The adsorption of oxygen on copper surfaces

121

Surface Science 118 (1982) 121-135 North-Holland Publishing Company

THE ADSORPTION OF OXYGEN ON COPPER SURFACES 1. ~~~1~) and CufllO) A. SPITZER

and H. LUTH

2. Physikalisches Institut der Rheinisch- Westfiilischen Aachen, Fed. Rep. of Germany Received

7 December

1981; accepted

for publication

Technisehen

24 February

Hochschule

Aachen,

D-5100

1982

Oxygen adsorption on Cu(lOO) and Cufl IO) is studied by means of UPS, ELS, LEED and work function measurements. At room temperature strongly chemisorbed atomic oxygen is identified, whereas in the temperature range between 100 and 300 K two other oxygen species are inferred additionally: a chemisorbed molecular 0, and a less strongly bonded 0 atom. Based on the spin multiplet structure of oxygen, the molecular adsorbate is interpreted in terms of chemisorbed singlet 0, (‘Xp’ ).

1. Introduction For several problems in catalysis research, the question of the oxygen interaction with Cu surfaces is of considerable interest. Accordingly a number of studies including LEED, UPS, XPS, inelastic electron scattering (ELS, HRELS), work function measurements and ion scattering deal with the adsorption of oxygen on Cu, in particular, in the range above room temperature [l-25]. Only little work has been published about oxygen adsorption on Cu at temperatures below 300 K [2, 26-281. On many metal surfaces oxygen has been observed to adsorb dissociatively or in an oxide-type configuration at and above room temperature. Molecular adsorption has been found only in rare cases: Besides a true physiso~tion-type bonding on Al and W at temperatures around 30 K 129-311, chemisorbed molecular 0, species have been detected by a variety of methods on Pt( 11 l), polycrystalline Pt, Ag(ll0) and Ag( 111) at temperatures between 110 and 150 K 132-371. As far as UPS and XPS studies are concerned, the analysis of data seems to be obvious for the cases of strong atomic chemisorption and for pure physisorption. The chemisorbed atomic oxygen is characterized by one emission band at about 5-7 eV below the Fermi level, whereas the photoemission spectrum of physisorbed 0, resembles that of the free molecule 129, 301. Oxygen on Cu surfaces is an interesting adsorbate system, since particularly 0039-6028/82/0000-0000/$02.75

0 1982 North-Holland

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at lower temperatures adsorption stages with adsorption bond strength in between strong chemisorption and physisorption might occur. The present paper shows that such adsorption phases do exist, and an attempt is made to interpret the observed UPS data in terms of different spin multiplet structures of the adsorbed oxygen species.

2. Experimental As experimental techniques, UV photoemission spectroscopy (UPS), electron energy loss spectroscopy (ELS, primary energy 50
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For the low temperature measurements with sample at 100 K, in particular with high exposures, the oxygen inlet is made with a cryopanel held at liquid nitrogen temperature. The ion pump is switched off, in order to suppress or at least to minimize exchange reactions, especially the production of CO. Then the system is pumped by the turbomolecular pump. During oxygen inlet the sample is not irradiated by electrons or UV light. The dosing procedure is checked by mass spectroscopy.

