Reactions of the Copper Dimer, Cu2, in the Gas Phase*

LI LIANt, FARAH AKHTARt, PETER A. HACKETT, and DAVID M. RAYNER Steacie Institute for Molecular Sciences, National Research

Council, 100 Sussex Drive, Ottawa, Ontario, Canada KIA OR6

Abstract

Reactions of Cuz with several small molecules have been studied in the gas phase, under thermalized conditions at room temperature, in a fast-flow reactor. They fall into one of two categories. Cup does not react with 0 2 , N20, N2, H2, and CHI at pressures up to 6 torr. This implies bimolecular rate constants of less than 5 X cm3 s-l a t 6 tosr He. Cuz reacts with CO, NH3, C2H4, and C3H6 in a manner characteristic of association reactions. Second-order rate constants for all four of these reagents are dependent on total pressure. The reactions with CO, NH3, and C2H4 are in their low pressure limit at up to 6 tom He buffer gas pressure. The reaction with C3H6 begins to show fall-off behavior at pressures above 3 torr. Limiting low-pressure, third-order rate constants are 0.66 2 0.10, 8.8 ? 1.2, 9.3 ? 1.4, and 85 ? 15 X cm6 s-l in He for CO, NH3, C2H4, and C3H6, respectively. Modeling studies of these rate constants imply that the association complexes are bound by at least 20 kcal mol-I in the case of C2H4 and C3H6 and at least 25 kcal mol-l in the other cases. The implications of these results for Cu-ligand bonding are developed in comparison with existing work on the interactions of these ligands with Cu atoms, larger clusters, and surfaces. 0 1994 John Wiley & Sons, Inc.

Introduction

The understanding of size-related properties forms a central theme of metal cluster research. Of these, chemical reactivity is of special interest due to the link to surface chemistry and thereby heterogeneous catalysis. Recent studies indicate that both electronic and geometric structure play a role in determining the chemical reactivity of metal clusters [l]. However, as the nuclearity of clusters increases, our ability to predetermine these properties, either experimentally or theoretically, diminishes rather rapidly. Consequently, one is led to study small clusters in order to develop concepts in metal-centered bonding that can be extended to larger systems. The systematic approach is to begin by comparing the reactions of metal dimers with those of the isolated metal atom.

The copper dimer is one of the best characterized metal dimers. Its electronic structure is well understood following spectroscopic studies using King furnace and, more recently, molecular beam techniques [21. It has distinctive transitions in the visible, convenient for either laser induced fluorescence (LIF) or resonant photoionization monitoring [3,41. The spectroscopic studies have provided accurate

*This article is submitted in the memory of Bob Back. He is much missed as a valued friend and colleague at NRC.

N. R. C. C. Research Associate, 1992-present. * N. R. C. C. Summer Student, 1992.

International Journal of Chemical Kinetics, Vol. 26, 85-96 (1994) 0 1994 John Wiley & Sons, Inc. CCC 0538-8066/94/010085-12

86 LIAN ET AL.

measurements of the Cu-Cu bond dissociation energy, bond length, and vibrational frequency [2]. Molecular structure calculations using both Hartree-Fock and density functional methods have been successful in reproducing these quantities [21. Bonding in the dimer is dominated by a ( 4 s ~ ~ ~ ) ~ single bond, giving it a closed shell electronic ground state in contrast to the open shell, 3d104s1 configuration of the Cu atom. As the simplest of the 3-d series dimers, Cu2 has been an important test molecule for theoretical descriptions of transition metal bonding. Small copper clusters have also been adopted as finite cluster models in quantum chemical studies of CO and NH3 chemisorption at copper surfaces [5-83.

The chemistry of larger copper clusters, Cum, has recently been studied in the gas-phase using small dimensioned flow reactors as extension nozzles for laser ablatiodsupersonic expansion cluster sources of the type pioneered by Smalley and co-workers [9]. In this manner, Riley and co-workers have studied the reactions of Cu, (n 2 12) with 0 2 and H20. They have interpreted their results in terms of the interplay between electronic and geometrical structure effects [lo]. H2O showed binding patterns consistent with site-specific chemisorption on icosahedrally packed clusters. 0 2 was unreactive with clusters with closed electronic shell structures and high ionization potentials. Kaldor and co-workers studied the reaction of Cu, with CO as part of a study of CO chemisorption on a series of metal clusters 1111. Reaction was only observed for clusters with n 2 4. However, the lack of reactivity of the smaller clusters was not attributed to thermochemical effects but to the inadequacies of the reactor which did not provide sufficient collisions with buffer gas to stabilize the collision complexes formed by small clusters. Indeed, recent quantum chemical calculations on geometry-optimized Cu clusters predict CO to bind more strongly to smaller clusters 181.

