XPS o ESCA UPS. Photoemission Spectroscopies. Threshold Spectroscopies (NEXAFS, APS etc ) The physics of photoemission.

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1 XPS o ESCA Photoemission Spectroscopies UPS Threshold Spectroscopies (NEXAFS, APS etc ) The physics of photoemission. How are photoemission spectra recorded: sources and analyzers Semi-quantitative analysis. Selected examples and measurements techniques.

2 Spettroscopic tecniques Electron out Photon out Electron in HREELS AES e - e - SOLID Inv. photoemission SXAPS e - SOLID hn Photon in XPS UPS EXAFS hn e - SOLID IRAS FTIR hn SOLID hn The mean free path of electrons undergoing inelastic scattering due to a material exhibits a minimum at ~100 ev. Electron spectroscopies operating around this energy are thus extremely surface sensitive. Single particle excitation Plasmonic excitation

3 Photoemission Spectroscopy : the ideal picture Single Particle Scheme of Energy Levels E kin E b F Many Particle Scheme: Total Energies E f (N 1) E kin E i (N) E kin = Final State Kinetic Energy = Work Function E bf (k) = Binding Energy of the k-th Initial State

4 Photoemission Spectroscopy Adiabatic vs. Sudden Approximation ADIABATIC approximation (ideal case): The process is slow and the system (isolated atom or solid) has the time needed to reach an equilibrium state. SUDDEN approximation (more realistic): The photionization-photoemission process is fast so that the final state can have electrons in bound excited states (shake-up) or in the continuum of non bound states (shake-off). This happens at expenses of the kinetic energy trasferred to the photoelectron.

5 Photoemission Spectroscopy How real spectra look like: Primary and Secondary Electrons

6 Photoemission Spectroscopy How single-particle and manyparticle mechanisms are reflected in a photoemission spectrum The adsorbed photon can cause: 1. Direct excitation of a core electron; 2. Direct excitation of a valence electron; 3. Auger process; 4. Inelastic processes (plasmon excitation and production of secondary electrons). The set of inelastic processes determines the asymmetric shape of XPS peaks (exhibiting a high binding energy tail)

7 Photoemission Spectroscopy: Photon Sources

8 Standard X-ray source

9 Standard monochromator for X-ray source Specs model

10 Ultra-violet photoemission spectroscopy A Standard UV Discharge Lamp

11 Typical experimental setup Hemispherical Analyzer of Electron Kinetic Energy with Entrance Optics Designed for Lateral Resolution X rays UV rays

12 The photoemission process is modelled by the Three-Step Model of Photoemission in Solids Photon Absorption - Photoionization Optical Absorption Machinery Selection Rules Electron Propagation within the Solid Inelastic Mean Free Path [l(e kin )] Electron Escape from the Solid Refractive Effects at the Surface (for low E kin electrons) Collection of photoemitted electrons

13 XPS as a core level spectroscopy Quantitative chemical analysis of surfaces. How sensitive is it? Which is the mimimal detectable concentration of an element? How easily this techinique can be made quantitative? How reliable is it as a quantitative analysis technique? A key argument to answer such questions is provided by the photoionization cross section of the different energetic levels of different elements One-particle approach Electromagnetic Field-Matter Interaction Semi-Classical Treatment of the Electromagnetic Field Quantum Treatment of the Solid H 1 2m e p e c A 2 e V By appropriate treatment and care quantitative analysis of XPS spectra can be performed with an accuracy of 5%-10% p A H V Electron momentum Vector potential Scalar potential Hamiltonian Potential energy

14 Phoionization Cross Sections for Free Atoms vs. Photon Energy (Yeh and Lindau) The Cooper mimimum in the cross section is observed for states having a node of the radial wave function. When operating with a tunable X ray source it can be of help to measure at energies corresponding to the Cooper mimimum to suppress an intense signal and to detect better the photoemitted intensity from other superimposed levels

15 Photoemission Spectroscopy: semi-quantitative analysis Once the photon flux f is given, the photoelectron current I i of the (nl) orbital of the i-th atomic species is approximately given by I i nl C i l E kin f( ) nl ( )T E kin Where C i l nl T Atomic Concentration of the i-th species Escape Depth Orbital Cross Section Instrumental Efficiency Once the efficiency of detection of an atomic species is calibrated via the sensitivity factors one gets I i C i i s iii s i Where C i s i I i Atomic Concentration of the i-th species Orbital Sensitivity Factor of the i-th species Spectral Intensity Related to the i-th species

