 2 x 106. Grids 1 and 2 were kept at, or slightly above, the potential of the target when the surface ion current was being observed. The potential barrier presented by these grids prevents ions formed in the gas phase from reaching the mass spectrometer. The ambient gas can be mass-analyzed by setting the potential of Grids 1 and 2 below the target potential (but above the potential of Plate
Fig.
I.
Schematic
diagram
of experimental
system
DISSOCIATIVE
1) thus permitting Plate
1 to reach
ions formed
ELECTRONIC
DESORPTION
OF co
in part of the volume
the mass spectrometer.
The ability
between
63
the target and
to observe
either
the
surface ion current or the gas-phase ion current, as the target temperature is increased at a controlled rate, has been very useful in this study. Grids 1 and 2 have also been used to record retarding potential plots of the surface ion current. The ion energy distributions which are obtained by numerical differentiation of these plots are, however, only approximations to the true energy distributions because of the unfavorable geometry in the ion collection system. The tungsten target was a ribbon 0.0076 mm thick, 6.3 mm wide and 25 mm long. Only the center 9.5 mm of the ribbon was subjected to electron bombardment. It was heated to about 2100°K for 24 hours in 10d6 Torr of oxygen to remove carbon impuritiesls) before the measurements were initiated. During the measurements, the target was periodically heated in oxygen to remove the carbon which remained on the surface after the dissociative electronic desorption of CO. Laue back-reflection photographs, taken of the target after all of the electronic desorption work was completed, revealed that the area of the ribbon subjected to electron bombardment consisted of two crystallites each with a (411) orientation. The crystallites were rotated approximately 135 ’ with respect to each other (in the plane of the ribbon). The vacuum system consisted of a 12.5 cm diameter high vacuum bell jar and accessories. The bell jar was made of 7052 glass sealed to Kovar which was in turn inert-gas welded to a 304 stainless steel flange. The baseplate and accessories were also constructed of 304 stainless steel. The background pressure in the system after a bake at 250°C was between 10m9 and lo-” Torr. A 5 liter per second titanium ion pump provided the pumping for the system and was operated continuously during the electronic desorption measurements. A Granville-Phillips Type C high vacuum valve was used to admit CO to the system at a constant rate. 3. Results and discussion 3.1. ELECTRONIC DESORPTIONEFFICIENCY The ion liberated by the electron bombardment of a CO covered surface is the O+ ion. The fact that no C+ or CO+ surface ions were is in agreement with the results of other workerslS3’9’10). Initially a surface ion current was observed and only by prolonged heating of sten ribbon was this current eliminated. The magnitude of the bombardment-induced Of surface ion current was not noticeably by bombardment of the tungsten ribbon with a current density
tungsten observed large F+ the tungelectronaffected of about
.I. W.
64
COBURN
100 PA/cm2 of 100 eV electrons for a few minutes. During preliminary measurements on other systems (CO and Cl, on Pd and 0, on Si) the surface ion current was decreased severely by electron bombardment. The decrease in these cases is attributed to reduction of the surface coverage caused by the bombardment. An analysis of this transient behavior provided a convenient estimate of the total cross section for electronic desorption: [a(O, on Si)=6 x 10-r’ cm2,0(COonPd)==lx10-‘7cm2,0(C120nPd)= 1.6 x lo- I7 cm2, (electron energy = 100 eV, surface temperature =350X)]. However, even though the gas pressure (with the exception of C1,) and the electron current density were approximately the same in ail this electronic desorption work, no transient behavior of the 0” current was seen for the CO on W system. Thus, in order to estimate the efficiency of the process, it was necessary to measure the collection efficiency of the ion collection system. A sodium zeolite ion sourcel9), placed in the system at the target location, was used for the calibration. The uncertainties caused by changes in the multiplier gain, the focusing voltages, and the mass spectrometer transmission were eliminated by using the first collector (fig. 11, with a large retarding voltage on Grid 4, for the calibration. The measured transmission from the ion source to the first collector was 0.2f0.05. The ratio of the surface ion current measured at the first collector to the incident electron current varied from 0.4 x lo-’ to 1.1 x 10e7. The lower values were recorded shortiy after a thermal cleaning of the target (but after adequate CO exposures to result in monolayer coverage of the surface), whereas the higher values were obtained when the target had been exposed to CO for hours or even days. Therefore, the measured electronic desorption efficiency for the desorption of O+ ions is 2 x lo-? ions per electron from a clean tungsten surface which has been subjected to an exposure of about 10e6 Tort--min of CO. The results obtained using the electron multiplier output instead of the first collector were consistent with the above figures but contained a larger uncertainty. The above efficiency includes desorption from c1and /? sitesso) and was determined using 100 eV electrons and a tungsten target temperature of about 340°K. 3.2. co’
AND o+
THERMAL DESORPTION SPECTRA
The ability to record the surface Of current as the target temperature is increased at a controlled rate permits the determination of the relative contribution of the c1 and P-bonded CO molecules to the O+ current. A typical ‘Q thermal desorption spectrum” is shown in fig. 2. The initial decrease in the Of current is attributed to the thermal desorption of cc-bonded CO molecules and the second step in the curve is believed to be caused by thermal desorption of &-bonded CO molecules. The contribution
DISSOCIATIVE
J,
a 125
MC.WMWS/d
‘i 2.7
I IO-’
ELECTRONIC
DESORPTION
65
OF co
,..’ ,,_...J
l -
P
7ORR
,.....”