Just as for the reactions of any other small molecule, control over reaction conditions is crucial to investigating the gas-phase chemistry of smaller clusters. More than just achieving conditions where reaction can be observed, it is desirable to target the fall- off and limiting low-pressure regimes in order to characterize the kinetics adequately. Unimolecular reaction theory can then be applied to attempt to extract elementary thermodynamic information that, for transient species like metal cluster complexes, is hard to obtain in any other way. Such an approach has recently proved successful in the study of a number of metal atom complexes [121. We have recently designed a fast-flow reactor specifically for the study of the reactivity of thermalized small metal clusters under well-defined and controllable conditions and have communicated preliminary results of a study of the reaction of Cuz with ethylene [13]. This was found to be an association reaction in its low pressure limit up to at least 6 torr with a limiting low-pressure third-order rate constant of 9.3 t 1.4 X cm6 s-l in He. This is, to our knowledge, the first limiting low-pressure rate constant to be reported for any neutral metal cluster. From it, reaction rate modelling studies implied that Cuz(C2H4) is bound by at least 20 kcal mol-l. Here we report on the extension of this work to the gas-phase chemical kinetics of the reactions of Cu2 with a variety of reaction partners. Reaction rate modeling is applied to extract estimates of binding energies, and comparison with the reactivity of Cu atoms is made to develop our understanding of the factors determining bond formation in these systems.

Experimental

The flow reactor used in these studies has been described elsewhere [13,141. Briefly,

REACTIONS OF CUZ IN GAS PHASE 87

it uses traditional flow reactor techniques adapted for neutral clusters. To reduce the loss of clusters to the walls, the flow tube dimensions are based on those established for studying other species with high sticking coefficients (molecular ions [151, cluster ions [161, and atoms [171). A large tube diameter to source-to-detection length ratio and a high buffer gas flow rate ensure that the cluster diffusion time to the walls is at least comparable to the transit time down the tube. At a buffer gas flow rate of 190 torr 1 s-l (15000 sccm) the minimum tube pressure attainable by the pumping system is 0.4 torr. An XeCl excimer laser vaporization source is used to produce and inject copper dimers into the flow tube. It uses a rotatingltranslating Cu metal target rod over which a continuous flow of inert buffer gas (He) is passed at ca. 50 torr in a 2 mm diameter channel. A 40 mm long clustering tube extends this channel, before the gas and entrained clusters expand mildly into the reactor tube which is at 0.2 to 6 torr. The copper dimer is monitored 1.3 m downstream by laser induced fluorescence (LIF) using an excimer pumped pulsed dye laser tuned to the dimer’s B-X(0-0) transition. LIF excitation spectra of the B-X and C-X bands in the region of 460 nm showed that Cu2 was vibrationally thermalized at even the lowest operating pressure of the reactor. Reagent gas is introduced through a multi-hole ring inlet, 0.7 m from the detection region, at flow rates up to 2.5 torr 1 s-l. The partial pressure of reagent gas was calculated from the total pressure and the mass flow rates of the reagent and buffer gas.

Helium (HP, 99.995%) and argon (HP, 99.998%) were obtained from Air Products. He was further purified by passing through a liquid N2 cooled molecular sieve trap. Reagent gases were obtained from Matheson and used directly from the cylinder without further purification. Purities were: CO (CP grade, 99.5%); NH3 (Anhydrous, 99.99%); C2H4 (Polymer grade, 99.9%); C3Hs (CP grade, 99.0%); Hz (Prepurified, 99.99%); and CH4 (CP grade, 99.0%). Laboratory air was used as the source for 0 2

and N2. Procedures used to obtain absolute rate constants with this apparatus have been

proven in a study of Al atom reactions [141. An important feature of the instrument is that it uses pulsed cluster injection and pulsed detection. This means the speed of the detected clusters is determined unambiguously by the delay between two laser pulses, removing the need to be concerned with details of the flow hydrodynamics. In this regard, the time spent in the clustering tube, ca. 30 ,us, is negligible com- pared to the time in the reactor which is 5.5 ms or more, depending on the tube pressure. The reagent contact time, 7, for detectable molecules is obtained trivially from their speed and the distance of the reagent inlet from the detection point. Experiments on the reaction of Al with 02, varying both the reagent pressure and the reagent inlet position, showed this approach to be effective and that mixing effects are not important, as is to be expected for a reactor of our dimensions 1141.

