Electron Emission Spectroscopy Flashcards Preview

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Flashcards in Electron Emission Spectroscopy Deck (30)
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1
Q

What does penetration depth of radiation depend on?

A

Penetration depth of radiation is dependent on the type of radiation and the energy

2
Q

Photons penetration depth?

A

Photons of high energy (gamma or x rays) penetrate deeper into materials and escape from greater depth (up to 10 micrometer in XRF)

3
Q

Electrons penetration depth?

A

Electrons (or beta radiation) have an escape depth of a few atoms only, the spectrum of electron energies can report the surface chemical information only (1-3 nm or about 10 atomic layers)

4
Q

How are photoelectron spectra interpreted?

A

Photoelectron spectra are interpreted in terms of an approximation called Koopmans theorem which states that the ionisation energy Ii is equal to the orbital energy of the ejected electron

5
Q

Why is electron emission fast?

A

Electron emission is very fast to give a snap shot of the atom or molecule

6
Q

What is the photo electric effect?

A

The photo electric effect each photon dislodges exactly one electron, the energy balance is based on excess energy going into kinetic energy, for electrons from molecules we also see vibrations but for the remaining cation

7
Q

How does electron emission spectroscopy work?

A

x ray sources = XPS or UV for UPS
Sample absorbs photon and emits electron, with applied potential across the analyser only a narrow range of electrons with equal kinetic energy hit the target scanning the potential gives XPS spectroscopy. Need to work under very high vacuum conditions for electron energy analysis, velocity of the electrons need to be measured to give escape velocity or kinetic energy, detector counts events

8
Q

XPS for solids?

A

X ray energy dislodges electrons from different orbitals electrons with different kinetics energy are separated and detected, measurements take typically 1-5 hours depending on the x ray source brightness

9
Q

Principles of photoemission spectroscopy?

A

Electrons absorb x ray photon and are ejected from surface atoms with following energy balance

10
Q

Important points about the photoemission spectroscopy spectra?

A

Spectrum - kinetic energy distribution of photo emitted electrons
Different orbitals give different peaks in spectrum
Peak intensities depend on photoionisation cross section (largest for C 1s) deepest electron in C
Surface potential or charging (impurities always there) can shift signals (calibration with C 1s = 288 eV very strong signal)

11
Q

Depth profiling principle in photoemission spectroscopy?

A

Emission of electrons possible only form surface layers (1-3 atoms), angle dependent depth information close to normal emission reaches deeper atoms (by changing angle can change depth of penetration), therefore information about compositional changes from top surface to deeper atoms can be obtained, some electrons deeper will escape eventually but lose energy from bumping into atoms and gives background noise (note dashed arrows for electrons that lost energy)

12
Q

Why is there only one line for the 4s orbitals in XPS spectrum?

A

There is no spin orbit coupling, not very strong

13
Q

Why are there two lines for the p, d and f orbitals?

A

Spin orbit coupling, the spin quantum number (S = 1/2) and the orbit quantum number coupled to give l + 1 and l - 1, magnetic moments couple

14
Q

Why is there a step in background intensity?

A

Due to lost energy, electrons tend to lose energy in collisions and therefore a step of tail is seen towards higher energy

15
Q

What are Auger electrons based on?

A

Secondary electron emission

16
Q

Chemical shift in XPS spectrum?

A

A reference peak is the C1s at 288 eV, electron withdrawing substituents increase the bidding energy of the core electrons (fine structure in the C1s line reveals different chemical environments) inner electrons give stronger signal = higher probability for emission

17
Q

Peak intensity in XPS spectrum?

A

For a given orbital equal intensities can be assumed, the intensity of the peaks is directly proportional to the number of atoms in the molecule

18
Q

Electron density at C1s?

A

Note that the electron density at the location of the C1s orbitals is associated with the chemical shift, lower electron density = more positive shift = more difficult to remove the electron

19
Q

What an important feature of XPS?

A

The possibility of permitting a chemical surface analysis

20
Q

What is qualitative XPS?

A

Qualitative XPS analysis for the identification of the elements through the position of the typical electron emission lines. Relative amounts of atoms are readily quantified

21
Q

What is quantitative XPS?

A

Quantitative analysis that needs an accurate data analysis accounting for the intensity of the different electron emission lines. The measured intensity depends in fact on a number of factors including the characteristic of the source, of the detector, of the geometry, and sample morphology. The use of standards with known concentration of the elements under investigation is particularly important for precise determinations

22
Q

Electron emission spectroscopy in the UV: UPS?

A

Uses valence electrons, Her gas sample excited with photons, UV photon dislodge valence electrons = electron fluorescence, kinetic energy analysis shows orbital energies and vibrations

23
Q

Summary of XRF?

A

X ray fluorescence, X ray in (or electron) = x ray out,

1-8 keV of incident energy, learn about the chemical elements but no chemical information (ionised state)

24
Q

Summary of XPS?

A

X ray photoelectron spectroscopy, x ray in = electron out, 1-8 keV of incident energy, learn about chemical state, composition at the surface

25
Q

Summary of UPS?

A

UV photoelectron spectroscop, UV photon in = electron out, 5-500 eV of incident energy, learn about valence band energies, vibrations, and chemical information

26
Q

Summer of Auger electron spectroscopy?

A

Electron (or x ray) in, electron out, radiation less process, filing of core hole, learn about composition for light atoms, depth profiling, leaving to secondary electron spectroscopy or XANES

27
Q

What does Koopmans theorem allow?

A

For monochromatic x ray excitation (for primary electrons) (non equilibrium) Koopmans theorem allows us to determine biding energies from the kinetic energy of emitted electrons

28
Q

Where do x ray photo electrons emit from?

A

X ray photoelectrons emit only from the top 1-3 atoms at the surface of the sample and therefore provide quantitative surface analysis (finger print pattern for atoms)

29
Q

How are charging effects removed?

A

Charging effects are removed by calibration and quantitative fitting can be used to provide the ratio between atoms in many types of sample in vacuum

30
Q

What information can be obtained?

A

Chemical information on oxidation states and neighbour atoms is obtained