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CHAPTER 5

PHOTOELECTRON SPECTROSCOPY A. Goldmann

1.

INTRODUCTION Considerable experimental and theoretical progress made within the last decade has promoted photoelectron spectroscopy (PES) - and recently also its timereversed counterpart, inverse photoelectron spectroscopy (IPES) - to the most important techniques to study the electronic states of metal surfaces - both clean and after interaction with adsorbates. Two features of PES and IPES are of particular interest: First, initial and final state energies of radiative transitions are directly determined by the experiment. Other methods, e.g. light absorption or reflection, can in general only determine the energy difference between initial and final state. Second, the electron momentum fi~ may be determined in angle-resolved experiments using single-crystal samples. Thus PES and IPES can supply the full information on the electron energy eigenvalues E(~) and their dependence on the electron wave vector k. In the context of the present book, where the use of polycristalline films it is mainly the first aspect averages out much of the detailed ~-information, which yields the useful spectroscopic information, in particular on occupied and empty level energies or the electronic densities of valence states. However, also the possibility to measure work functions and their adsorption-induced changes, or to exploit cross-section effects to deduce detailed information on orbital character, are of particular interest. Furthermore, in many cases PES and IPES peak intensities allow direct determination of surface concentrations or adsorbate coverages, make possible depth profiling and related techniques, and provide a convenient monitor for adsorption kinetics. It is the aim of the present contribution to illustrate some typical applications of PES and IPES to chemisorption studies on metals. We do not intend to describe the techniques in an exhaustive way, since several reviewing articles and detailed monographs are available (ref. 1-8). Therefore, only a very basic overview of the methods will be given in section 2. Examples of clean surface investigations are presented in section 3.1. In section 3.2 we discuss several selected adsorption studies to illustrate the use and versatility of PES and IPES. Finally, section 4 summarizes some future possibilities.

161 THE METHODS Principles of Photoemission and Inverse Photoemission The typical PES experiment is illustrated in the upper half of Fig. 1. The sample is irradiated by photons of energy nw. If a photon is absorbed in an occupied initial state Ii>, at energy Ei below the Fermi level EF (E i ~ 0 at E F), 2

2.1

E

PES

IPES

Fig. 1. Schematics of photoemission (upper half) and inverse photoemission (lower half). The shaded area of the energy diagram is accessible to the respective technique. an electron is excited into the empty final state If> at Ef. Energy conservation gives Ef - Ei ~ hw, using the convention that Ef > 0 and Ei ~ O. If Ef > Ev' the energy of the vacuum level, the electron in the excited state may leave the sample. The emitted photoelectrons are then analyzed with respect to their intensity, kinetic energy Eki n and other variables of interest (e.g. emission direction, spin-polarization). PES thus gives information on the occupied states below EF and empty states at Ef > Ev' From energy conservation we find fiw ~ E - Ei ~ Eki n +

162

is evident that both Ei and Ef(>E v) are obtained if ¢ is known. The determination of ¢ by PES is discussed below. Note that the PES initial state energy Ei(;O) is often also named binding energy EB, with the convention that EB = IEil. Inverse photoemission is illustrated in the lower half of Fig. 1. An electron at Ei = E + ¢ impinges on the sample, penetrates the surface and enters the ki n previously empty state Ii> at Ei > Ev' By a radiative transition of energy nw this state is connected with the empty state If> at Ef > EF. The emitted photon is registered in a detector. Again by energy conservation Ei and Ef are determined once Eki n, nw and ¢ are known. IPES can probe all states above EF, while states below EF and above Ev are accessible to PES. A suitable combination of PES and IPES can thus investigate all electronic states. Most PES experiments measure an electron distribution curve (EOC), i.e. the number N(E ki n) of emitted electrons, see Fig. 2. If nw is sufficiently large, E

Ej

I----+-

Fig. 2. Illustration of the fact that in PES the density of occupied states N(E.) is often approximately reflected in the emitted electron energy distributio~ curve N(E ki n). emission out of core levels is observable. The area of the corresponding peak (shaded in Fig. 2, and superimposed to a continuous background of inelastically scattered electrons) is proportional to the number of emitting atoms. Its binding energy Ei identifies the emitting element and very often ("chemical shift") also the chemical environment. Emission from occupied valence states in PES or into empty valence states in IPES yields information on the density of states (DOS). In general, however, the EOC does not directly reflect the density of states N(Ei), as idealized in Fig. 2. In the following we will discuss

163

this point for PES in some detail. PES of bulk states can transparently be described by a three-step model (for more refined treatments we refer to Ref. 1-4): photoexcitation of an electron, travelling of that electron to the surface, and escape through the surface into the vacuum. Beyond the low-energy cutoff at Ev travelling through the solid and escape are described by smooth functions of E and will not give rise to structure in N(E ki n). Therefore primarily the photoexcitation process determines the shape of the EDC. For bulk states, where crystal momentum n~ is a quantum number conserved in the reduced zone scheme ("vertical transitions" in Fig. 1) we then find for the distribution of photoexcited electrons 3k 2 (1) N(E k" , Fiw) 'V I f d l

'V

I f i, f

3

d k 01 02

(2)

We will then expect that at low photon energies (typically nw < 20 eV) the EDC does generally not reflect the density of occupied states, since only few final states for photoexcitation are available. However, at increasing Nw , the number of accessible final states tncreases and the intensity modulation through these If> states becomes less important. The EDC will then progressively become a replica of the initial density of states (DOS), as long as Mf i = constant. We will discuss below experimental results under this aspect. Similar considerations are applicable to IPES (Ref. 5-8). Things get much easier for the investigation of 2-dimensional adsorbate states: these can couple directly to freeelectron states in the vacuum. The EDC will then replicate the density of occupied (PES) initial and of empty (IPES) final states, modulated in intensity by the corresponding matrix elements Mf i. A useful byproduct of PES is the determination of 1>. The underlying energetics are explained in Fig. 3. Fig. 3a shows how an electron is excited from the sample (work function 1>s' Ei = EF = 0) to a maximum kinetic energy Em = Nw - 1>s' The EDC, however, is measured in the analyzer (A). If 1>A < 1>s' the photoelectron will be accelerated towards the analyzer and gain kinetic energy 61> , compare Fig. 3b. Then the width of the EDC, as measured from the experimentally observed Fermi level at Em to the low-energy cutoff at Eki n = 0

164

E /

Em

/

Em

I

nw-*s nw
s
Ev *