Cu2 Reactivity

Reactions of Cu2 in the gas phase at room temperature proved to fall into one of two categories. On one hand, 0 2 , NzO, NZ, H2, and CH4 showed no reactivity to Cu2 at pressures up to 6 torr, implying bimolecular rate constants of less than 5 X cm3 s-l at 6 torr He and 295 K. On the other hand, CO, NH3, C2H4, and C3H6 reacted, all showing behavior characteristic of association reactions. Bimolecular rate constants for these reactions were obtained over the total pressure range 0.4 to 6 torr from the dependence of the relative Cu2 concentration at the detection point, [Cuzl, on the

88 LIAN ET AL.

reagent partial pressure, p(L). Under pseudo-first-order conditions [Cuzl is given by

(1) In([Cu2l/[Cu2lo) = - I Z ( ~ ) ~ ( L ) T

where CCu210 is [Cuzl in the absence of added reagent, L, k(2) is the effective second- order rate constant for the reaction, p(L) is the partial pressure of the reagent and 7 is the reagent contact time. A plot of ln([Cu~l/[Cu~l~) against p(L) is expected to be linear, passing through zero with a slope of - k ( 2 ) ~ . In addition, association reactions are expected to show a dependence of on buffer gas pressure, p(Q), which, in the simplest approximation, can be deduced from the Lindemann mechanism,

k lP(L) +

cu2 + L [Cu2L]*kz2Q) CU2L 9

k-1

as

The reaction of CUZ with C2H4 has already been shown to fit well with this mechanism [131. Figure 1 shows plots based on eq. (1) for CO as the reagent gas. The fit to first- order kinetics is apparent as is the fact that k(') depends on p(Q) . These first-order plots are typical of those found for NH3 and C3H6, as well as those published for C2H4 U31. In the low pressure limit, i.e., when k-1 >> &I(&), eq. (2) reduces to

(3) k

k-1 k(') = -'k2p(Q) = k(3)p(Q)

where k(3) is the third-order limiting low pressure rate constant for the reaction. Like CzH4, both CO and NHa show a linear dependence of k(') on p(Q), consistent with eq. (31, over the accessible pressure range up to 6 torr. This behavior is typified by the results for NH3 shown in Figure 2. Table I summarizes the third-order rate constants

I I I I

0 1 2 3 4 5 6

p ( ~ ~ ) / cm-3 Figure 1. the flow reactor at 2 (+), 5 (W), and 7 (0) torr of He buffer gas.

Cu2 depletion as a function of the pressure of carbon monoxide, p(CO), in

REACTIONS OF C U ~ IN GAS PHASE

2.0 1 I /

89

0.0 0 2 4 6 8

p(He)/ Tom

Figure 2. reaction of Cuz with NH3.

Buffer gas pressure dependence of the second-order rate constant for the

found from the slopes of such plots using He and Ar as buffer gases. The rate constants are higher in Ar, as expected for a more efficient collisional relaxation partner.

It is a feature of association reactions that, as the reaction partners become larger, the high pressure limit is reached at lower pressures. This is due to the increased number of internal modes of the collision complex which increases its density of states and results in a lowering of k-1, allowing collisional deactivation to compete more readily with spontaneous dissociation back to products. For the reaction of Cu2 with alkenes, substitution of one hydrogen atom by a methyl group is sufficient to make this observable. Figure 3 shows the pressure dependence of k(') for the reaction of C3Hs with CUZ. At He pressures greater than 3 torr k(2) begins to show fall-off behavior. Our data does not extend to high enough pressures to extrapolate to the high pressure limiting rate constant with any confidence. From the slope at low pressures we obtain the value for k(3) given in Table I.

I 1

I I 0.0 0 2 4 6

p(He)/ TOIT

Figure 3. Buffer gas pressure dependence of the second-order rate constant for the reaction of cuz with C3Hs.

90 LIAN ET AL.

TABLE I. in He and Ar at 297 K.

