Decomposition of CO2 on (100) W

SURFACE

SCIENCE 33 (1972) 1 l-26 0 North-Holland

DECOMPOSITION L. R. CLAVENNA**

OF CO,

Publishing Co.

ON (100) W*

and L. D. SCHMIDT

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A. Received 19 April 1972; revised manuscript received 23 May 1972 The binding states and condensation kinetics of CO2 and its decomposition products CO and 02 on (100) W are examined by flash desorption mass spectrometry. Carbon dioxide desorbs almost entirely as CO and oxygen at all coverages. At saturation there are three major states of CO. These correspond to the high temperature states of CO alone although all peaks are shifted and amounts are altered slightly. Carbon monoxide exhibits four major binding states. All of these obey first order desorption kinetics as expected except the most tightly bound state which obeys second order kinetics. Desorption from a bimolecular complex is postulated to explain these results. Initial sticking coefficients of COZ, CO, and 02 at 300°K are 0.85,0.49, and 1.Orespectively. Coverage dependences indicate precursor intermediates for the states occupied at low coverages. The initial sticking coefficient of CO2 varies rather slowly with substrate temperature, falling to 0.3 at 1200°K.

1. Introduction Considerable information concerning the binding states and kinetics of simple adsorbates on single crystal surfaces has been accumulated, but as yet little comparable data exists for multiple species and chemically reacting systems. One of the simplest surface reactions, except possibly isotopic exchange, is the unimolecular dissociation into products. Most of the literature dealing with CO, on tungsten suggests that CO, dissociates according to the reaction CO2 + CO (ads) + 0 (ads). Evidence of dissociation comes from the work of Hayward and Gomerl) who inferred from work function changes that in a temperature range of 400 “K to 750 OK, CO2 on a tungsten field emitter existed as CO and 0. They also observed that when a surface fully covered with CO, was heated above 400”K, a slight amount of reversible desorption of undissociated and loosely

* This work partially supported by the National Science Foundation under Grant No. GK 16241. ** Present address: Corporate Research Laboratory, Esso Research and Engineering Company, Linden, N.J. 07036, U.S.A. 11

12

L. R. CLAVENNA

AND

L. D. SCHMIDT

chemisorbed CO, occurred. Collins et al.2) studied CO, chemisorption on evaporated metal films in the temperature range of 195 “K to 343 “K. They found that on tungsten there was a fast irreversible chemisorption which at 195°K corresponded to a 4-5 site mechanism and that at 343°K there was a slight amount of reversible chemisorption. Adsorption studies on evaporated metal films by Brennan and Hayward3) indicated that at room temperature the major part of the CO2 adsorbed on a W film dissociates according to the above reaction. They found extremely fast initial rates of adsorption along with a large initial heat of adsorption, - 122 kcal/mole. This large heat of adsorption is expected for dissociation into adsorbed CO and 0. They also found the ratio of the maximum amount of COZ adsorbed to that of CO was about 1: 2.3. A study of CO2 adsorption on a polycrystalline ribbon in this laboratory‘i) using flash desorption techniques indicated that only a few percent of adsorbed COZ is desorbed as undissociated COZ. Adsorption pressure-time measurements on the polycrystalline tungsten ribbon gave a high initial sticking coefficient of 0.89 which remained constant to a coverage of -0.4. In the following study, the interactions of CO2 and its dissociation products (CO and oxygen) are examined on a tungsten substrate. In order to minimize the influence of crystallographic differences and thereby facilitate the interpretation of binding states and kinetics, the measurements were performed on the high symmetry (100) plane of a tungsten macroscopic single crystal. This study was initiated as a preliminary study to determine the feasibility of a more detailed investigation of the decomposition kinetics of CO2 on (100) W in a molecular beam. The properties of the COZ decomposition products, CO and oxygen, have been studied, especially on polycrystalline W. Carbon monoxide on W exhibits a number of binding states, conversion between which depends sensitively on the substrate temperature and deposition conditions5.s). Oxygen on W desorbs largely as atoms and tungsten oxides, making desorption measurements rather difficult. However, from extensive studies of this system’-10) a fairly detailed picture of the desorption kinetics of oxygen on W has emerged. 2. Experimental The apparatus and procedure have been described previouslyll-13). Condensation and desorption kinetics of gases on a single crystal disk of W exposing primarily (100) planes were monitored from partial pressure changes in a Pyrex ultrahigh vacuum system. Crystal temperatures between 78 and 2800 “K were obtained by cryogenic cooling of the lead assembly and heating by electron bombardment or a focussed light beam. The crystal was