OF CO _, .,,,,
. . . ...“’
a= dt - 7*K/SEC
0
I 400
I
I
I
I
500
600
700
800
ADSORBENT
Fig. 2.
I SO0
I 1000
SURFACE TMFERATLRE
O+ and CO+ thermal desorption
1 1 II00 1200 TCK-NONLINEAR)
spectra for CO on W.
of the former to the O+ current is usually about three times that of the latter. A typical “CO+ thermal desorption spectrum” is also shown in fig. 2. It consists of a plot of the CO pressure immediately in front of the target surface as the surface is heated, whereas the O+ thermal desorption spectrum is a plot of the O+ surface ion current under identical conditions. The target is bombarded during the recording of both spectra. The effect of the bombardment on the thermal desorption spectra is negligrble for the amount of bombardment required to record the spectra. Before these spectra were recorded, the ribbon was heated to about 2100°K for a few seconds and then exposed to a pressure of about lo-’ Torr of CO for 10 minutes. The horizontal axis of the XY recorder was driven by a voltage proportional to the angular displacement of the motor-driven autotransformer used to heat the W ribbon. Therefore, the temperature scale of fig. 2 is nonlinear. The time taken to record the spectra was varied but was typically between 2 and 3 minutes. The small, broad a peak is difficult to see clearly in this spectrum, The nonlinear temperature scale makes it appear broader than it would appear on a linear scale. There are several features of these thermal desorption spectra which require discussion. First of all, the failure of the O+ current to reach a zero value at the high temperature end is discussed in section 3.3. Secondly, the CO+ retrace curve shown at the right side of the spectrum is the CO+ current as the surface is cooled. The maximum temperature reached in the thermal desorption spectra is about 1400°K (as opposed to 2100°K for thermal cleaning) and the retrace curve is recorded after approximately one
66
J. w. COBUKN
!‘1‘i;++ ;: ,;
2V,
\ ,
_---__--.---\ : it:
I: 100 VOLTS :
Je r SO MiCR~~~~* \ ’ -
P p 1.8 X id7 dT E q S*K/SEC
TORR (CO AND Ot
MIXTURE1
:
I \
,I’
‘.l_ _, .____.--
.j I 400
0
I
L
500
1
I
I
SOo7w8cam AWORMiNT
Fig. 3.
I
loo0
SURFACE TEWERATURE
--r--y
‘\
----___ __
I
A 1200
I100 (°K-NGtdLINEAR)
for;CO and 02 on W.
O+ and CO+ thermal de~orption~s~tra -T_-
4
:: :
\
I
I
163076
--Y
5r---
-I
2-
\
Ja p 120 MICROAMP~CM’ P
B 2.1
II
IO-?
3
s @‘K&EC
TORROF
CO
.
\ \
I-
\ \ \
I 400
3--
I 500
600
I 700
ABSORBENT
Fig. 4.
\
I ‘._ 800 900 SURFACE
OI and CO+ thermal desorption
I 1100
TEMPERATURE
I 1200 PK-MONLINEAR
SCALE)
spectra for CO on carburized W.
The retrace curve is believed to be an indication of the rate of adsorption into fiZ and p3 sites and may give information about the surface cleanliness. Note the shape of the retrace curves for figs. 3 and 4, which show thermal desorption spectra for a mixture of 70% CO and 30”//, 0, on W, and CO on carburized W respectively. The CO adsorption into p2 and p3 sites appears to be blocked in these examples, probably by 0, and C respectively. minute
at this temperature.