Limiting low-pressure, third-order rate constants for association reactions of the copper dimer

k(3)/10-30 06 3-1

He Ar

co 0.66 2 0.10 0.84 2 0.15 NH3 8.8 ? 1.2 11.5 ? 2.0 CZH4 9.3 ? 1.4 11.4 ? 2.0 C3H6 85 2 15

Binding Energies for Cuz Association Complexes

An association reaction of the type described above is the reverse of the unimolecular dissociation reaction of the complex. The kinetics of the two reactions are linked through the equilibrium constant, K, by

(4)

With K available through statistical mechanics, modeling of Kuni, the rate constant for unimolecular dissociation of the complex, using unimolecular reaction theory presents a route to predicting over the entire pressure range. Troe has discussed the predictive possibilities of unimolecular rate theory in this respect and has presented an analytical factorization of the rate constant kuni in the low pressure limit,

K = k"/kUni = k ( 3 ) p ( Q ) / k , , i , ~ , in the low pressure limit.

which reveals how this quantity, and therefore k(3) , contains information on the complex binding energy, Eo [191. The product PZLJ is a rate constant for collisional stabilization, Pvib(E0) is the vibrational density of states of the association complex at the dissociation limit, Eo, and Q u , c ~ is the vibrational partition function of the complex. The factor, F, is a product of several correction factors for which prescriptions are given by Troe 1191. From this relationship, the Whitten-Rabinovitch semi-classical approximation to Pvib(E0) and the relationship for K from statistical mechanics, we obtain

F . (Eo + aEJS-' Qe,CL Q ~ , c L Q ~ , c L

Qe,CQe,L Q t , c Q t , ~ Q r , c Q r , ~ Q u , c Q u , ~ (6) k(3) = PZLJ k B T

T(s) fi hui i = l

where s is the number of vibrational modes in the complex, a is a factor between 0 and 1 given by the prescription of Whitten and Rabinovitch [191, E, is the zero-point energy of the complex and QCL, Qc, and QL are partition functions of the complex, isolated cluster and ligand, respectively, with the subscripts e , t , u, and r referring to the electronic, translational, vibrational, and rotational degrees of freedom. The dependence of k(3) on Eo is clearly shown by this expression. The other unknown parameters are the vibrational frequencies of the complex, ui, and the rotational partition function of the complex which, in turn, depends on the geometry of the complex. If these molecular properties can be accessed in some way, then modeling either through the above expression or more rigorously using "exact" numerical RRKM calculations, will lead to Eo. Ideally one would like to have access to spectroscopically determined frequencies and structures. Cryogenic matrix isolation experiments have

REACTIONS OF CUZ IN GAS PHASE 91

the potential to supply such information but there are no reports for the 1:l dimer- ligand complexes of the reagent molecules studied here. In the gas phase, the direct spectroscopic observation of M, L complexes remains an unanswered challenge from n = 1 and up. Alternatively one could rely on molecular structure calculations. In this regard, we have begun a study of Cu2- CO, - NH3, and - C2H4 complexes using density functional theory but the results are, as yet, unavailable.

Without accurate frequencies and geometries, we rely on limiting models to estimate the strength of Cu2-L interactions. For each complex, we have taken the vibrational frequencies, vi, as the frequencies of the reactants, available from the literature, plus one four or five-fold degenerate vibration, v,, to represent the four (in the case of a diatomic ligand forming a nonlinear complex) or five (in the case of a diatomic ligand forming a linear complex and in the case of any nonlinear ligand) additional internal degrees of freedom in the complex. For two geometries in each case, Eo was found as a function of v, by fitting to the experimental value of k(3) using eq. (6). The representative geometries are given in Figure 4 and the results are shown in Figure 5. It is clear that Eo depends strongly on the choice of v, in all cases and also strongly on the geometry in the cases of CO and NH3. A reasonable estimate for v, is between 200 and 300 cm-'. The geometric mean for the additional vibrational modes in related Cu atom complexes is in this range [201. Preliminary results of our molecular structure calculations also support this estimate. On this basis we have argued that our results show that Cuz(C2H4) is bound by at least 20 kcal mol-l;

cu-cll---- c=o

I H

CHZ

C b II cu - cu ----

FH2 FH

cu - cu ----

CH3

End-on, Type I Geometries Side-on, Type I1 Geometries

Figure 4. Trial geometries used to estimate CuzL binding energies from third-order rate constants.