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17 X-RAY Photoemission Spectroscopy Elemental Sensitivity via Core Level Binding Energies Chemical Environment Sensitivity via Core Level Chemical Shift Quantitative Evaluation via Core Level Intensity Analysis & Cross Section Evaluation Access to Many Body Realm via Spectral Line Shape Analysis

18 XPS A few examples: Core Levels and Core level shift Wide XPS spectrum of graphite (C) C(1s) region SENSIBILITA CHIMICA The singlet C 1s line is characterized by: 1) A finite width, reflecting instrumental resolution, lifetime broadening and other many-body effects. 2) A specific binding energy which reflects the specific atomic species (C) in a specific chemical environment (core level shift). The vacuum level would be a reference to determine precisely E b e DE b. It is however quite sensitive to work function changes due to modification of the surface. For the sake of simplicity the Fermi level is often used because it is apparent from the XPS spectrum itself.

19 XPS: Core Levels and Spin-Orbit Splitting Quantum Numbers j Total Angular Momentum l Orbital Angular Momentum s Spin Angular Momentum j l s p-symmetry state l = 1 s = ±1/2 Degeneracy = 2j+1

20 XPS A few examples: Core Levels and Core level shifts Si(100) oxidized in O 2 : The Si 2p line is characterized by the occurrence of 5 chemically distinct components which reflect different chemical states of the Si atoms at the interface.

21 XPS A few examples: Core Levels and Core level shift Si 2p 3/2 Core level shift with resect to bulk Si for: a) Films of different thickness grown on Si(100); b) Films ( 5 Å ) grown on different surfaces.

22 XPS A few examples: Core Levels and Core level shift

23 Photoemission Spectroscopy: Surface Core Level Shifts Surface sensitivity is achieved by exploiting the inelastic mean free path vs. hn. Below threshold: high chance to penetrate bulk photoelectrons dominate At the mimimum of the penetration depth the sensitivity to surface photoelectrons is maximised.

24 Background Subtraction in Core Level Photemission Spectra 1) The yellow areas correspond in principle to the primary photoelectrons 2) The case of Shirley base-line corresponds to the integral line of the pristine spectrum. Mechanisms Governing the Linewidth and Lineasymmetry in Photoemission Spectroscopy

25 How the Instrumental Resolution can be determined by measuring the Fermi Edge of a Metallic Sample Ag 4d-related manyfold Ag 5(sp)-related manyfold

26 Photoemission Spectroscopy: Valence Band States Any valence band spectrum brings altogether contributions related to differing atomic sites and orbital symmetry. Thereby, one has to properly design ad hoc experiments aiming at disentangling the various spectral components. Cooper Minimum Photoemission and Resonant Photoemission are two examples of such ad hoc experiments.

27 Cooper Minimum Photoemission Possible when one of the valence band orbital shows a Cooper minimum in the photoionization cross section Cooper minimum in the Pt 5d cross section A joint analysis of VB photoemission spectra taken at and off the Cooper minimum enables to disentangle the differing site- and orbital-specific contributions. Pt 5d bonding Pt 5d non-bonding Pt 5d anti bonding The Pt 5d and Si 3p cross sections are comparable

28 Valence band photoemission Usually excitation sources with less than 150 ev are employed (UV lamp or synchrotron radiation) Ultimate resolution: ~15 mev with a conventional UV lamp. Small integration in k // so that it is possible to measure the photoemision signal from a specific part of the Brillouin zone. The valence band spectrum is characterised much better than with X-rays. UPS information: Measurement of the valence band states of a surface (ARUPS). Investigation of bands and molecular orbitals of the adsorbates.

29 Angle Resolved UPS Mapping of the valence band states vs wavevector. The small integration window in k // allows to sample continuously the entire Brillouin Zone by measuring the band structure in the energy allowed range. Volume and surface states can be identified a) By comparison with theoretical predictions; b) By their different behaviour following gas adsorption (which obviously affects only the surface states).

30 Volume and surface states can be identified a) By comparison with theoretical predictions; b) By their different behaviour following gas adsorption (which obviously affects only the surface states).