DECOMPOSITION OF c02

ON (It)o)

w

13

Much of the data was repeated on twu (100) W crystals, Evidence that the ~~~v~or reported here is characteristic of the (tQO) plane comes from the fact that ~rev~ous~~ reported fIash d~so~t~on spectmll* fe) of HZ and N2 were reproduced on these surfaces, Binding states and desorption kinetics af the various species were examined by flash desorption. Tantafum film getters provided sufficiently high pumping speeds (z&O.05 set) that d~~er~~~~~~d~o~t~o~ spectra were obtained+ Crystaf ~em~rat~r~s were measured by a W-Re ~hermo~up~e or tan o@icaf pyrometer, Heating rates were in the range of 300_6OO”/sec for most Slash desorptian spectra shown, but lower rates were used to resolve the low temperature a states. An emission controller circuit maintained constant current for each d~so~~~on sequence Desorption salvation energies were d~~~rrn~~~ using the heating rates at the deso~tio~ temperatures. Sticking coefficients were determined from the pressure versus time curves or by flash desorptian after adsorption for known times and pressures”1* 14). The pumping time constants of the system for various gases were determined by ~~~b~~~the efectron bombardment filament and measuring the decay of the partirtJ pressure. Plots of IO&P--~,) versus time gave the time constants during each run. Relative adsorbate densities for CO and CO2 were determined by calibrating the mass spectrometer against an ion gauge> taking appropriate sensitivity factors, Amounts adsorbed were then obtained from the areas under aash desorption curves, k~o~~~~ the system vohnne and piping time constsnts. The II&&~ saturation densities of CB, CO, and 0, were also obtained by comparing the areas under the partial gressure versus time curves during adsorption; pumping time constants for each gas were measured before and after each run.

The flash desorption spectra obtained after exposing the surface to CO at 2 x 10-s torr for varying times are shown in 5g_ 1. FarfiaX pressures of other gases (main& Hz ano CO,) in these m~suremen~s were ‘Iessthan one percent of the tot&_ Flash d~sor~~~on spectra at masses of 2, 12, 14, 16, f 8, and 44 indicated. that the coverages of gases other than Co were much less than one percent. Four major peaks are evident in the spectrum; a low temperature

14

L. R. CLAVENNA ANDL. D.SCHMIDT

o. state and three high temperature states labelled PI, pz, and p3. The relative amounts, estimated from the areas under the peaks (dashed curves), are indicated in table 1. Adsorption at 195 “K produced identical desorption spectra except that the relative amount in the low temperature a states is increased from 3.0 to 4.1. The additional CO at low temperatures consisted

dn dt

0

300 400

600

600

1000

1200

1400

1600

I600

T (“K) Fig. 1. Flash desorption spectra of CO from saturation at 3OO”K.The dashed curves indicate assuming first order desorption kinetics for for the pa

(100) W for various CO coverages up to theoretical fits to the PI, Bz, and 83 states the 01 and PZ states and second order state.

TABLE1 Parameters

for CO on (100) W at 300°K

State

Ed0

Desorption order

B3(<0.1)

93f5 74*4 62h-4 57+4 21+2

lj

4.1

? 1 1

1.1 ) 1 i 3.0 4.1*

F:(‘O.l’ B1 a

* After saturation at 195°K.