I 1000
DISSOCIATIVE
In addition
ELECTRONIC
DESORPTION
in fig. 2, note that the CO+ current
OF co
between
67
the c1and the pi
peaks is less than the steady state value at the left side of the trace. This observation is believed to be an indication of the electronic desorption of neutral CO molecules from c( sites. The steady state CO+ current is due to both the ambient pressure in the system, plus a fluxofmoleculeselectronically desorbed from the surface, whereas the CO+ current between the tl and the pi peaks is due only to the ambient pressure, since the a sites which are presumed to be the source of these electronically desorbed molecules have been thermally emptied. The CO+ and O+ thermal desorption spectra for a mixture of 70% CO and 30% 0, on tungsten is shown in fig. 3. Note that the contribution of the c1sites to the Of current has been eliminated and also that the CO+ current does not fall below the steady state value. The absence of an a contribution to the O+ current indicates that the number of a-bonded CO molecules has been decreased appreciably and as a result it might be expected that the number of neutral CO molecules electronically desorbed from a sites would be reduced. The fact that in fig. 3 the CO+ current does not fall below the steady state value supports the idea that neutral CO molecules are electronically desorbed from a sites. An order of magnitude estimate for the electronic desorption efficiency of 3 x low3 CO molecules per electron (for 100 eV electrons and a surface temperature of 340 “K) can be obtained from the contribution to the CO+ current attributed to electronically desorbed CO molecules. Another important aspect of the spectra shown in fig. 2 is their relative misalignment along the temperature axis. This observation is in contradiction to the work of Sandstrom and co-workersis) where the disappearance of the ion current coincided precisely with the onset of a-phase CO desorption. The only other known work in which both surface and ambient thermal desorption spectra are plotted so they might be compared, is that of Lichtman. In some cases the alignment of these spectra is goods) and in other cases there appears to be noticeable misalignment5,s). Intuitively, one would expect the peaks in the time derivative of the O+ spectrum to occur at the same temperature as the peaks in the CO+ spectrum. This expectation is based on the assumption that the O+ current due to a-bonded (or PI-bonded) CO molecules is proportional to the density of a-bonded (or fir-bonded) CO molecules. However, this expected alignment has not been observed. The CIpeak in the CO+ spectrum, although not well defined in fig. 2, occurs at a lower temperature than the largest negative slope of the 0’ spectrum in the CI region. However, the opposite situation is observed for the /3i sites. It has not been possible to explain the misalignment in terms of defects in the experimental technique. The conditions under which the O+ and CO+ spectra are recorded differ in only one respect. The O+ ions are electronically
68
1. W.
COBURN
desorbed from one side of the center 9.5 mm of the 2.5mm long W ribbon, whereas the CO molecules are thermally desorbed from both sides of the entire ribbon. That is, the temperature of the source of O+ ions is more uniform than the source temperature of the CO molecules. However, this difference is not believed to be significant for the following reasons: i) The thin (0.0076 mm) ribbon appeared to be isothermal over at least 75% of its surface area when it was observed with an optical pyrometer at temperatures above 1000°K. This may be indicative of the temperature distribution at 920”K, the temperature at which the & peak occurs in the CO” spectra. If temperature nonuniformities were influencing the position of the 01peak in the CO+ spectra, the actual misalignment of the O+ and CO+ spectra in the CIregion would be greater than the observed misalignment. ii) The sensitivity of the CO ionization facility is greatest for CO molecules that are thermally desorbed from the center of the front side of the target. iii) The activation energies for desorption calculated from the CO’ thermal desorption spectra are 1.2 eV and 2.4 eV for the Mand fil sites respectively. These values agree quite well with the results obtained from thermal desorption spectra recorded on polycrystalline wires under more ideal conditions. [&(a)=1.26 eV and &(&)=2.48 and 2.65 eV (ref. 20); Ed(g)= 1.0 eV and E,@,)=2.74 eV (ref. 21)-j. Some additional information which supports the conclusion that the observed misalignment of the 0’ and CO+ spectra is not a spurious effect is presented in figs. 3 and 4. Fig. 3 shows the CO+ and O+ spectra observed when the ambient gas was about 30% O2 and 70% CO. The spectra of fig. 4 are for pure CO on the W ribbon after the ribbon had been carburized by heating in propane. The relative alignment of the O+ and CO+ spectra is, therefore, influenced by the condition of the adsorbent surface and by the composition of the ambient gas and does not appear to be peculiar to the experimental apparatus. In fig. 3, dO*/dt has a maximum 22°K below the temperature at which the CO’ current has a maximum. For convenience, the temperature at which the M(or p,) contribution to the Oi current has decreased by a factor of two has been used as a basis for determining the misalignment. In fig. 3, this misalignment is 35X, whereas in spectra for pure CO on W the misalignment in the b1 region was about 80°K. An observation which is believed to be related to the misalignment is illustrated by figs. 5 and 6, These figures illustrate the dependence of the two types of spectra on the duration of the thermal cleaning. Note that the O+ spectra are influenced much more noticeably than the CO” spectra. If all the adsorbed CO molecules contributed equally to the O+ ion current, one would expect that perturbing factors such as the thermal cleaning procedures woutd cause equivalent changes in the O+ and CO+ spectra.
DISSOCiATlVE
ELECTRONIC
DESORPTION
69
DF co
163052 .Y
6
94 8 2lOCPK
&
FOR 5 SEC
i3 s
12 0 +‘8
CO EXPOSURE:
i .4 x
300
600
ADSORBENT
Fig. 5.