3. Results The results obtained after room temperature exposures to oxygen resemble those of other authors [ 1-4, 8, 19, 201 and shall therefore be discussed only briefly. Both on Cu(100) and Cu(ll0) in UPS, an oxygen characteristic emission band at about 6 eV binding energy (BE) and additionally a structure above the Cu d-bands near 2 eV BE is found, similarly to the 300 K spectra in figs. 1 and 2 (right part). With increasing oxygen coverage on both surfaces the work function increases and reaches saturation changes A+ = 0.28 eV near 500 L and A+ = 0.26 eV near 10 L on Cu(100) and Cu( 1 lo), respectively (fig. 4). On Cu(100) a (OX 0) R45’ superstructure with an additional “four-spot” structure is observed in LEED after a 1000 L exposure, whereas on Cu(ll0) a (2 X 1) superstructure occurs for dosages higher than 0.4 L, first streaky and faintly pronounced. and above 5 L as sharp as the substrate spots. In ELS room temperature exposure to oxygen gives rise to one characteristic new loss at 9.3 eV on Cu( 110). Low temperature results for Cu( 100) are shown in fig. 1. At 100 K in UPS up to 100 L a spectrum develops which is characterized by one peak at 5.8 eV BE and a shoulder above the Cu d-band at 1.6 eV BE. A detailed analysis by means of difference spectra also reveals a little shoulder near 9 eV. For higher coverages up to 1000 L, a “three-peak” spectrum with emissions at 5.8, 9.4 and 12.5 eV is observed; the shoulder near 2 eV on top of the d-band also remains. The energetic position of the structures are obtained from difference curves. Below 100 L in ELS no new oxygen characteristic losses can be resolved, only the surface plasmon-like loss [38] decreases in intensity and slightly shifts from 7.3 to 7 eV, thus indicating an adsorbate coverage. For exposures higher than 100 L, the 7 eV loss shifts further to 6 eV and new losses emerge at 10 and 13.6 eV, the latter having a little shoulder on the low energy side (fig. 3). The work function change A+ clearly shows the different adsorption phases. Up to 100 L an increase up to 0.43 eV is followed by a decrease down to 0.13 eV (fig. 4). The exact A$ values reached are dependent on the time of measurement, i.e. the changes are not stable under the conditions of measurement. Up to 100 L a weak (fi X 6) R45’ superstructure occurs, which looses its contrast at higher exposures, and at 1000 L it has vanished, thus leaving a substrate pattern very poor in contrast.

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hv:408eV ! 1

i

BINDING

ENERGY

&VI

Fig. 1. UPS spectra (energy distribution curves. EDC”s) of a clean Cu(IOOf surface and after several exposures to oxygen at 100 K (left part). UPS spectra after a I000 L exposure at 100 K measured at sever& annealing temperatures (right part). The zero level of each spectrum is marked

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Fig. 2. UP5 spectra of a clean and oxygen covered Cu( 1 IO) surface at 100 R (left part); spectra measured at sever4 anneafing temperatures after a 1000 L exposure at 100 K (right part). The spectra are referred ta different zero levels marked on the ordinate.

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1 Cu(lOO)OXYGEN

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0

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Fig. 3. Electron energy loss spectra of a clean Cu(100) surface and after exposures to oxygen (left part); E,, is the primary energy; the factor indicates the amplification for each spectrum with respect to the elastic beam. ELS spectra after a 1000 L exposure at 100 K measured at different annealing temperatures (right part).

Upon annealing the sample from lOOK, the “three-peak” UPS spectrum (after 1000 L), remains with slightly decreasing intensity up to about 145 K. Between 145 and 210 K, a “two-peak” spectrum is observed with emissions at 5.8 and 9.1 eV. The 9.1 eV peak gradually looses intensity with increasing temperature and finally near room temperature only the “one-peak” spectrum (peak at 5.8 eV BE) with an additional shoulder near 2 eV BE is observed. This spectrum is already known from room temperature exposures. In ELS, simultaneously with the “three-peak” UPS spectrum, the 13.6 eV loss decreases in intensity (fig. 3). In the temperature range 145-210 K (in UPS: “two-peak” spectrum) only one oxygen loss is scarcely seen around 10 eV; above 240 K this loss clearly develops but decreases slightly in intensity around 300 K. Up to 210 K the surface plasmon-like loss shifts from 6 to 6.3 eV, where it stays up to 300 K. The value of the work function after exposure at 100 K changes with time: within about 10 min, e.g., a decrease of 0.1 eV occurs. Upon annealing the work function change A+ shows a monotonous behaviour in the whole temperature range, increasing from its negative value at 100 K to a saturation A+ = 0.23 eV near 260 K (fig. 5). In LEED a first very weak (a X 0) R45” superstructure occurs around 185 K, which develops around 240 K into a sharp

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Fig. 4. Change of work function versus oxygen exposure of a Cu( 100) surface (upper part) and of a Cu( 110) surface (lower part). Open circles indicate results for a substrate temperature of 100 K and triangles those for 300 K.

