THE NATURE OF THE EXCITED STATES OF ADSORBATES AND THEIR DECAY MECHANISMS

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THE NATURE OF THE EXCITED STATES OF ADSORBATES AND THEIR DECAY MECHANISMS

Ph. Avouris, J. Demuth, N. Dinardo

To cite this version:

Ph. Avouris, J. Demuth, N. Dinardo. THE NATURE OF THE EXCITED STATES OF ADSOR-

BATES AND THEIR DECAY MECHANISMS. Journal de Physique Colloques, 1983, 44 (C10),

pp.C10-451-C10-454. �10.1051/jphyscol:19831088�. �jpa-00223546�

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JOURNAL DE PHYSIQUE

Colloque C10, suppldment au n012, Tome 44, ddcembre 1983 page CIO-451

T H E NATURE OF T H E EXCITED S T A T E S O F ADSORBATES AND THEIR DECAY MECHANISMS

Ph. Avouris, _{J . E .} Demuth and N.J. DiNardo

IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, U.S.A.

Re'sume - Nous pr6sentons des spectres de perte d16nergie dlBlectrons pour (A) des gaz rares sur des mktaux, (B) CO chemisorb6. A partir de ces spectres nous discutons la nature des 6tatsexcitEsde ces s y s t h e s a i n s i que leursprocessusdedEexcitation.

A b s ~ l -t - We present electron energy loss spectra and discuss the nature and relaxation mechanisms of the excited states of (A) noble gases on metals and (B) chemisorbed CO.

In recent years, there has been a growing interest in understanding the nature and the non-radiative decay pathways of electronically excited adsorbates. In previous work we have discussed the excited states of weakly adsorbed and have evaluated the importance of non-radiative decay mechanisms involving coupling to surface and bulk electron-hole (e-h) pair excitations of the s u b ~ t r a t e . ~ ? ~ Here we will consider briefly the excited states of two different types of adsorbate systems: (A) noble gas atoms on metals and (B) chemisorbed CO. The spectra are obtained using high resolution electron energy loss spectroscopy. The experimental arrangement has been discussed previously.1,2

I. Excitations of Physisorbed Noble Gases

Noble gas atoms in their ground state have a full np6 outer shell and interact with metal surfaces primarily via dispersion forces. Their excited states are Rydbergs, the lowest one having the alkali-like configuration n p 5 ( n + l ) s (e.g., Xe looks like Cs). Alkalis adsorb ionically on metals and under appropriate conditions excited noble gases may behave analogously so that ionic excited states will be formed. A necessary condition is that the ( n + l ) s level be located above the Fermi level of the metal, i.e., the metal work function +>I*, the excited state ionization potential. Using optical reflectance techniques Flynn and coworkers4 studied several combinations of noble gases and metal surfaces and concluded that excitonic absorptions of monolayers are observed only when I*>+. Theoret- ical work by Lang et a ~ . ~ , however, cast doubt on the interpretation of the optical experiments. Since the results of optical spectroscopy can be complicated by local field effects6, we have decided to use low energy, non-dipolar EELS to study the electronic excitations of noble gas/metal surface systems. In Fig. 1 we show results for Xe/Cu, where I*<+. We have also performed analogous studies for Ar and Xe on Al, Ag and A U . ~ Since knowledge of surface coverage is essential we have used the different final state shifts in photoemission (UPS) of mono- and multi-layers to delineate these two coverage regimes (Fig. 1A).

In Fig. 1B we show the electronic excitations of monolayer and two layers of Xe along with a diagram of the free Xe transitions. Also shown are the shapes of the lowest

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831088

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5P6+5P3,26s (J=1 and J = 2 ) transitions after background subtraction. Important conclu- 5 sions can be reached by simple inspection of the spectra: (1) the excitonic absorptions of the Xe monolayer are clearly seen (even though I*<+) and can be correlated with groups of free atom transitions, (2) the lineshape of the 5p6+5p56s transition of monolayer Xe/Cu is quite symmetric and has a linewidth (FWHM) of -0.6eV. The maximum of the band is blue shifted with respect t o the gas phase value by -0.2-0.3eV. The corresponding two layer spectrum is narrower and transitions to both J = l and J = 2 levels begin to be resolved.