Saturation atom density relative to PI

SO

0.49 -0.4 -0.1

Coverage dependence ofs Precursor 1-e -1-e

DECOMPOSITION

OF co2

ON (100)

15

w

of several small peaks between 200 and 300 “K. These are probably

associated

with other crystal planes as they are much smaller than the major a peak. The shape and absence of a shift with increasing coverage in the a state indicates first order desorption kinetics. From the peak temperature the desorption activation energy was estimated to be 21 kcal mole-’ assuming a pre-exponential factor of 1Ol3 set -I. The B1 and Bz states also desorb with first order kinetics with activation

energies of 57 and 62 kcal mole-‘,

respec-

tively. The desorption kinetics from the B3 state appear quite different than from the other states of CO. For e(B,)O. 1. It was necessary to heat the crystal to ~-2350°K to obtain the CO spectra shown in fig. 1. If this had not been done, the pl and B2 states were as shown in fig. 1, but the CYand B3 states were much smaller or were not populated at all. Anderson and Estrupl5) observed only two high temperature states for CO on (100) W. Madey and Yatesis) observed a large c1 peak and two incompletely resolved B peaks. Armstrong17) also observed an a peak near saturation at 300 “K. Our results appear to be consistant with those of Madey and Yates and Armstrong, although our increased peak resolution reveals three distinct B states. The absence of an cr state in the spectra of Anderson and Estrup could be due to a crystal temperature above 300°K or contamination. The two l3 states observed by Anderson and Estrup could be either

16

L. R. CLAVENNA

AND L. D. SCHMIDT

our p1 and p2 states or incompletely resolved p1 and pz plus the &. The desorption temperatures agree better for the latter while the shapes of peaks indicate the former. Incomplete cleaning should inhibit formation of the p3 state as well as the a state. Considerable care was used to show that the p3 state of CO was not due to

n. v. = 37 x IO” set-’

5.8

6.0 f,

62 6.4 10-s OK-’

66

6.6

7.0

Fig. 2. Second order plot for the j3s state of CO on (100) W. Each point is obtained from the area under a flash desorption spectrum. Points indicated as A and B are for the curves shown in fig. 1. The vertical line for e(Bs)
DECOMPOSITION

QF co2

ON (100)

17

w

another adsorbate (e.g. NJ or a state of CO induced by the presence of another adsorbate. The desorption peak at mass 12 (C’) was identical to that at mass 28 while peaks at& 14, and 44 were much smaller than those from CO and its fragmentation products. The spectra from the clean surface were precisely reproducible over extended periods of time. It was also established that electron bombardment heating did not significantly perturb the adsorbate. This is especially important for the weakly bound a states of CCW 1s) (and probably CO,) for which the electron impact cross sections for conversion into p states and desorption are very high. The absence of such effects was demonstrated by obtaining desorption spectra using focussed light beam heating and by bombarding the adsorbate covered crystal with a low current of electrons for sufficient time to obtain the same total electron flux used in Aash desorption. Only minor changes in the desorption spectra were noted. Fig. 3 shows a plot of s versus total coverage for CO on the crystal at 300 “K. Three distinct coverage regimes are evident: at low coverage s is constant indicating a precursor state, at intermediate coverage s decreases tinearly,

0.5

0.4

S

0.3

0.2

0.1

0 0

.2

.4

.6

.8

1.0

i3 co

Fig. 3. Sticking coefkient versu5 coverage for CO on (100) W at 300”R, Data were obtained from pressure-timeCLUTE.. The three regions of the curve correspond ta populations of the p3, Bl+Ba, and a states in flash desorption. Approximate sr(&) curves for these states assuming sequential population are indicated by dashed lines.