(a X \/z) R45” pattern. Up to 300 K this pattern changes into a “four-spot” structure. Exposure of a Cu( 100) surface at 100 K to 100 L oxygen only, instead of 1000 L, and a subsequent annealing to 300 K, yield the same results in AES, LEED and UPS at room temperature. The results on Cu(ll0) at 100 K (fig. 2) are similar to those on Cu( 100) as far as the spectral information is concerned, only the kinetics of the adsorption process are much “faster”. In UPS one peak at 6.2 eV with a shoulder near 9 eV (from difference curves) develops up to a 0.8 L exposure. In ELS the 9.3 eV loss occurs simultaneously. A “two-peak” UPS spectrum is observed up to 50 L. Between 100 and 1000 L, UPS shows the characteristic “three-peak” spectrum (emissions at 6,9.5 and 12.7 eV) and in ELS the corresponding two losses at 9.3 and 13.5 eV (shoulder near 12.3 eV) appear. The surface plasmon-like loss shifts from 7.1 eV on the clean surface to finally 5.8 eV similarly to Cu(100). The work function change shows a corresponding behaviour, it reaches a maximum of about 0.3 eV after 1 L exposure and then

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Cu (lOO)-OXYGEN(lOOOL)

100

300

200 TEMPERATURE

Fig. 5. Change of work function versus part) and on Cu(l10) (lower part).

fill temperature

after a 1000 L exposure

on Cu(100)

(upper

slightly decreases between 5 and 50 L (fig.4). Between 100 and 1OOOL a further decrease of A+ down to = 0.13 eV follows. The work function, however, is not stable; A+ measurements at later times show a further decrease similar to the behaviour on Cu( 100). The different curve branches for Cu( 110) at 100 K (fig. 4) result from different runs, i.e. the break between 10 and 100 L is caused by the time instability of A+. Within the whole exposure range no LEED s~perst~cture occurs, only the substrate pattern looses contrast with increasing oxygen doses. Annealing of the Cu( 110) surface after exposure to 1000 L oxygen at 100 K yields the three types of oxygen characteristic UPS spectra in different temperature ranges: the “three-peak” spectrum between 100 and 180 K, the “twopeak” spectrum between 180 and 250 K, and finally the “one-peak” spectrum up to 300 K. Like on Cu( lOO), the “one- and two-peak” spectra are accompanied in ELS by the characteristic loss at 9.3 eV, whereas the “three-peak“ UPS data correspond to the losses at 9.3 and 13.5 eV (with shoulder). For temperatures between 100 and 150K the loss spectrum is unchanged, and with increasing temperature the 13.5 eV loss decreases in intensity. These spectral changes are correlated with the dependence of the work function changes on temperature {fig. 5). With increasing temperature A# decreases from its initial

value 0.1 eV reached after 1000 L at 100 K; a minimum follows at 150 K where the “three-peak” UPS spectrum has reached its maximum intensity, and then up to 300 K, A$ gradually increases and saturates at a value of = 0.32 eV. In LEED for temperatures up to 235 K no superstructure is seen, and the substrate spots are poor in contrast: around 240 K very weak additional spots appear, which finally form a c(6 X 2) superstructure between 250 and 300 K. The formation of this c(6 X 2) superstructure requires an initial exposure at 100 K exceeding 10 L; for a 1 L oxygen exposure, e.g., a (2 X 1) superstructure starts to form for temperatures higher than 195 K. The initial oxygen exposure at 100 K influences also the detailed shape of the Arp versus temperature curves: the higher the initial dose, the lower the temperature of the work function minimum. For initial exposures of 10, 100 and lOOOL, minimum values A&+ of 0.16, 0.13 and 0.0 eV are reached at 190, 170 and 150 K, respectively. In experiments like the present ones, where exposures higher than 100 L are used in an ion pumped stainless steel UHV system, exchange reactions in the pumps and on the walls might interfere with the surface reactions being studied. In the present case, in particular H,O and CO had to be taken into account as severe contaminants. Careful investigations therefore were made to rule out an effect of those gases on the oxygen adsorption studies, especially, at low temperature. Fig. 6 shows UPS difference curves obtained from a series of experiments, in which the coadsorption of oxygen with H,O and with CO,

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Fig. 6. He II photoemission difference spectra of a predosed (10 L oxygen at 100 K) surface with an additional dosage of (a) 990 L oxygen, (b) 0.8 L H,O, and (c) 0.5 L CO. The difference curves (scale enlarged by a factor of 2) are calculated with respect to the spectra of the predosed surface. Binding energies are referred to the Fermi energy.