31 UPS for the study of adsorbed molecules Chemisorbed C 6 H 6 Condensed C 6 H 6 The comparison of spectra for benzene chemisorbed on Ni(111) and for benzene condensed on the same surface (i.e. for physisorbed benzene, not chemically bound to the surface) shows: 1. at least three structures corresponding to the molecular orbitals; 2. A shift of the p orbital toward more negative energies for the chemisorbed phase, indicating that interaction with the surface involves mainly this orbital Gas-phase C 6 H 6 The assignment of peaks due to adsorbates are made mainly by comparison with theoretical predictions or with experimental data available either for the gas phase or for adsorption on previously studied surfaces

32 Threshold Spettroscopies In XPS the energy of the impinging photons may be much higher than the photoionization threshold. When operating with tunable sources (synchrotron) the energy of the photons is chosen in such a way to maximise the photionization cross section and the surface sensitivity. Alternatively, it is possible to study core levels looking for their photionization threshold by a source at tunable energy. The overcoming of the threshold for photoionization will be detected either by a decrease of the impining flux or more often by the emission of photons or of Auger electrons associated with the filling of the vacancies produced by the impinging photon beam. There are several tecniques employing different detection techniques. They are collectively addressed as APS (Appearance Potential Spectroscopy). The excitation source (50 ev to several KeV) can be provided either by photons or by electrons. In the former case a synchrotron light osurce is needed.

33 NEXAFS (Near Edge X-ray Absorption Fine Structure) opp. XANES (X-ray Absorption Near Edge Structure) Working with tunable X ray sources (Synchrotron Radiation) one may chose the energy at which surface sensitivity is maximised. Alternatively one may look at the threshold energy and look for the onset of photoemission or AES signals. Such technique is called APS (Appearance Potential Spectroscopies). In the near edge region the excitation probability depends on the density of available empty states which may be strongly modulated below and at the vacuum level. Biatomic molecule: Keeping in mind the relevant selection rules the transition of an 1s electron to p and levels is modulated by the photon energy. Fig. 3 Schematic potential (bottom) and corresponding NEXAFS K-shell spectrum (top) of a diatomic molecular (sub)group. In addition to Rydberg states and a continuum of empty states similar to those expected for atoms, unfilled molecular orbitals are present, which is reflected in the absorption spectrum.

34 NEXAFS (Near Edge X-ray Adsorption Fine Structure) opp. XANES (X-ray Adsorption Near Edge Structure) Fig. 1. Schematic molecular potentials (bottom) and K-edge spectrum (top) of a diatomic molecule XY. The K-edge features are due to transitions from the ls core level of atom X to the following partially filled or unfilled molecular orbitals: p* orbitals in the bound state, Rydberg states at energies just below the Fermi level, and * resonances in the continuum state. (From St6hr [1]; copyright Springer-Verlag.) The threshold energies depend on the material 2p levels in non metals: K-edge (1s 2p)= 285 ev for C 400 ev for N 530 ev for O 685 ev for F 3p levels in non metals: K-edge (1s 3p)= 1830 ev for Si 2140 ev for P 2470 ev for S 2830 ev for Cl L-edge (2p 4s, 3d) in the range ev. 3d metals : 4d metals: L III,II (2p 3d) <1000 ev K-egde (1s 4p) >4500 ev M III,II (2p 4d) <1000 ev K-edge (1s 4p) >4500 ev

35 X-ray absorption cross section x p h e 2 A p f 2 i m hc 1 ( E) f h ( h E i E f ) h = incident photon energy e, m= charge and mass of electrons (E)= energy density of the final state I<f f A p f i > =dipole matrix (h +E i -E f )=delta function for the conservation of energy. The matrix describes the dipolar interaction between the electron (momentum p) and the electric field (vector potential A). The orientation of the molecule determines the parity of initial state (generally a symmetric s level) and final state. Photoemission and hence X ray absorption takes place only when the matrix element is even. Angular momentum conservation implies moreover Dl=±1 between initial and final state.

36 Information about the molecular orientation In the case of linearly polarized light, the angular dependence of the matrix element of interest: f e assumes a simple form. For a 1s initial state and a directional final state orbital the matrix element points in the direction of the final state orbital O and the transition intensity becomes cos 2 f p f (with angle between the electric field vector, e, and the direction of the final state orbital). Therefore, the intensity of a resonance is largest when e lies along the direction of the final state molecular orbital and vanishes when e is perpendicular to it. i 2 f f p f i Schematic representation of the origin of the angular dependence of NEXAFS resonances for a p-bonded diatomic molecule adsorbed with its molecular axis normal to the surface. As a result of the different overlap between the electric field vector and the direction of the final state orbitals the p *-resonance is maximized at normal incidence (left), while the *- resonance is maximized at grazing incidence (right).