18

L. R. CLAVENNA

AND

L. D. SCH.UIDT

and at high coverage a smaller slope is observed. As indicated in the figure these regimes correlate well with the populations of the states observed in Bash desorption. As the p3 state populates s is constant, s decreases linearly as the p2 and PI states populate, and approaches zero as the a state populates. There is of course no certainty that the populations of the various states at 300°K are the same as those at the flash desorption temperature because some conversion between states may occur during desorption. However most datas~e~1s-20) (work function change, electron impact desorption, adsorbate mobilities) indicate rather abrupt changes as the total coverage increases. Assuming that this curve does represent sequential population of the various binding states, the s versus Bj curves should have the properties indicated in fig. 3 and table 1. The initial sticking coefficient was 0.49 +O.l. This is slightly higher than the value of 0.3 obtained by Madey and Yatesel). 3.2. OXYGEN Since oxygen desorbs from W largely as atoms and oxides’-lo), flash desorption spectra are quite unreliable indicators of binding states and coverages. However we observed small but reproducible desorption spectra at mass 32 upon exposure of the (100) W crystal to 0,. The peak heights were only

T(“K) Fig. 4. Flash desorption spectra of 0% (mass 32) from (100) W. Spectra were obtained after 0~ exposures of 2 x lO-6, 5 x IO-‘, and 2 x 10-T torr set with the crystal at 300%. The abscissa is linear in desorption time. N 1% of those for other gases, and therefore it was necessary to pump out O2 before obtaining the spectra. The flash desorption sequence at mass 32 is shown in fig. 4. There are two definite peaks observable at approximately 1100 and 1920 “K. The peak labelled pz exhibits a definite shift with increasing coverage possibly indicating second order kinetics or a variation in Ed with

DECOMPOSITION

coverage,

OF coz

ON (100)

19

w

while the PI peak does not shift. The heating rate could not be made

linear with time at high temperatures because of radiation cooling by the crystal; the abscissa of fig. 4 is linear in time. No state of oxygen desorbing at 1000°K has been reported previo+-lo), although the high temperature state occurs in the temperature regime where most of the oxygen desorbs (largely as 0 atoms and oxides). We do not regard these data as in any sense definitive measures of the binding states, coverages, or desorbing species, although spectra were fairly reproducible and correlated fairly well with total coverage determined by condensation measurements. A possible cause of O2 desorption peaks is replacement of adsorbed gases on walls by atomic oxygen or oxides; there were no corresponding CO peaks and the 0, pressure before flashing was very low to minimize weakly bound 0, on the walls.

0

.2

Fraction Fig. 5.

.4

.6

of Saturation

.6

1.0

Coverage

Sticking coefficient versus coverage curves for CO2 and 02 on (100) W at 3OO”K, obtained from pressure-time curves.

The sticking coefficient versus coverage curve for 0, on (100) W at 300 “K is shown in fig. 5. The value of so obtained was very close to unity. The saturation atom density at 300”K, determined from adsorption measurements, was 0.90 of that of CO at 300 “K. These data are summarized in table 2.