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respectively, is studied at 100 K on Cu( 110). The procedure for obtaining the difference spectra in fig. 6 consists of predosing the clean Cu surface at 100 K by 10 L 0,, then exposing to 990 L O,, to 0.8 L H,O or to 0.5 L CO. The difference curves are calculated by subtracting the UPS spectra of the predosed surface from the final data. For the comparison, spectra after a 10 L predose are subtracted, since a 1000 L 0, exposure at 100 K yields, after annealing to 300 K, the same oxygen coverage (based on AES) like a 10 L dose. The “three-peak” spectrum due to O,, therefore, appears to be superimposed on the 10 L data. From fig. 6 it is spectrum (a) cannot be obtained by an concluded that the “three-peak” additional I-I,0 or CO adsorbate layer. Those gases are therefore ruled out as contaminants in the present oxygen adsorption studies. There is further support for this conclusion; upon annealing adsorbed molecular O,, H,O and CO are no longer detected in UPS at different temperatures: 180, 155 and 190 K, respectively. Also the work function changes upon annealing an H,O or CO covered surface from 100 to 300 K are not identical with those in fig. 5.

4. Interpretation and discussion Preferentially UPS and ELS data of the different oxygen adsorbates are used to identify the particular species. After roopn temperature adsorption the adsorbed oxygen species is characterized by one characteristic emission near 6 eV BE (“one-peak” spectrum) and an additional shoulder (or peak) near 2 eV BE on top of the Cu d-band. The 6 eV emission is a characteristic pattern for strongly chemisorbed atomic oxygen, well known on other transition metal surfaces, too 127, 39-411. Therefore, the 6 eV line is interpreted as due to 012~). The structure near 2 eV has already been observed by Ling et al. [S] and has been attributed to an occupied antibonding orbital, which results from the Cu(d)-O(2p) interaction. This interpretation is supported by calculations of Liebsch [42]. In spite of a careful study by means of angular resolved UPS a splitting of the O(2p) emission line due to a different bonding symmetry of the 2p orbitals has not been observed 181. The “one-peak” photoemission spectrum is accompanied in ELS by one characteristic loss near 10 eV, which is ascribed to an electronic transition of oxygen in its atomic form, as is derived from gas phase data [43]. The observed increase of the work function corresponds to a negatively charged 0 atom, which is in agreement with observations on other metal surfaces [44-461. The results on Cu( 100) and Cu( 110) at 300 K are consistent with the genera1 trends being observed in adsorption of oxygen on other transition metals. In contrast, no other UPS results on the gas phase or on other metal-oxygen systems can be used as a “finger print” for the interpretation of the “three-peak” spectrum. This spectrum is obviously not due to atomic oxygen. There are too many emission bands and also the work function decrease is hard to under-

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stand in terms of adsorbed oxygen atoms. A pure physisorbed 0, species can also be ruled out, since at least four emission bands resembling in energy position the five bands of gas phase 0, would be expected [47-491. The following new interpretation of this “three-peak” UPS spectrum is based on the spin multiplet structure of 0,. Spin multiplicity is already responsible for the five emission bands of gaseous triplet 0, (32). Emission of an electron, e.g. from the ?r,(2p) level (fig. 7) can leave the 0, ion in two energetically different final states, a state with one unpaired spin or a state with three unpaired spins. Therefore, both the m,(2p) and the us(2p) orbitals give rise, each, to two emission bands and the rs(2p) orbital to one band in UPS. The corresponding five final states of the 0, ion are X211,, a411,, A2111,, b4Z;, B2Z, [49]. According to fig. 7 three emission bands of adsorbed oxygen can easily be explained by 0, in its singlet state (‘2: ). Emission of an electron from rs(2p), 7rJ2p) or us(2p) of the singlet always leads to a final state with one unpaired spin and only three photoemission bands result. Since to our knowledge there exists no complete experimental spectrum of gas phase singlet O,, theoretical considerations are necessary to support the interpretation of the “three-peak” spectrum in terms of chemisorbed singlet 0,. In a rough estimation, therefore, the ionization energies for singlet 0, have been deduced from those of triplet 0, in the following manner. Emission of an electron from 7rJ2p) or from us(2p) (fig. 7) results in two final states of the triplet 0, ion which are different in energy because of the interaction of the remaining unpaired spins in 7rJ2p) or us(2p) with those in 57&2p). The final state with higher multiplicity has a lower energy than the final state with lower multiplicity. Because of spin compensation in 7r&2p), this spin interaction does not exist in singlet 0,. To approximate the singlet levels, we assume the spin

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1

I

Fig. 7. Electron spin scheme of the oxygen molecule and of the oxygen atom, each in their triplet and singlet configuration. The number on the right side of the levels indicate the numbers of photoemission bands expected due to spin multiplicity. In the last row the total number of photoemission bands originating from the plotted molecular or atomic orbitals is given.