700
800
Effect of thermal cleaning treatment
t 5
t
IGWK
SW
SURFACE
t
Km0
TEMPERATURE
flO0
1200
T W- NONLINEAR
SCALE)
on the CO+ thermal desorption
/
/
I
PULSE
spectrum. 163052 I
f V*
= too VOLTS
J,
c 130
mlcROAMps/cM*
dT * __.. ._-_ z = 6’KISEC co EKlvSURE=
I
Of, 400
i
500
600
ADSORSENT
Fig, 6.
L
700
SWFACE
Effect of thermal cleaning treatment
8
I
800
900
I
/
I000
TEMPERATURE
I .43 x 10-6
It00 TPK-
J
t
mo
RONLIWEAR SCALEl
on the 0” thermal desorption
spectrum.
An attempt to explain the misalignment of the spectra, based on a ternperature dependence of the electronic desorption efficiency, was unsuccessful, However, it is possible to explain the observations qualitatively with a model which is based on the assumption that the electronic desorption efficiency for production of an 0 + ion (~‘1 is not the same for all CO moleeuIes that are adsorbed in a given type of site. Both the misalignment of
70
1. W.
the O+ and CO+ spectra, duration
of the thermal
COBUKN
and the greater sensitivity cleaning,
of the O+ spectra to the
suggest that not all of the cl-bonded
CO
molecules (nor all of the PI-bonded molecules) contribute equally to the O+ current. It has been suggested21) that there exists a distribution in desorption energies for CO adsorbed on W. In addition, the desorption energies for x-bonded CO on W can be calculated from the O+ thermal desorption spectra (figs. 2 or 6) using the first order Polanyi-Wigner equation: dQ/dt = - v0 exp (-
E,/kT)
The value of the activation energy for desorption (Ed) that is obtained varies from about 1.3 to 1.7 eV depending upon the point on the curve used for the calculation. This result suggests that the desorption energy of the a-bonded CO molecules which contribute to the O+ current is not constant. The reason for the variation in Ed (i.e. principally in the W-C bond) is believed to be interaction with adjacent adsorbed molecules, Rigbysr) has observed what is believed to be the result of an interaction of this type, using N, and CO on W. Evidence that such influences on the W-C bonding can extend to the C-O bond is provided by the infrared absorption work of Eischensss). The latter author found several values for the stretching frequency of the C-O bond, each presumably corresponding to a distinct state of adsorption of CO on Pd. Thus we may expect that molecule-molecule interactions which lead to a range of thermal desorption energies will also produce changes in the C-O bond which will be evident as a range of values observations which suggest such a of the quantity yl+. The experimental variation in yl+ will now be discussed further. The misalignment of the O+ and CO+ spectra indicate that only cc-bonded CO molecules with desorption energies at the high-energy end of the distribution of Ed(a) contribute appreciably to the O+ current. That is, the CO+ thermal desorption spectrum indicates that a substantial number of the a-bonded CO molecules have been thermally desorbed before any appreciable decrease in the O+ ion current is seen. The opposite situation is observed in the case of the fir-bonded CO molecules (fig. 2). Here apparently only the PI-bonded CO molecules with desorption energies at the lower end of the distribution of Ed@‘,) contribute to the O+ current. Figs. 5 and 6 can be used to support the ideas introduced above. The dotted spectra in the two figures were recorded under identical conditions, as were the two solid spectra. In these figures a change in the thermal cleaning time has caused significant changes in the O+ spectrum but only small changes in the CO+ spectrum. In fig. 6, increasing the duration of the thermal cleaning time from about 0.5 seconds to 5 seconds has caused a significant decrease in the CIcontribution to the O+ current and an increase in the /I1 contribution.
DISSOCIATIVE
ELECTRONIC
D&SORPTION
OF co
71
However, in fig. 5, the same change in the cleaning time caused only a barely perceptible decrease in the high temperature end of the CIpeak of the CO+ spectrum and a more pronounced increase in the low temperature end of the fil peak, whereas, the height of the fll peak was unaffected. If all Pi-bonded CO molecules were contributing equally to the O+ current, it would be expected that a factor of two increase in the B1 contribution to the O+ current would require a factor of two increase in the fi, peak in the CO+ spectrum. A similar argument can be applied to the E-bonded molecules. Since this expectation is contrary to the observations, figs. 5 and 6 lend some support to the concept of a variation in the electronic desorption efficiency among CO molecules adsorbed in a specific site. It is somewhat reassuring that the thermal desorption of a-bonded molecules terminates at about the same temperature in both the 0’” and the CO” spectra. Also the thermal desorption of PI-bonded molecules begins at about the same point in both types of spectra. The fact that the duration of the thermal cleaning influences the results is disturbing. The temperature at which the surface is cleaned, provided that it is greater than about 2OOO”K, does not influence the results noticeably. If the cleaning time is reproduced exactly the reproducibility of the O+ thermal desorption spectra is quite good. The cause of the dependence of the O+ spectra on the cleaning time (fig. 6) is believed to be CO adsorption that occurs at elevated W ribbon temperatures. Ideally the ribbon should be cleaned in vacuum and then be allowed to cool to the ambient temperature before any adsorption occurs. However, in this work the ambient pressure is about lo-’ Torr of CO and is influenced by the duration of the thermal cleaning. If the thermal cleaning is brief, the large surfaces nearby, such as Plate 1, remain below the minimum temperature required desorption. However, for longer thermal cleaning times these become hot enough for thermal desorption to occur and a large the CO pressure is seen. In addition, the increased temperature
for thermal surfaces do increase in of the sur-
roundings caused by prolonged thermal cleaning of the ribbon decreases the cooling rate of the W surface. Both the higher pressure and the smaller cooling rate lead to more CO adsorption at elevated temperatures. An extreme example of this situation will now be discussed. 3.3. Of CURRENT AT HIGH SURFACE TEMPERATURES It has been observed that an appreciable 0’ surface ion current is detected even when the W ribbon temperature is greater than the temperature required for the thermal desorption of CO molecules from pi sites. If the W ribbon is heated to temperatures in the range of 1000 to 1300°K for about 10 minutes in about toe7 Torr of CO, the high temperature 0’ current caused by