37 Experimental detection of X-ray absorption The absorption of a photon implies the excitation of an electron from a core level to the valence state. This excited state will decay either by AES or by photoluminescence and be detected. Energy liberated in the deexcitation process Fluorescence ( f ) Auger electron ( a ) f + a =1 Fluorescence: - Photon-in /Photon-out technique no UHV required; - The penetration depth of photons vs electrons allows to distinguish between bulk and surface processes; - It is insensitive to charging problems. But measuring thresholds in the soft X-Ray region may be complicated.

38 Experimental detection of X-ray absorption In NEXAFS the final state of the photoelectron may be a bound state such as an exciton. Measuring fluorescent photons or Auger electrons allows to tune the sensitivity of the technique. The signal corresponds to the sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures is the total joint density of states of the initial core level with all final states, which are consistent with the conservation rules. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum. These narrow characteristic spectral peaks give the NEXAFS technique. A lot of its analytical power is illustrated by the B 1s π* exciton shown below. The great power of NEXAFS derives from its elemental specificity. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal.

39 Experimental detection of X-ray absorption Comparing fluorecence and electron yield K-edge of O in 50 Å NiO(100)/Ni(100) The peak positions and the relative intensities of the O K-edge features are identical in the two measurements. The NEXAFS features labeled A, B and C are assigned to the one-electron transition to the 3e g, 3a 1g and 4t 1u orbitals, respectively. The peaks labeled B* and C* are related to multielectron configuration interactions.

40 Comparing fluorecence and electron yield: the information you can get. NiO/Ni(100) Preparation at 300 K followed by annealing to T=800 K Flourescence NiO clusters + c(2 2) O chemisorbed phase; the O signal decreases in AES or XPS. Such decrease can originate either because of the formation of thicker NiO(100) clusters or by diffusion of oxygen deep into the metal bulk. Which one of the two? Electron yield NEXAFS: The fluorescence yield remains unchanged after annealing to 800 K while the electron yield decreases. Oxygen remains near the surface region (mean free path of the O K-edge photons 2000 Å). The thickness of the O containing region is larger than the electron escape depth (10-15 Å at 500 ev).

41 NEXAFS: characterization of Molecules POLYMERS NEXAFS spectra differ significantly even for rather similar molecular structures, so that they can be used as a fingerprint of each polymer. In many cases, enough is known about how chemical structure and X-ray absorption spectral features are related to allow one to identify unknown species from measured NEXAFS spectra. Individual spectral features, particularly the low energy p* features, are often sufficient for qualitative identification in reasonably well characterized systems, and they can serve as useful energies for selective chemical contrast in X-ray microscopy. K-edge for C

42 Photoelectron Diffraction: Basic Principles c(2x2)s/ni(100)

43 Photoelectron Diffraction: Basic Principles Condition of Constructive Interference

44 Chemically-Shifted P 2p Components of PF x Fragments Chemisorbed at the Ni (111) Surface

45 Photoelectron Diffraction: Experiment vs. Simulation PF 3 /Ni(111) Experiment Theoretical Simulations Best Fit

46 Photoelectron Diffraction: Experiment vs. Simulation PF 2 /Ni(111) Experiment Theoretical Simulations Best Fit

47 Photoelectron Diffraction: Experiment vs. Simulation PF/Ni(111) Theoretical Simulations Best Fit Experiment

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49 2 photon photo emission 2PPE

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51 For negative delays only the probe laser is in action. Electrons are then promoted from bulk band to the n=1 and 2 image potential states by the 2PPE mechanism and pumped from there by a third photon. The electrons in the surface Shockley state are photoemitted by the same 2PPE mechanism ending up at halfway between the n=1 and n=2 photoemission energies. At time 0 the pump laser enters in action. The n=0 state is then rapidly depleted of electrons (vanishing of the n=0 signal). Such electrons are promoted to the states n=1 and n=2 (increasing photoemission from such states induced by the probe laser). At positive delays the probe laser probes the surviving electron density ins taes n=1 and 2 The Xe layer decouples the image potential states from the Cu surface so that the lifetime of electrons in the image states becomes longer.