Fig. 1 (A) The ultraviolet photoemission spectra (UPS) of a monolayer and two layers of Xe on Cu. (B) The corresponding electron energy loss spectra. Also shown are the lineshapes of the 5p6 + 5p:,26s transition after background subtraction and the free Xe excitations.

BINDING ENERGY (eV) ENERGY LOSS (eV)

From the above results we can conclude that the energetic criterion I*<+ is not sufficient for the production of ionic excited states but the coupling strength to the substrate empty conduction band states

I

k>, VkA, plays a crucial role. Our experiments suggest that for excited noble gases at the ground state (Franck-Condon) adsorption geometry VkA is small. For large VkA the discrete atomic state will be heavily mixed with the continuum of metallic states and result in an asymmetric Fano-type lineshape, not the symmetric lineshape we observe. A transition to an ionic excited state will be subject to image shift effects which will induce a blue shift4 of e2/4z, i.e., -2eV. The observed blue shifts, however, are - l o x smaller. Finally, the oscillator strength of the 5p6+5p56s excitation of Xe in the first layer and subsequent layers is found to be nearly the same.

The non-radiative decay rate, kNR, of the excited state, presumably involving electron tunneling to the metal can be estimated via linewidth analysis. Using the observed linewidth T=0.6eV the upper limit t o this rate is kNR = ~ / R - & x ~ o ~ ~ s - ~ . More detailed consideration of the different contributions to the observed width suggest that kNR is -2x smaller. Besides electron tunneling another possible decay path involves field coupling of the adsorbate excitations with the substrate e-h pair excitations. The theory of Persson and

an^^

describes the relaxation to both bulk and surface e-h pairs. The correlation of the .theoretical predictions with experimental results on N2/A1(1 1 1 j 2 has been very good.

In the present case the decay rate via this mechanism is calculated to be - 2 x 1013s-l.

11. Excitations of Chernisorbed C O

We have studied the valence excitations of molecularly chemisorbed C O on transition metals and Cu. Figure 2 shows the EEL spectra of a clean Cu surface (A) and after

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exposure to 1 L C O (B), 2.5L C O (C) and 10L C O (D). C O absorption enhances the intrinsic Cu excitations at -2.2eV (L3+Fermi surface) and a t -4.2eV ( L ~ , + L ~ ) . ~ At 1L C O new excitations appear a t -8.5eV and -6eV (shoulder). We also observe analogous excitations for C O o n transition metals at room temperature. At higher CO coverages transitions with characteristic vibronic structure are observed at -6eV and -8.5eV which are due respectively t o triplet and singlet coupled, 50+2nf excitations (in the range 11-14eV perturbed Rydberg states are observed).

Fig. 2 The electron energy loss spectra of CO on C u at -10K. (A) Spectrum of clean Cu surface, (B) Exposed t o 1 L (1 ~ a n ~ m u i r = l ~ - ~ torr-s) CO, (C) Exposed to 2.5L CO, and (D) Ex- posed to 10L CO. By UPS we deter- mine that spectrum B corresponds to about one monolayer.

2 4 6 8 10 12 14 16

ELECTRON ENERGY LOSS (eV)

The accepted picture of C O chemisorption involves charge transfer t o the metal from 50 and backdonation to the 2a* orbital9 Therefore, both the 5u9 and 2n*1° orbitals are perturbed by the chemisorption process. This realization along with misconceptions regarding the importance of relaxation effects in neutrg elez$ronic excitations has led to considerable speculation regarding the location of the ^So+271 excitations of chemisorbed

~ 0 . l ~ We-belize that although both the 50 and 2n* orbitals are perturbed by chemisorption the 5 o + 2 n excitation energy is essentially unchanged from its free molecule value.