J. W.
72
COBURN
electron bombardment reaches a steady state value which is comparable in magnitude to the steady state O+ current observed when the ribbon is at the ambient temperature of about 340°K after a thermal cleaning and a CO exposure of about low6 Torr-min. The following are the characteristics of this steady state high temperature 0’ current: i) It decreases until it is no longer observable when the W ribbon temperature is greater than about 1500°K. ii) It has a dependence on the ribbon temperature which can be characterized by an Arrhenius plot with an activation energy in the range of 0.4 to 0.8 eV over a limited temperature range (fig. 7). The reason for the spread in activation energy is not known. iii) It is independent of CO pressure. iv) It is independent of the magnitude or direction of dT/dt, the rate of
IO9-
\
0
B-
0’ \
?-
=
C,
EXP(-E,/kT)
WHERE
0
L= C,=
0.65 eV CONSTANI’
\
6-
5-
4-
3-
V,
=
100
VOLTS
J,
?
140
MICROAMPS/CM’
P
s
2
IO-‘TORR
$+
z -5’K/SEC
x
OFCO
2-
Ia;O°K
,
13yOeK
,
12q0°K,
;I~““”
ISZC
7.5
8.5
8.0 IO?T
Fig. 7.
Temperature
dependence
9.0
,
1
9.5
(OK-‘)
of the O+ current
at high tungsten
surface
temperatures.
DISSOCIATIVE
ELECTRONIC
DESORPTION
OF co
73
change of the ribbon temperature provided the temperature has not exceeded about 1400 “K. v) If the ribbon is cooled to the ambient temperature after the steady state high temperature O+ current has been established, the resulting 0’ current is much lower (a factor of 5 or 10) than the O+ current observed after a normal thermal cleaning and CO exposure. Additional exposure to CO does not appreciably increase this small 0” current. Fig. 8 shows an Of spectrum taken after this treatment, and the relatively small contribution of the o! and &-bonded CO molecules to the Oi‘ current can be seen. The high temperature O+ current is believed to be caused by electronic desorption of CO molecuIes which are adsorbed in sites with large desorption energies [p2 or p3 sitesae)]. Heating the W ribbon in CO at temperatures of the order of 1000°K apparently results in more CO adsorption of this type and fewer molecules adsorbed in the a and pi sites than would be the case if all the adsorption occurred at the ambient temperature of 340°K. At 340°K it is believed that some of the p2 and /I3 sites which are available for CO adsorption can be eliminated by CO adsorption into a ,8i site. It has been suggested 21) that the differences among the /I sites lies in the spacing of the W atoms to which the bridged P-bond is formed. Therefore, if a particular W surface atom has adjacent W atoms at distances suitable for both p1 and fi3 bonds, it is not difficult to see how the occupation of the pi site can eliminate the /I3 site when only one bond per surface W atom is allowedzl). Thus, when adsorption takes place at a surface temperature above the thermal desorption temperature of the pi sites, more of the /I3 sites will be occupied. The a sites are though to be a result of adsorption onto a W atom whose nearest neighbors are all involved with b-bonded CO molecules20). Since CO molecules are mobile on the W surface at temperatures above about 7OO”Kes), it is not surprising that heating the surface at 1000 “K would result in a more closely packed P-adsorbed array of CO molecules, thus reducing the number of CI sites. The temperature dependence of the high temperature Of current does not appear to be due to coverage changes. It is believed to be a result of thermal excitation of the adsorbed CO molecule into higher vibrational energy states from which electronic desorption is more probable. 3.4. THEENERGYTHRESHOLD
AND THEENERGY
DEPENDENCE OF THE 0"
CURRENT
The 0’ current has been observed for electron energies up to 550 eV and the results are shown in fig. 9. In order to focus the electron beam onto the target at different electron energies, the potential difference between Plates 1 and 2 (fig. 1) must be changed. Unfortunately this results in an ion collection
74
J. W.
COBURN
6
s-
ve -
100
VOLTS
J, E 160 MICROAhWWf P = 6.6 x lO*TORR
4-
% -
OF CO
5 IZ*K/SEC
HEATlffi
SEVERAJ_ HOURS AT
134O.K
3-
600
700
600
SO0
1000
ADSORBENT SURFACE TEMPERATURE
Fig. 8.
