III. Analytical Aspects Photoelectron Spectroscopy Cheetham & Day, Chapter 3 Surface Technique: cannot provide completely reliable analysis for bulk samples Alternatives include (a) Electron Microprobe (b) Scanning Electron Microscopy (SEM) (c) Inductively-Coupled Plasma/Mass Spectrometry (ICP-MS) (d) Atomic Emission (e) X-ray/Neutron Diffraction

III. Analytical Aspects Photoelectron Spectroscopy: Background Cheetham & Day, Chapter 3 Energy of a photon:E = h E  eV (X-rays): XPS (core and valence e  ) E  eV (Vacuum Ultraviolet): UPS (valence e  ) Minimum energy needed (threshold frequency): h c = e  ;  = work function (2-6 eV) Maximum kinetic energy of the photoelectron is E kin max = h  e  (electrons close to E F ) E kin = h  E B  e  (electrons in states E B below E F ) E F = “Fermi level” = Energy of “HOMO” in metal; E B = Core electron “Binding Energy” Hand-Outs: 23

III. Analytical Aspects Photoelectron Spectroscopy: Background Cheetham & Day, Chapter 3 Characteristic core electron binding energies for each element: Electron Spectroscopy for Chemical Analysis (ESCA; P. Siegbahn) Chemical Shifts Multiplet Structures Satellites XPS Hand-Outs: 24

III. Analytical Aspects Photoelectron Spectroscopy: Background Cheetham & Day, Chapter 3 Characteristic core electron binding energies for each element: Electron Spectroscopy for Chemical Analysis (ESCA; P. Siegbahn) Chemical Shifts Multiplet Structures Satellites UPS Hand-Outs: 24

(1) Source of radiation (a) X-rays: high photoelectron flux (I   3.5 ) and good resolution (line-widths) K  of low-Z elements: L(2p)  K(1s) Mg (1254 eV) and Al (1487 eV) Al: K  1,2 (2p  1s) shows   0.4 eV; lifetime width of 1s hole state  0.47 eV resolution  0.9 eV Improve resolution by focusing using Bragg reflections, but sacrifice intensity. (b) UV: gas discharge lamps He(I):21.21 eV;1s 1 2p 1  1s 2 He(II):40.8 eV;2p 1  1s 1 Better resolution (ca eV) but distorted backgrounds due to “degraded” electrons. III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3 Hand-Outs: 25

(2) Spectrometer (XPS): Deflection spectrometer III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3 Hand-Outs: 25

(3) Sample Preparation: how stable is sample toward decomposition at low p (< 10  7 torr) XPS (ESCA) provides information about a thin surface layer. Electrons from the bulk are inelastically scattered and produce broad structure extending toward greater binding energy (lower kinetic energy). Sample often requires careful surface preparation: cleaving, Ar-ion sputtering, deposition on a substrate. Often have surface oxide, CO 2, grease (C can be a “reference”). Nonmetallic samples: Au film provides metallic surface. III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3 For Al K  : h = 1487 eV Mean Free Path ~ 7-18 Å Hand-Outs: 26

III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3 (4) Spectrum Analysis (Deceptively simple; traditionally complex) Binding energies: reference level and calibration (?); metal vs. nonmetal (charging effects); multiplets, satellites, shake-off; … Differences in PE cross-sections M(s) + h  M + (s) + e  h = E KE (e  ) + E(M + )  E(M) = E KE (e  ) + E B Spectrum plotted as Intensity (Counting rate) vs. Binding Energy E B = h  E KE (e  ) Hand-Outs: 26

Mg K  III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s) eV 690 eV 720 eV 910 eV 920 eV 581 eV Hand-Outs: 27

Mg K  III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s) eV 690 eV 720 eV 910 eV 920 eV Transform E KE (KE) to E B (BE = h  KE) 343 eV 333 eV 534 eV 561 eV 581 eV 673 eV 920 eV 0-8 (4-12) eV (4d, 5s) 54, 88 eV (4s, 4p) Hand-Outs: 27

Mg K  III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s) eV 690 eV 720 eV 910 eV 920 eV Transform E KE (KE) to E B (BE = h  KE) 343 eV 333 eV 534 eV 561 eV 581 eV 673 eV 920 eV 0-8 (4-12) eV (4d, 5s) 54, 88 eV (4s, 4p) Tail: E loss from solid Hand-Outs: 27