Here we can only sketch our arguments. The essential physics of the problem can be described by a Newns-Anderson Hamiltonian for a two level,

I

I > and 12>, adsorbate system coupled to the substrate metal.12 The adsorbate ground state has two electrons in ) 1 > and an energy Eg=2(s1+3Vi)+(ul-2Vi), while in the excited (singlet) state one electron has been promoted t o 1 2 > and the energy is

E,=(sl +3Vi)+(e2+3Vi)+(ul2-2Vi); s1 and e2 are the one-electron level energies, ul and u I 2 represent the Coulomb repulsion for two electrons in level

I

1 > or one in

I

1> and the other in 12>, respectively, and Vi represents the image interaction (Vi is about the same for similar size orbitals). The excitation energy E,-Eg=e2-~1 -uI + U 12 therefore does not depend on image screening effects. In addition, screening via charge transfer from the substrate to the affinity level 12> (2nf in CO) is, unlike in photoemission, prohibited by a u2 repulsion because 12> has been populated by the excitation process itself (for CO uzW*-l3eV). The same conclusions can be reached by studying the excitations of physisorbed systems such as N2 on ~ 1 ( 1 1 1 ) . ~ Since the relative bonding shifts of e l and s2 are

!nearly zero in this case, the observed near coincidence of the excitation energies in the free and physisorbed states shows the absence of significant screening effects on the transition energy.

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In addition t o the Coulombic terms, the exchange interaction between two electrons with parallel spin, one in

1

1 > , the other in ) 2 > , maintains its free molecule value (for C O Ip,2v:-2eV); i.e., spin-quenching by the substrate is negligible. This argument is based on t e unfavorable energetics for metal spin localization near the adsorbate.

Based on photoemission9 and recent inverse photoemission10 results on chemisorbed C_O and b_y*making the reasonable assnmption that the final state (relaxation) shifts for the 50 and 2 n levels are similar, we can conclude that the corresponding bonding shifts are also about equal (-3eV). This result coupled with our conclusions regarding the Coulomb and exchange interactions in the chemisorbed state lead us to assign_ the-transitions of chemisorbed CO at -6 and -8.5eV to triplet and singlet coupled 5 u + 2 n excitations, respectively.

Since in the chemisorbed C O the 2n* level is above EF or in our model12 E ~ - ~ ~ - U , + U ~ ~ > E ~ - ~ ~ - U ~ - V ~ , the main relaxation mechanism is again expected t o involve electron tunneling to the metal. The width of the excitations (-1.5-2eV) cannot, however, b e linked in a direct manner to the non-radiative lifetime of the excited state. In the noble gases the np level remained sharp so that the lifetime broadening of the np(n+ 1)s excited state could be associated with the width of the (n+ 1)s level. In the case of CO, however, the 50 is mixed (in the ground state) with the continuum of metallic states, thus acquiring a significa_nt widtk; Therefore, the observed excitation widths contain contributions from both 5u and 271 levels. The situation becomes similar t o that of adsorbed noble gases when trazsitions in_vpIving C O core levels are considered. For C O / N ~ ( ~ O O ) ~ ~ the width of the 20(C1,)*2n excitation is T-2eV (FWHM). This large width may be considered the result of the stronger VkA coupling of C O t o the substrate (r-0.3eV for 5p+6s in noble gases). However, besides electron tunneling to the substrate other Auger-type mechanisms may become important in the decay okthese cor5:xcitations.

This is suggested by the fact that the width of the higher energy lu(01,)-+.2m excitation of CO/Ni(100) is even broader -4eV.

References

Ph. Avouris and J. E. Demuth, J. Chem. Phys. 75 (1981) 4783; J. E. Demuth and Ph.

Avouris, Phys. Rev. Lett. 47 (1981) 61.

Ph. Avouris, D. Schmeisser and J. E. Demuth, J. Chem. Phys. 79 (1983) in press.

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J. E. Cunningham, D. Greenlaw and C. P. Flynn, Phys. Rev. B22 (1980) 717.

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