20

L.R.CLAVENNAANDL.D.SCHMlDT TABLE

Adsorption

2

of COa, CO, and 02 at 300°K Saturation coverage relative to CO2

~~~

Gas adsorbed

So

0.85 0.49 1.0

co2

co 02

3.3.

CARBON

Adsorption

1. 2.4 2.2

Flash desorption 1.0 2.4

B only

1.0 1.5 -

DIOXIDE

The flash desorption spectrum of CO2 after saturation of the crystal at 300 “IX is shown in fig. 6. This represents approximately 4% of the total CO2 adsorbed, the rest presumably desorbing as CO, 0, O,, and tungsten oxides.

300

6ocl

900

1200

TPK) Fig. 6. Flash desorption spectra of CO2 (mass 44) from (100) W at 300°K. The upper curve was obtained after saturation and the lower curve at 0.8 of this coverage. The area under the upper curve is only 0.04 of that for CO from COz, indicating that most of CO2 adsorbed at 300°K desorbs as CO and oxygen.

For coverages less than 80% of CO2 saturation, less than 1% of the adsorbed CO2 desorbed as COz. The flash desorption spectrum of CO obtained after saturation with CO, is shown in fig. 7 along with the CO spectrum after saturation with CO. In both cases three high temperature states of CO are observed, although the states from CO2 are shifted from those from CO. The major difference is that only a small amount of CO desorbs from the low temperature a state after CO2 saturation. The flash desorption spectra of CO after increasing exposures to CO, at 300°K are shown in fig. 8a. These are qualitatively similar to those for CO alone, fig. 1. Again the p3 state exhibits a shape and shift indicative of second order kinetics, although as will be

DECOMPOSITION

OF co2

ON (loo)

21

w

dn dt

300

600

900

1200

1500

I800

-VK) Fig. 7. Flash desorption spectra of CO from (100) W after saturation with CO or COz at 300°K. No a-CO is observed from COZ and &CO peaks are shifted from their positions when CO alone is adsorbed.

later, the inffuence of oxygen is evident in reducing the desorption temperature or coverage of CO in this state at higher coverages. The relative saturation atom density of COz was determined from the area under the adsorption curve and found to be 0.42 of that for CO alone. The amount of CO desorbing from the CO, saturated surface was also 0.4 of that desorbing from the CO saturated surface. Agreement in amounts by the two methods confirms that most of the carbon in adsorbed COz desorbs as CO (96%) or COz (4%) rather than as species which are not stable gas molecules (C, carbides, &OS, or electronically excited molecules). As indicated in table 2 the agreement between the different techniques provides confidence that the adsorption and Bash desorption methods are accurate measures of stoichiometries for various species (except for flash desorption of oxygen). The sticking coefficient of CO2 at 300 “K versus coverage is shown in fig, 5. The initial sticking coefficient se was 0.85 at 300°K and decreased with increasing substrate iemperature as indicated in fig. 9. Data indicated by the solid circles were obtained by flashing the crystal to 2500”K, ahowing it to cool to the desired temperature, and maintaining that temperature by electron bombardment or light beam heating while exposing to CO,. By

discussed

22

L. R. CLAVENNA

AND L. D. SCHMIDT

measuring the pressure for the clean and saturated surfaces, s,, could be obtained. An alternate method, shown by the open circles in fig. 9, consisted of admitting CO2 into the system to a low pressure, flashing the crystal, and measuring the CO, pressure versus time as the crystal cooled. At high temperatures no adsorption occurred so there was no pumping by the crystal, but at lower temperatures condensation occurred and the pressure decreased. As long as the coverage remained low, these measurements give s0 versus temperature. As shown in fig. 9, data obtained by the two methods were in reasonable agreement. Coadsorbed oxygen causes the desorption of the most tightly bound state of CO to occur at lower temperatures than for CO alone as seen in fig. 8a. The influence of oxygen was examined further by repeatedly saturating the surface with CO2 and heating to - 1800 “K to desorb only the CO. Since most

dn dt

000

loo0

1200

1400

1600

1400

1600

1600

T(OK)

800

1000

1200 T PK)

Fig. 8. Upper graph shows desorption spectra of CO after CO2 adsorption on (100) W at 300°K to various coverages. Lower graph shows flash desorption spectra of CO after saturation with COZ. Top curve is the spectrum after saturation of the clean surface, and the lower curves were obtained after successive saturations after heating only to - 1800°K so as not to desorb oxygen. After four such cycles only a small amount of CO2 adsorbs and the CO desorbs exclusively from the pz state. The vertical scale for the low coverage curves is identical to that for the other curves.