O+ thermal desorption
1100
TPK-NONLINEAR
1200 SCALE)
spectrum after heating the W surface in CO.
efficiency which varies with electron energy. The resulting distortion of energy dependence curves is shown in fig. 9 by the comparison of the measured CO+ current from the ambient with a published CO+ ionization efficiency curve24). The principal effect is seen to be a progressive lowering of the apparent yield at higher energies. In contrast to the q+ versus V, curve shown on fig. 9 (maximum at about 320 V), most published datal-sT14) sh ow maxima between 100 and 150 V. Recently however, Redheadd) has published a curve showing a maximum at an electron
energy greater than 280 eV. The reason for these discrepancies
is
not known. The behavior of the O+ current in the region of the energy threshold is illustrated in fig. 10. Curve A of this figure is a plot of O+ versus V, whereas Curve B is a plot of (O+)+ versus V, .The energy threshold ek’, can be determined quite easily from Curve B. The reason for the cubic dependence of O+ on the excess energy is not clear and may not be significant. It is interesting to plot V,, the threshold voltage for the formation of an O+ ion, versus V,, the retarding potential applied to Grids 1 and 2. The results that have been obtained are shown in fig. 11. Plots of this type have been used often in studies of dissociative ionization processes25) in the ambient and RedheadzsJ) has also made such plots in his studies of the electronic desorption of O2 from MO and CO from W. For a dissociative ionization event of the type XY+e-+X++Y+2e
DISSOCIATIVE
ELECTRONIC
DESORPTION
15
OF co
163070 163092 I
I
I
I
I
160
140
120
4c
o+ 1
2(
co+:
P 6 9x
IO-*TOFIR
2
*
J
J,
P=
1x10-7
5 f
Je * 35
TORR
100
The dependence
OF
CO
MICRa4MPS/CM’
200 Ve
Fig. 9.
OF CO
MIC!?OAMPS/CM2
1
I
I
300
400
500
I
I
0
I5
(VOLTS)
of the Of and CO+ currents
on the electron
energy.
the threshold energy depends on the kinetic energy of the dissociation products X+ and Y. The total kinetic energy can be expressed in terms of I&(X+), the kinetic energy of the ionized fragment, by invoking momentum conservation. The threshold energy eV, can then be expressed in terms of E,(X+) and eV,,, the threshold energy when the kinetic energy of the fragments is zero. eV, = elf& +
m(X)+-4Y) l&(X+) -~~-~ [
m(Y)
. 1
Fig. 11 is a plot of eV, versus Ek(Ot) where E,(O+)=eV, (Vr=retarding potential applied to Grids 1 and 2). If Ek(Of) has a minimum value, the threshoId energy ek’, will be independent of eV, for eV~<&(O+),,i,. Thus,
76
J. W.
P z 2 x lO-7 20
= J,=
36
TORR
COBURN
OF CO
MICROAMPS/CM2
ELECTRON
Fig. 10.
The dependence
the following 4 ii) iii)
ENERGY
(~‘4)
of the 0+ current on the electron energy in the region of the energy threshold.
information
can be obtained
from fig. 11:
V,, = 16.2 V, E, (O’),i,
= 4.7 eV,
nz(O+)+m(ConW)_l m(ConW)
’
Therefore the mass of the 0’ ion is much smaller than the mass of the other dissociation product. The latter probably includes the tungsten lattice as well as the carbon atom. The sensitivity of the threshold measurements was limited by photoemission from the first dynode of the electron multiplier which was caused by the electron-bombardment induced-soft X-rays. A deflection type of mass spectrometer would eliminate this problem. Fig. 12 is a plot of the energy distribution of the 0’ ions obtained by differentiating an O+ retarding potential curve for three values of the electron energy. The fact that a substantial number of the ions appear to have energies less than the 4.7 eV minimum implied by fig. 11, is believed to be a result of the unfavourable geometry of the system. The geometry about the ribbon is planar instead of spherical, and in addition, there is considerable collimation of the ion beam (by the mass spectrometer apertures) beyond the retarding grids. Any electrostatic focusing effects which occur in the vicinity of these grids will be dependent on the ion energy and will result in a larger fraction
DISSOCIATIVE
ELECTRONIC
DESORPTION
OF co
77
6+:?. _.Mm. 2 4>’
,’ /’ /Y’
: 4
./’
2,/%LOfE
8
= I
.’ i E 3
I/’ o.--E; p:
L -2 -
P E 2 I IO-’ TOM
P. ‘D; #i .:
-4
OF CO
20 6 Je = 36 MICROAMFS/CMr Q I 17
’ I:’
1 IS
1 16
I 20
THRESHOLD
Fig. 11.