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interaction between Q2p) and the other orbitals of the triplet to be “switched off’. Emission of an electron from 7rJ2p) or us(2p) now would give rise to one final state only (as for the singlet), whose energy level would be located energetically between the final states of the triplet, namely between the quartet and the doublet terms. This is true, because parallel spins cause an energy increase and antiparallel spins a decrease as compared with the situation of “switched off” spin interaction. Moreover the fictitious molecule with “switched off’ spin interaction is a triplet, since the parallel spins in na(2p) are parallel. In order to simulate the singlet, one has to take into account the energy which is necessary for a spin flip in the 7rs(2p) orbital. From ELS results on the gas phase molecule this energy is known to be about 1.6 eV [50]. The term scheme being obtained by “switching off’ the spin interaction between 7rs(2p) and the other orbitals and by taking into account the energy for a spin flip in 7rp(2p),is now identified with that of singlet 0,. Assuming the doublet final states of the singlet molecule to be half-way between the doublets and the quartets which arise from excitation of triplet Oz, the following approximate ionization energies are obtained for singlet OZ (‘2,‘): X2111,, 10.5 eV; A211,, 14.9 eV; and B2Zl, 17.6 eV. A comparison with the emission lines of the observed “three-peak” spectrum of OZ is possible when the corresponding energies are related to the vacuum level, to yield the ionization energies: 10.2, 13.8 and 16.9 eV for 0, on Cu(100) and 10.8, 14.3 and 17.5 eV for OZ on Cu(ll0). With the assumption of a uniform relaxation/pol~zation shift [Sl] of about 0.5 eV, the two high BE levels A211, and B2Zl fit the experimental bands within 0.5 eV. Then the low BE orbital of the adsorbed species has to undergo a binding shift of about 0.5 eV to obtain agreement between theory and experiment. Taking into account the uncertainties of the theoretical estimation, the agreement seems satisfying and might be taken as a support for the interpretation of the “three-peak” UPS spectrum as being due to singlet OZ adsorbed on Cu( 100) and Cu( 110). The work function decrease (fig. 4) accompan~g the “three-peak” spectrum is consistent with molecular adsorption. The decrease in A# suggests at least a neutral O,.spccies with dipole moment directed from the vacuum to the Cu surface. The same sign of A+ is observed for physisorbed 0, on W( 110) [31]. Furthermore, in XPS on Cu(100) the 0( 1s) level for high O2 exposure has been found at 533.6 eV rather than near 530 eV [28]. This might also be taken as a hint for a molecular OZ species. It should be emphasized that in the presence of the “three-peak” UPS spectrum too the shoulder near 2 eV BE is observed, which is ascribed to an antibonding Cu(d)-O(2p) orbital. This fact, and the change of the relative intensities of the three photoemission bands with coverage and with temperature, suggest that, additionally to the molecular 0, singlet species, atomic 0 is chemisorbed. This might explain why both with the “one-peak” and with the “three-peak” photoemission spectrum in ELS a loss near 9.5 eV is observed. The additional loss near 13.5 eV only occurring with the “three-peak” spec-