DECOMPOSITION

of the oxygen desorbs

OF

23

Co2 ON (100) W

at > 1900”K, such experiments

give CO2 adsorption

on an oxygen covered surface. Results of these measurements are shown in fig. 8b. The upper curve is the CO desorption spectrum obtained after saturation of the clean surface with CO, the next curve represents saturation of CO, on a surface containing the residual oxygen from the previous saturation, etc. It is seen that oxygen efficiently blocks the p3 and PI CO sites and that only at high oxygen coverages was the pZ state not populated.

0 crystal coaling .

300

400

600

constant crystal temperature

000

1000

1200

T (“K) Fig. 9. Initial sticking coefficient SO versus crystal temperature for CO2 on (100) W. Data indicated by open points were obtained from pressure-time curves while the crystal was cooling after flashing. The solid points were obtained while constant crystal temperatures were maintained by electron bombardment or light beam heating.

4. Discussion These measurements are only of binding states and condensation kinetics, and far more extensive experiments will be necessary to completely characterize the adsorbed states and the reaction kinetics of this or any other surface reaction. The present measurements provide no direct evidence regarding the adsorbed species at the CO2 deposition temperature, although the high sticking coefficient of CO2 and the complete absence of desorbed CO,

24

L. R. CLAVENNA

AND L. D. SCHMIDT

except near saturation argues for complete dissociation. Evidence for this has been cited from the work function change upon CO, adsorption on field emittersl) and the high heat of adsorptions). 4.1. STICKING COEFFICIENTOF CO, The high initial sticking coefficient of CO,(O.85 at 300°K) is somewhat surprising considering that, in contrast to CO, the gas molecule has no dipole moment and might have steric limitations to condensation, i.e. may require bonding at the carbon atom and possibly bond breaking. The high value of s implies a weakly bound precursor intermediate state, and the independence of s on coverage up to 0.2 of saturation is interpretable in these terms. The sticking coefficient for CO, remains higher at high temperatures than for the simple chemisorbed diatomics. For N,is) at substrate temperatures between 300 and 1200”K, s varies from 0.41 to -0.07 while for CO, it varies from 0.85 to 0.3. 4.2. SURFACE SITES The ratio of the saturation density of CO, to that of CO in the p states is very nearly 1.O to 1.5. This is just as expected if CO occupies two surface sites and an oxygen atom one site (or if undissociated COP occupies three sites). Further the measured ratio of the saturation density of OX to that of CO is 0.90 to 1, very near to the 1: 1 ratio expected. It should also be noted that while there are four states of CO from CO adsorption, most of the CO desorbs from the p3 and a states. The total saturation densities in all states on the basis of the parent molecules for CO, O,, and CO2 on (100) W at 300°K are in ratios of 1.0: 1.06:0.74, in fair agreement with two sites for CO in the l3 states, one in the a state and one site for the 0 atom. The present measurements on a single crystal plane thus agree generally with stoichiometries reported previously on polycrystalline W 4, and evaporated films2). The effects of coadsorbed oxygen on the binding states of CO from adsorbed CO, are clearly evident, figs. 7 and 8. There is very little CO or CO, desorbing at temperature between 300 and 900°K in contrast to the large a state observed after CO adsorption. The population of the p3 state of CO is reduced somewhat from that for CO alone. The desorption activation energy of p,-CO is apparently lowered while those of the pi and pz states are increased by the presence of coadsorbed oxygen. Larger coverages of preadsorbed oxygen decrease the amount of CO2 which can be adsorbed. (The areas under the CO flash desorption curves of fig. 8b are proportional to the CO* adsorbed.) Oxygen first blocks CO adsorption in the p3 state, then the p1 state, and finally the pz state. It has been observed in several mixed adsorption systems, H, + COsa) and H, + N, ss), for example, that a strongly bound

DECOMPOSITION

OF coz

ON (100)

w

25