The retarding
I 21 VOLTAGE
22 V+
voltage V, versus Vt, the threshold of an Of ion.
I 24
I 23
_
MLTS)
voltage for the production
163046 5
P 5 E
1.6 I
I
I
I
IO -7
I
TOAR OF CO
4-
3
o.5
Fig. 12.
’ I
I 2
I 3
B.
v,
C.
Ve -265
= 98 VOLTS.
4
VOLTS,
Je r
170 MICROAMPS/CM’
5
6
RETARDING
VOLTAGE
The approximate energy distribution
7 Vr
6
9
(VOLTS)
of the 0+ current at three electron energies.
of the low energy ions being removed from the ion beam by collimation. This effect would result in a spreading of the ion energy distribution curve to lower energies. However the data of fig. 3 1 should not be influenced by this geometrical difficulty since the threshold energy is determined for ions with energy eV,. If a fraction of these ions is removed from the ion beam by collimation beyond the retarding grids, the only effect is a reduction in
78 the sensitivity
.I. W. of the ion collection
CORURN
system. In order to check the accuracy
of
the retarding potential measurements, a retarding potential curve of the Naf and Kf ions that are emitted from a fresh tungsten ribbon2s) was measured. The energy distribution obtained in this way had a maximum at an accelerating potential of about 0.5 V instead of at a retarding potential of about 0.1 V. Redhead4) observes a most probable ion energy of 7 eV (compared with 6 eV in fig. 12) and a second peak at t eV which is believed to be due to electronically desorbed CO+ ions. No electronically desorbed CO+ ions were seen in the present work; the reason may be that the surface temperature of 340 “K is too high. Yatesll) also found two peaks in the ion energy distribution at 0.4 and 6 eV and assigned the low energy peak to ions desorbed from M sites and the high energy peak to ions desorbed from p, sites. This assignment disagrees with the present work in which ions originating at a sites were observed to have large kinetic energies. It is tempting to apply conservation of energy to the electronic desorption process as others3,4y9) have done. If there are no energy loss mechanisms in the interaction and ionization of the bound oxygen results in a free electron of zero kinetic energy, then el/,, = D(C0)
+ ev,(O+)
+ E,(CO)
- Ed(C),
where V,, = 16.2 V, D(C0) = dissociation energy of the CO molecule (11.1 eV) (ref. 26), Vi(O+) = ionization potential of the oxygen atom (13.6 V), E,(CO) = desorption energy for CO on W (1.2 eV) (ref. 20), E,(C) = desorption energy for C on W. The value of E,(C) that is obtained (9.7 eV), is not an unreasonable value in view of the tenacity with which carbon adheres to tungsten27). 3.5. MISCELLANEOUSOBSERVATIONS The magnitude of the O+ current was measured as a function of electron current density and of the CO pressure in the system. The Of current was proportional to the electron current and was almost independent of the CO pressure. A slight decrease in the O+ current was observed at high CO pressures (> 10e7 Torr). The desorption energy of cc-bonded CO molecules was determined from the transient response of the O+ current to a sudden increase in the W ribbon temperature. The O+ current decrease is approximately exponential with a time constant: l/z = v exp(If the ribbon
temperature
E,/kT).
is kept below the thermal
desorption
temperature
DISSOCIATIVE
of the /Ii-bonded can be obtained.
ELECTRONIC
CO molecules, values
79
OF co
a value of Ed for the a-bonded
The results obtained
agreed quite well with spectra (1.3-1.7 eV).