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trum might be related to a transition near 14 eV which is known for gaseous singlet 0, f43,52]. Upon annealing to 145 K on Cu( 100) and to 180 K on Cu( 1 lo), the “three-peak” UPS curves change into “two-peak” spectra with bands at 5.8 and 9.1 eV on Cu( 100) and at 6.1 and 9.1 eV on Cu( 1 lo), respectively. The emission near 2eV BE remains, thus indicating at least a cont~bution of strongly chemisorbed 0 atoms. The relative intensities obtained from difference curves, however, indicate that in addition a species is present which must be characterized by emission bands near 6 and 9 eV. Since in ELS only one loss near 9eV is observed, and since the work function increases with respect to the molecular adsorbate, an atomic 0 species is suggested as being responsible for this “two-peak” spectrum. OH contaminations as a possible reason for the “two-peak’ spectrum can be ruled out; the spectrum already occurs on Cu(ll0) at oxygen dosages as low as 0.8 L, and H,O as the only reasonable source for OH could not be detected (see section 3). According to fig. 7, emission of an electron from the 2p level of atomic 0 in its triplet state gives rise to two different emission bands because of the spin multiplicity. An interpretation of the “two-peak” photoemission spectrum as due to atomic 0 in its triplet state (similar to free 0) is further supported by the fact, that in the gas phase the 3P-4S and 3P-2D transitions are found at 13.6 and 16.9 eV, respectively [53]. The energetic difference exactly coincides with the relative distance of the two emission bands observed. A bonding shift would affect both emission lines in the same way, since they originate from the same orbital. Agreement with the absolute gas phase ionization potentials is given if a reasonable relaxation/polarization shift of about 3 eV is taken into account. The loss near 10 eV in ELS on the adsorbed species fits well with an optical transition near 9.5 eV of triplet 0 in the gas phase [43]. Adsorbed atomic 0 could yield a “two-peak” photoemission spectrum also for two other reasons: (1) Due to a different bonding symmetry of the 2p,, 2p, and the 2p, orbitals, the O(2p) emission line might be split into two components. The splitting of about 3 eV, however, is too large for a species which is less strongly chemisorbed than the atomic species at room temperature (“one-peak” spectrum) 1421. (2) The two emission lines might arise from final state effects. In addition to the 6 eV line, a satellite near 9 eV might be due to an emission process with uncomplete relaxation (unscreened hole) [54,55]. Uncomplete relaxation, however, indicates a less strongly bonded species. In conclusion, the “two-peak” spectrum in any case must be attributed to atomic oxygen, which is less strongly chemisorbed than the atomic 0 at room temperature (“one-peak” spectrum). Further experimental support for the interpretation of the “two-peak” spectrum in terms of atomic 0 is also obtained by other authors from high resolution electron energy loss spectroscopy [26]. On Cut 1 lo), the vibrational

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spectrum after a 20 L exposure at 100 K (where the “two-peak” spectrum appears) reveals only atomic 0 on the surface. It is worth mentioning that Schmeisser and Jacobi have observed the same “two-peak” spectrum in UPS for oxygen adsorbed on Cu films at low temperatures [56]. Thus, mainly from spectroscopic data, three different kinds of adsorbed oxygen have been detected on Cu(100) and Cu(ll0): molecular 0,, weakly bonded atomic 0, and strongly chemisorbed 0. The conclusions concerning bonding strength are consistent with the observations upon annealing an oxygen covered Cu(100) or Cu(ll0) surface from 100 to 300 K. At low temperature strongly bonded atomic 0 and molecular 0, are present. With increasing temperature the atomic 0 stays and the 0, molecules are desorbed or dissociated to form less strongly bonded 0 atoms. Finally, near 300 K this species has disappeared and only the strongly chemisorbed atomic 0 remains on the surface. From a comparison of the observed LEED patterns with the different species being adsorbed, one can conclude that only atomic oxygen can form well defined superstructures; the necessary surface mobility seems to be reached only for temperatures higher than 100 K.

5. Conclusion The main contribution of the present work to the problem of oxygen adsorption might be the identification of chemisorbed molecular 0, in the singlet configuration (‘2: ). Singlet 0, in the gas phase is metastable and is supposed to induce oxidation reactions on organic molecules [57]. On the copper surface singlet 0, seems to be stabilized by adsorption and for that reason easily accessible to other reaction components. Interesting implications concerning heterogeneous catalysis might be given. Furthermore, both less strongly adsorbed oxygen species, the molecular 0, and the atomic 0 at temperatures below 300 K, might be thought of as intermediates in the dissociative adsorption of molecular 0, on transition metal surfaces. Those species, therefore, require more attention. Molecular orbital and cluster calculations should give further insight into the adsorption models discussed in this paper.

Acknowledgement The work was schungsgemeinschaft.

financially

supported

by

the

Deutsche

For-

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of oxygen on copper. I

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A. Spitzer, H. Liith / Adsorjuion

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