92

7% -

LIAN ET AL.

c2H4

further refinement awaits molecular structure calculations [131. Similar arguments can be applied to CO and NH3 complexes although the lower limit has to be set at 15 kcal mol-' to account for the possibility of side-on bonding combined with extraordinarily soft complex frequencies. The results for CO and NH3 demonstrate the inadequacies of this "blind" approach to modeling binding energies which is arguably at its worse in the case of a dimer interacting with a diatomic molecule because of the large difference in rotational partition functions between the extremes of linear and T-shaped structures. Preliminary results from our molecular structure calculations show that both Cu2CO and Cu2NH3 have linear end-on structures which implies that they are much more strongly bound and allows us to raise our estimated lower limit to a value of 25 kcal mol-'.

To at least a first-order7 we expect ethylene and propene to interact in the same manner with Cu2 and therefore to have similar binding energies. This is, for example, the case found for Ni atoms interacting with C2H4 and C3Hs [21]. The order of magnitude difference in k(3) for the reactions of the two alkenes with Cu2 must therefore be largely associated with the additional internal modes in the C3Hs complex. If the above approach to estimating Eo is at all consistent we expect the results for the two alkenes to return similar results for Eo, and its dependence on vx, despite the large difference in rates. As seen in Figure 6 this is indeed the case. From this result we can also put the same limit of > 20 kcal mol-l on the C U ~ ( C ~ H ~ ) binding energy as found for the C2H4 complex.

Reactivity of Cu2 in Comparison with Cu Atoms, Larger Clusters, and Surfaces

In Table I1 we compare the reactivity of Cu2 with that of Cu atoms in the gas- phase at room temperature. Several differences are apparent. The drop in reactivity with 0 2 an going from the atom to the dimer is consistent with what is known about 0 2 reactions with atoms of the 3-d transition metal series. Only atoms with a d"s' configuration form association complexes with 02. This is understood in

REACTIONS OF C U ~ IN GAS PHASE 93

0 100 200 300 400 500

v,/ cm-'

Figure 6. Sensitivity of the binding energy, Eo, calculated from K(3), to the assumed vibrational frequency, vz, for Cuz association complexes of C3H6( -) and CzH4 ( - - - -). Type I geometry was assumed in both cases.

terms of simple molecular orbital considerations concerning the initial interactive potential which is attractive in the case of a singly occupied s orbital overlapping the singly occupied in-plane n-*-antibonding orbital of 0 2 [221. For atoms with dns2 configurations, the second s electron must occupy an antibonding molecular orbital between the metal atom and 0 2 leading, to a less attractive or repulsive potential. Cu2 is a closed shell molecule with a (3d10) (3d10) ( 4 s ( ~ ~ ) ~ configuration 121 and consequently has no singly occupied orbital available for this radical-radical combination type of interaction. The inertness of larger closed-shell Cu clusters to 0 2 has been interpreted through an electron transfer mechanism forming 0, which

TABLE 11. Reactivity of Cuz at room temperature in the gas phase at room temperature compared to Cu atoms.

Reagent cu; c u

0 2 No reactionb k(3) = 2 x 10-31 cm3 s - l c

NzO No reactionb k(2) < 10-18 cm3 s-ld co Fast association reaction Weakly bound, reacts slowly

CZH4 Fast association reaction Weakly bound, reacts slowly

C3H6 Very fast association reaction

and adds a second COe

and adds a second C Z H ~ ~

in the fall-off region above 4 tom He

NH3 Fast association reaction Hz No reactionb CH4 No reactionb Nz No reactionb

a This work. k(2) < 5 X

Reference [221. Reference [231. Reference [201.

cm3 s-l (at 6 torr He, 297 K).

94 LIAN ET AL.

lowers the barrier to 0-0 bond breaking and leads to dissociative chemisorption and even bulk oxide formation [lo]. Such electron transfer is taken to be less likely for the closed-shell clusters with their associated high ionization potentials. It is also possible that, as in the atom, the interactive potential does not favor the initial approach of 0 2 when there is no singly occupied orbital available. In either case, our result for Cu2 emphasizes the importance of an available unpaired electron to the interaction of copper with molecular oxygen from the atom through to large clusters.