Spin-orbit splitting of 3d peak Mg K  III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s) eV 690 eV 720 eV 910 eV 920 eV Transform E KE (KE) to E B (BE) = h  KE 343 eV 333 eV 534 eV 561 eV 581 eV 673 eV 920 eV Pd + : 2 D 5/2 (6 levels); 2 D 3/2 (4 levels) (3d) 9 > ½-filled: E(J=5/2) < E(J=3/2) Hand-Outs: 27 Pd(s)  Pd + (s) + e 

Mg K  III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s) eV 690 eV 720 eV 910 eV 920 eV Transform E KE (KE) to E B (BE) = h  KE 343 eV 333 eV 534 eV 561 eV 581 eV 673 eV 920 eV Auger Process (2-electron process) Hole created in M (n = 3) shell; Relaxation from N (n = 4) shell into M; N  M (E released) E < E overcomes binding from N shell (Auger Electron) First Hole Second Hole Third Hole MNN Auger Transition Hand-Outs: 27-28

Chemical Shifts (Binding Energies) Measured binding energy of core electrons  Calculated energy of the core state Z (core/valence)  Z+ (core  1/valence): (lower observed E B ; higher measured E KE ) valence electrons in final state feel lower screening from the nucleus, behaves like Z+1 element (in metals, this is only seen for elements with states in the conduction band near E F ) Intra-atomic: single-ion shift to core electron if valence electron is missing (constant potential, ca. 10 eV for free atoms) Inter-atomic: response of the environment to ionization of atom. Polarization of neighboring ions in insulators; of conduction electrons in metals. III. Analytical Aspects Photoelectron Spectroscopy Inorg. Chem. 1984, 23, Hand-Outs: 29

The Binding Energy of an electron depends on: (1) Formal oxidation state of the atom; (2) Local chemical and physical environment Typically, E B increases with oxidation state e.g., compare Ti (metallic) with TiO 2 (insulating) III. Analytical Aspects Photoelectron Spectroscopy Cheetham & Day, Chapter 3 2p 1/2 2p 3/2 ca. 5 eV Hand-Outs: 30

III. Analytical Aspects Photoelectron Spectroscopy Cheetham & Day, Chapter 3 Examine Madelung potential and local environment: PbO vs PbO 2 (ca.  2.3 eV shift in 4f binding energies from PbO (Pb 2+ ) to PbO 2 (Pb 4+ ). Pb O O Pb: 4 n.n. O 4 n.n. Pb PbO PbO 2 Pb: 6 n.n. O Hand-Outs: 30

III. Analytical Aspects Photoelectron Spectroscopy Cheetham & Day, Chapter 3 Changes of relative position of Fermi level can lead to “negative” chemical shifts (especially in valence band spectra): e.g. CdCl 2 Energy Metals E B relative to E F Semiconductors Insulators Build-up of + charge at surface; dipole layer; E F depends on dopant concentrations Hand-Outs: 30

III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3 Au: metallic No states at the Fermi level 7 eV broad 5d Band; Satellites (shoulders); 6s contributions Narrow 5d Band -- Weak interatomic overlap in CsAu E B increases as Au is “reduced” Au 0  “Au  1 ” CsAu: transparent, red insulator Au Cs 2.89 Å 4.26 Å Hand-Outs: 31

III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3 (2-electron processes) Ni  Cu  Zn Increasing E B 3d band narrows (Nuclear charge increases) 3d3d 3d3d 3d3d 4s + 4p Hand-Outs: 31

III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3 ReO 3 Calculated Density of States Curve Valence Band XPS Spectrum Mg K  Re 5d (t 2g orbitals) Re-O antibonding O 2p nonbonding O 2p Re-O bonding States with Re contributions have stronger intensities than O states Different cross-sections for emission of photoelectrons from Re vs. O WO 3 Hand-Outs: 31

III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3 VO 2 Rutile-type (Tetragonal); Metallic Monoclinic; Insulating Hand-Outs: 31

Occupied MOs in Li x Nb 3 Cl 8 x = 0: 7 electrons x = 1: 8 electrons Nb 3 Cl 8 Li x Nb 3 Cl 8 Mg K  He(II) He(I) III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3 Mg K  He(II) He(I) Nb MOs HOMO Hand-Outs: 32

III. Analytical Aspects Related Spectroscopies Cheetham & Day, Chapter 3 Photoelectron Spectroscopy – occupied electronic states; Bremsstrahlung Isochromat Spectroscopy (BIS) – empty electronic states; Auger Spectroscopy – surface chemical composition; Rutherford Backscattering – chemical composition (heavy elements); Extended X-Ray Absorption Fine Structure (EXAFS) – local structure (SRO) Electron Energy Loss Spectroscopy (EELS) – excitation spectra