DESORPTION
with this approach
derived
from
molecules
(with v = 1013 set- ‘)
the O+ thermal
desorption
As was mentioned earlier, the Of current is not noticeably affected by electron bombardment in the amount required to record the desorption spectra. In this work apparently the CO pressure in the system is large enough (lo-’ Torr) to maintain nearly full coverage of the surface during the electron bombardment. However, if the ribbon is bombarded for times of the order of one hour, some effects are seen. The reproducibility of the bombardment effects is poor but the total ion current is decreased by about 20% by electron bombardment for one hour with a current density of about 130pA/cm2 of 1OOeV electrons. Even though the total Of current is decreased by electron bombardment, the p1 contribution to the O+ current is increased substantially (of the order of 50%) as indicated by subsequent O+ thermal desorption spectra. The CO+ spectra are only very slightly affected as might be expected, since only about 20% of the total ribbon surface is subjected to electron bombardment. The observation that the p1 contribution to the O+ current is increased by electron bombardment, whereas, the c( contribution is decreased, implies an a to p1 transition. This is probably a result of electronbombardment-induced reordering of the adsorbed CO molecules. 4. Summary The efficiency for the production of an O+ ion as a result of the dissociative electronic desorption of a CO molecule on a W ribbon determined m this study is about 2 x lo-’ ions per electron for 100 eV electrons at a surface temperature of about 340°K. About 75% of the ion yield is a result of desorption of the cc-bonded CO molecules and the remaining 25% is due to the /II-bonded CO molecules. This efficiency agrees well with Redhead’s value of 3 x lo-’ ions per electron for CO on MO 3). Lichtman has obtained a value of 1.3 x 10m4 ions per electrong) for CO on MO from LYsites only. It has not been possible to determine the density of adsorbed CO molecules in each of the adsorption sites and as a result it is difficult to estimate a cross section for the dissociative electronic desorption process. Redhead4) and Menzel and Gomerl4) obtain a total cross section for electronic desorption of CO from W of 3 x lo-‘* cm’. This figure includes the desorption of neutral atoms or molecules as well as the O+ ion. The 20.9 eV value for the energy threshold for the formation of an O+ ion agrees well with the 20 eV threshold observed by Lichtman for CO on Meg). However, the 16.2 eV threshold energy for the formation of an O+ ion
80
J. W.
with
zero
kinetic
energy
COBURN
is considerably
lower
than
Redhead’s
18.7 eV
value for CO on W “). The comparison of the W results with those for MO is questionable. In fact, it might be expected that the results for identical adsorbate-adsorbent systems will vary substantially as a result of variations in adsorbent crystallme structure and surface cleanliness. In this study the W ribbon was heated in oxygen to remove carbon impurities. This treatment caused severe changes in the electronic desorption characteristics and these changes were reversed by the intentional carburization of the W ribbon. The introduction of oxygen with the CO also resulted in severe changes in the behavior of the O+ current. The misalignment of the O+ and CO+ thermal desorption spectra that was observed in this study, although not reported by other workers, is not believed to be an experimental artifact. A tentative explanation has been presented, which is based on the assumption that the electronic desorption efficiency is not constant for CO molecules adsorbed in a specific type of site. Some additional observations support the proposal but the mechanisms involved are not understood in detail.
Acknowledgement The author is indebted to Professor W.T. cussions throughout the course of this work.
Peria
for many
helpful
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
G.E. Moore, J. Appl. Phys. 32 (1961) 1241. P.A. Redhead, Can. J. Phys. 42 (1964) 886. P.A. Redhead, Appl. Phys. Letters 4 (1964) 166. P.A. Redhead, Suppl. Nuovo Cimento 5 (1967) 586. D. Lichtman, J. Vacuum Sci. Technol. 2 (1965) 70. D. Lichtman and T.R. Kirst, Phys. Letters 20 (1966) 7. D. Lichtman, T.R. Kirst and R.B. McQuistan, Phys. Letters 20 (1966) 129. D. Lichtman and T.R. Kirst, J. Vacuum Sci. Technol. 3 (1966) 224. D. Lichtman, R.B. McQuistan and T.R. Kirst, Surface Sci. 5 (1966) 120. D.R. Sandstrom, J.H. Leek and E.E. Donaldson, J. Appl. Phys. 38 (1967) 2851. J.T. Yates, T.E. Madey and J.K. Payn, Suppl. Nuovo Cimento 5 (1967) 558. R. Comer and D. Menzel, J. Chem. Phys. 40 (1964) 1164. D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3329. W. Ermich, Philips Res. Rept. 20 (1965) 94. L.A. Petermann, Suppl. Nuovo Cimento 1 (1963) 601. G.F. Sauter, R.A. Gerber and H.J. Oskam, Rev. Sci. Instr. 37 (1966) 572. J.A. Becker, E.J. Becker and R.G. Brandes, J. Appl. Phys. 32 (1961) 411. R.E. Weber and L.F. Cordes, Rev. Sci. Instr. 37 (1966) 112. P.A. Redhead, Trans. Faraday Sot. 57 (1961) 641. L.J. Rigby, Can. J. Phys. 42 (1964) 1256.
dis-
DISSOCIATIVE
22) 23) 24) 25) 26) 27) 28)
ELECTRONIC
DESORPTION
OF co
R.P. Eischens and W.A. Plisken, Advan. Catalysis 10 (1958) 1. R. Klein, J. Chem. Phys. 31 (1959) 1306. J.T. Tate and P.T. Smith, Phys. Rev. 39 (1932) 270. H.D. Hagstrum, Rev. Mod. Phys. 23 (1951) 185. M.A. Fineman and A.W. Petrocelli, J. Chem. Phys. 36 (1962) 25. N. J. Taylor, Surface Sci. 2 (1964) 544. D. Lichtman, J. Vacuum Sci. Technol. 2 (1965) 91.
81