In contrast to earlier transition metal clusters, where size-selective, dissociative chemisorption is observed, all copper clusters are reported not to react with hydrogen [9]. This is explained as a consequence of the 3d band being filled and lying below the Fermi level. Nz chemisorption follows the same patterns. Our results for CUZ confirm that the inertness extends to the smallest Cu clusters and that kinetic effects do not mask the reactivity.

In its association reactions CUZ shows stronger interactions than the atom. The atom complexes Cu(C0) and Cu(CzH4) are both bound by only ca. 6 kcal mol-1 [201. In both cases this has been attributed to cT-repulsion between the s orbital of the Cu and the donor orbital of the ligand. An appreciable hybridization energy must be spent to polarize the s electron away from the incoming ligand. In the dimer, with its closed shell configuration, the s electrons are prelocalized in the 4sv, bonding orbital. Also the dimer structure might present an opportunity for polarization at a lower cost in promotional energy. The result is a much stronger bond in both dimer complexes. Bonding interactions in the Cuz(C0) and C U ~ ( C ~ H ~ ) complexes can involve both u-donation to the dimer c ~ * or 3p orbitals and .rr-backdonation from the dimer 3d orbitals. The result for Cuz(NH3) shows that .rr-backdonation is not necessary for the formation of a relatively strongly bound complex. Bagus, Hermann and Bauschlicher have made an ab initio study of lone pair ligands which includes Cu(NH3) and Cug(NH3) [51. In contrast to CO they find no significant Cu to ligand .rr-donation, as expected considering the energy of the lowest unoccupied .rr orbital in NH3. They also find an attractive electrostatic interaction due to the large N-H+ dipole moment which accounts for most of the bond strength in C U ~ ( N H ~ ) and Cu(NH3). There is no reason to suppose that such a mechanism does not make a significant contribution to the bonding in Cu2(NH3).

Of the three ligands which form association complexes, only the reaction of CO has been studied with larger Cu clusters. Kaldor and co-workers have demonstrated that, unlike 0 2 , CO demonstrates no striking size-selective reactivity patterns with Cu, (n 2 4) [ill. Indeed, this is the case for CO reactions with a range of transition metal clusters. Our finding, that CO is relatively strongly bound to Cu2, is consistent with kinetic effects masking the reactivity of the smaller clusters in earlier experiments. If we take our third-order rate constant and Kaldor and co-workers’ estimated experimental conditions, only assuming a reactor temperature of ca. 300 K at which our measurement was made rather than 400 K, we obtain a reactivity, defined by them as = -ln([Cu~l/[Cu~l~), of 0.06 which agrees with their experimental value of 0.1 +- 0.2. Nygren and Siegbahn, in their recent ab initio study of Cu,-CO systems, find appreciably stronger binding (ca. 23 kcal mol-I), for n = 3, 4, and 6 when compared to larger clusters and to the experimental value for bulk surfaces [81. Unfortunately, their theoretical results do not overlap with our experimental results for n = 2 but both are consistent with the conclusion that small Cu clusters bind CO significantly more strongly than both the isolated atom and extended systems.

REACTIONS OF C U ~ IN GAS PHASE 95

TABLE 111. Binding energies of small ligands at Cu atoms, dimers, and surfaces.

Binding energy/kcal mol-l Ligand CUa CU; Cu surfaces

co ca. 6 >25 11-20c C2H4 ca. 6 >20 Md NH3 >25 10, 14e C3H6 >20

a Reference [201.

CReference [24]. The range covers measurements on single crystal (11-16 kcal mol-') and polycrysta- This work.

line (18-20 kcal mol-l) Cu surfaces. Reference [25]. Polycrystaline Cu surface. Reference [261. Cu(100) single crystal surface.

We have already remarked that CzH4 is significantly more strongly bound in the dimer complex, CUZ(C~H~) , than to the atom or to Cu surfaces 1131. Table I11 compares binding energies for CO, CzH4, and NH3 to Cu atoms, dimers, and surfaces. The pattern found for CzH4 is repeated for the other ligands. Although not observed in related experiments on larger Cu clusters [71, size-effects are clearly important in determining the bonding in small copper clusters. Clusters as small as Cu5(CO) and Cug(NH3) have been proposed as models for the CO- and NH3-Cu (100) surface interactions [5,61. It will clearly be important to extend our measurements to the trimer, tetramer, and pentamer to test this proposal and develop our understanding of bond formation in these systems.

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Received March 11, 1993 Accepted May 24, 1993