Ele lectron and ele lectromagnetic radiation Generation and - - PowerPoint PPT Presentation
Ele lectron and ele lectromagnetic radiation Generation and interactions with matter Interaction with sample Response Stimuli Stimuli Waves and energy The energy is propotional to 1/ and 1/ 2 1 1 > 2 Electromagnetic
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λ
Electromagnetic waves: E= hc/λ =hf =hcν
h: Plancks constant, f: frequency, ν: wave number
Electron waves :E= eVo, E=½ mv2 = ½ m(h/λ)2
Stimuli Matter waves are referred to as de Broglie waves where λ=h/p and p=mv.
U (Volt) k = λ-1 (nm-1) λ (nm) m/mo v/c 1 0.815 1.226 1.0000020 0.0020 10 2.579 0.3878 1.0000196 0.0063 102 8.154 0.1226 1.0001957 0.0198 104 81.94 0.01220 1.01957 0.1950 105 270.2 0.00370 1.1957 0.5482 2*105 398.7 0.00251 1.3914 0.6953 107 8468 0.00012 20.5690 0.9988 Relationship between acceleration voltage, wavevector, wavelength, mass and velocity Stimuli
The speed of the electron is approaching the speed of light.
Gamma Hard X-rays Soft X-rays Visible light
E = Extreme N= Near F= Far HF = high freq. MF= medium freq. LF= low freq.
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When an electron is slowed down (accelerated) and the energy of the electron drops (speed is reduced), the energy can be transformed into electromagnetic radiation.
Electromagnetic waves: E= hc/λ Electron waves :E= eVo What is the peak energy of the bremsstrahung in
(Much stronger interaction compared to the interaction with X-rays and neutrons) The Coulombic force F is defined as: F = Q1Q2 / 4πεor2 r : distance between the charges Q1 and Q2; εo: dielectric constant.
http://www.microscopy.ethz.ch/downloads/Interactions. pdf
Interaction with sample
Interaction with sample
E0=20 keV : Typical energy of electrons used for analytical scanning electron microscopy studies. TEM ~200keV
t: up to a few hundred nm. t of interest much less. X-ray penetration depth: The depth at which the intensity of the radiation inside the material falls to 1/e (about 37%) of its original value at just beneath the surface.
wiki
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E1 E2 If E1= E2 If E1> E2
Interaction with sample Z+
Back scattered electrons.
Interaction with sample
Interaction cross-section (σ, Q) and mean free path (λmfp) represents the probability of a scattering event.
Illustration based on figure in: http://www.microscopy.ethz.ch/ downloads/Interactions.pdf
*t
t*
t: thickness of the specimen
Interaction with sample
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i.e. The initial electromagnetic wave is absorbed.
i.e. The electron continues with less speed/energy Interaction with sample
Inelastic scattering
E1 E2
(Lattice vibrations are more temperature dependent than molecule vibrations) Ref. Ch. 9.0 - 9.1.3.
Interaction with sample Inelastic scattering Quantified energy states
Phonon electron energy losses ~ 0.1 - 0.5 eV, Electromagnetic absorption (Molecules: 200-4000 cm-1) (Lattice: 20-300 cm-1)
Energy: Ep=(h/2π)ω ~10-30 eV
Plasmon frequency: ω=((ne2/εom))1/2 n: free electron density, εo: dielectric constant
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Which energy do 1000 cm-1 correspond to? Electromagnetic waves: E= hc/λ =hf = hcν
h: Plancks constant, f: frequency, ν: wave number
ν=100000 m-1 : λ=0.00001 m 1 J= 6.2415 e18 eV
Wiki magnunor
Similar to the absorption spectra of the electromagnetic radiation.
Thin specimen Inelastic scattering Responce
The progress has taken place on three principal fronts: (1) the energy resolution of EELS carried out in the electron microscope has been improved to around 10 meV; (2) the EELS–STEM instrument has been optimized so that the electron probe incident on the sample contains a current sufficient to perform EELS experiments even when the energy width of the probe is ∼10 meV and its size <1 nm; and (3) the tail of the intense zero loss peak (ZLP) in the EELS spectrum has been reduced so that it does not obscure the vibrational features of interest. Inelastic scattering Responce
Inelastic scattering Responce
(Lattice vibrations are more temperature dependent than molecule vibrations) Ref. Ch. 9.0 - 9.1.3.
(Above 50 eV and typically more than thousand eV for the ionization of inner electron shells (core electrons).) Interaction with sample Inelastic scattering Quantified energy states
Energy losses ~ 0.1 eV
Energy: Ep=(h/2π)ω ~10-30 eV
Plasmon frequency: ω=((ne2/εom))1/2 n: free electron density, εo: dielectric constant
K L M
1s2 2s2 2p2 2p4 3s2 3p2 3p4 3d4 3d6
Electron
K L M
Photo electron
x-ray
Secondary electron
Interaction with sample Inelastic scattering
EELS X-ray photo electron spectroscopy and X-ray absorption spectroscopy
https://xpssimplified.com/elements/germanium.php https://xpssimplified.com/whatisxps.php http://www.fis.unical.it/files/fl178/9232XASChap6.pdf
When the energy of the photons increases, the absorption coefficient μ(ω) decreases.
Synchrotron radiation Singe wavelength X-ray Commonly: Al Kα Can also probe
valence states More on XPS later in the semester!
http://pd.chem.ucl.ac.uk/pdnn/inst1/filters.htm The absorption edge of nickel metal at 1.488 Å lies between the Kα (λ = 1.542 Å) and Kβ (λ = 1.392 Å) X-ray spectral lines
be used to reduce the intensity of the Cu Kβ X-rays
Anode Cu Co Fe Cr Mo Filter Ni Fe Mn V Zr
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K L M
Characteristic x-ray
The probability to emit an Auger electron or X-ray
Siegbahn notation Ex.: Kα1 Intensity: α>β>γ> and 1>2>3
Fluorescence: electromagnetic radiation generate new electromagnetic radiation
http://www.emeraldinsight.com/journals.htm?articleid=1454931&show=html
Continous X-ray energies The cut-off energy for continous x-rays. Characteristic X-ray energies.
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http://www.emeraldinsight.com/journals.htm?articleid=1454931&show=html
Characteristic X-ray energies.
Two peaks Limited resolution of the detection method (EDS)
Improved resolution with wavelength dispersive spectroscopy
Stimuli Interaction with sample
Excitations: phonon, plasmon, ionization Zero, single, multiple scatteing events Kinematic condition Dynamic conditions