How deep into a solid sample does XPS (ESCA) probe? Incident x-rays Emitted electrons How far can the x-ray beam probe? From what depth can electrons escape without loss of energy? Several microns (µm) A few nanometres (nm) Escape Depth I= I0 e -αx α is the inelastic scattering cross-section (m-1) α = 1/λ x λ is the escape depth I= I0 e -x/λ When x = λ, I Increasing α (or decreasing λ) The escape depth, λ, is that depth at which the probability of escape is reduced to 1/e (~36.8%) compared to emission from an atom at the surface (x=0) λ I = I0 e -1 (e = 2.718) x I = I0 e - λ . 1 I = I0 e 15Å 10Å 5Å 0Å 1 10 100 1000 eV I = I0 exp(-x/λ) For the clean metal surface x θ Imet = I0 0∫ ∞exp(-x/λ) dx = λ I0 d/cos θ d For the metal signal in the presence of an overlayer of thickness d I’met = I0 d/cos θ∫ ∞exp(-x/λ) dx = λ I0 exp(-d/λcos θ) I’met /Imet = exp(-d/λcos θ) ln(Imet /I’met) = d/λcos θ Be XPS Angle-dependent XPS of a Silicon Wafer with a Native Oxide θ=0o θ=80o cos(θ) 80o 0o http://www.phi.com/publications/ 9603.htm XPS Imaging Cr O Al A scanned X-ray image of 100 µm x 100 µm Cr pads on Al2O3 Application Note Physical Electronics. Auger Auger Analysis of a Multi-Layer Paint Cross-Section NB All elements can be detected except H Atomic Concentration (%) Analysis Area C O N Cl Substrate 100.0 ----- --Adhesion Layer 90.0 ----- 10.0 Base Coat 72.0 16.4 3.5 3.3 Clear Coat 70.6 22.2 7.2 --- Si ----2.6 --- Al ----2.2 --- X-ray spectroscopy is primarily concerned with the behaviour of CORE level electrons Vacuum level LUMO Valence levels Energy HOMO hν (x-ray) Emission spectroscopy Core levels “Core hole” Isolated molecule (ionised) “Fluorescence” Created by emission of electron following bombardment ofsample by X-rays (XPS), high energy electrons, protons, ions etc http://www.amptek.com Conservation of angular momentum Photon has one unit of angular momentum Electron orbital angular momentum Allowed transitions l s p d f etc 0 1 2 3 s ---> p p ---> s d ---> p f ---> d etc p ---> d d ---> f f ---> g Δl = ±1 XRF analysis of St Gaudens US $20 gold coin with 90% Au and 10% Cu http://www.amptek.com/5x5x500.html Energy dispersive X-ray fluorescence 4 8 keV 12 http://www.thebritishmuseum.ac.uk/science/techniques/sr-tech-xrf.html#