species (in this case oxygen) first blocks the most tightly bound state of a a more weakly bound species (CO). However in this case the a and & states of CO are exceptions to this trend. 4.3. BINDING STATESOF CO Most (75%) of the CO adsorbed at 300°K desorbs from the ct and p3 states, 21% desorbs from the p1 and I+2states, and 4% from other states. The dashed lines in fig. I indicate the computed peaks for the hr, l’&and S3 state, assuming the rate parameters indicated in the text. Some broadening of the l& peak near saturation is expected due to variation in the pre-exponential factor. Including broadening for the p3 state, no other binding states need be considered to fit compietely the data for the high temperature states. It has been shown by Gomer and coworkers536) that considerable rearrangement of the weakly bound a states of CO occurs upon heating the W substrate. The present experiments represent desorption after saturation at 300”K, and the low temperature states should according to their results come from virgin and a states. The multiplicity of peaks for T< 1000°K may be associated with sim~taneous conversion and desorption, as noted by Bell and Gomerr3), although the major a state has a width and shape characteristic of a single first order state. A remarkable finding from the CO desorption results is the apparent second order desorption kinetics from the p3 state. Since other measurements and thermodynamic considerations indicate that CO does not dissociate on W, the simplest interpretation of second order desorption kinetics would involve a bimolecular complex on the surface. Such a complex was proposed by Madey, Yates, and Stern25) to explain their observation that isotope exchange occurs upon CO adsorption on polycrystalline W. It is assumed that CO can exist on the surface as an isolated species (CO) and as a dimer (CO),, and that these species are in equilibrium

2 (CO) P (CO), with equilibrium constant K. If desorption can occur either from the monomer or dimer species,

(CO)2 (CO),

3

(CO), 1:2 (CO),, then the rate of desorption is given by an expression of the form -

dB (CO) -

dt

= k10 (CO) + k,K [0 (CO)]” a

26

L.R.CLAVENNA

AND

L.D.SCHMlDT

This expression predicts first order kinetics as 6 + 0 and can yield second order kinetics at high coverage if the second term is large. However in the absence of energy barriers for some of the steps, the activation energies in the two terms should be identical. The measured activation energies of 93 and 74 kcal mole-l would imply either an energy barrier for monomer evaporation or a sudden decrease in the binding energies at coverages >O.l in the p3 state. The second alternative is essentially equivalent to the assumption of two distinct binding states for the p3 state, a monomer with Ed= =93 kcal mole-’ and a dimer with I&=74 kcal mole-‘. Equilibrium populations of these states results in desorption largely from the weaker state as long as the other state remains almost saturated, after which the population of the tightly bound state decreases giving first order kinetics and the higher Ed. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25)

D. 0. Hayward and R. Gomer, J. Chem. Phys. 30 (1959) 1617. A. C. Collins and B. M. W. Trapnell, Trans. Faraday Sot. 53 (1957) 1476. D. Brennan and D. 0. Hayward, Phil. Trans. Roy. Sot. London A 258 (1965) 375. L. R. Clavenna and L. D. Schmidt, unpublished. C. Kohrt and R. Gomer, J. Chem. Phys. 48 (1968) 3337. D. Menzel and R. Gomer, J. Chem. Phys. 40 (1964) 1164; 41 (1964) 3329. B. McCarroll, J. Chem. Phys. 46 (1967) 863. Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 6 (1967) 42. N. P. Vaiko, Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 14 (1969) 448. D. A. King, T. E. Madey and J. T. Yates, Jr., J. Chem. Phys. 55 (1971) 3236, 3247. P. W. Tamm and L. D. Schmidt, J. Chem. Phys. 51 (1969) 5352. L. R. Clavenna and L. D. Schmidt, Surface Sci. 22 (1970) 365. P. W. Tamm and L. D. Schmidt, J. Chem. Phys. 54 (1971) 4775. P. A. Redhead, Trans. Faraday Sot. 57 (1961) 641. J. Anderson and P. J. Estrup, J. Chem. Phys. 46 (1967) 563. J. T. Yates, Jr. and T. E. Madey, J. Chem. Phys. 54 (1971) 4969. R. A. Armstrong, Can. J. Phys. 46 (1968) 949. P. A. Redhead, Nuovo Cimento Suppl. [I] 5 (1967) 586. A. E. Bell and R. Gomer, J. Chem. Phys. 44 (1966) 1065. J. T. Yates, Jr., T. E. Madey and J. K. Payn, Nuovo Cimento Suppl. [I] 5 (1967) 558. T. E. Madey and J. T Yates, Jr., Nuovo Cimento Suppl. [I] 5 (1967) 483. J. T. Yates, Jr. and T. E. Madey, J. Chem. Phys. 54 (1971) 4969. J. T. Yates, Jr. and T. E. Madey, J. Vacuum Sci. Technol. 8 (1971) 63. G. Ehrlich, T. W. Hickmott and G. H. Hudda, J. Chem. Phys. 28 (1958) 506. T. E. Madey, J. T. Yates, Jr, and R. C. Stern, J. Chem. Phys. 42 (1965) 1372.