Electron Emission Spectroscopy

Question 1

What does penetration depth of radiation depend on?

Answer 1

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

Question 2

Photons penetration depth?

Answer 2

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

Question 3

Electrons penetration depth?

Answer 3

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)

Question 4

How are photoelectron spectra interpreted?

Answer 4

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

Question 5

Why is electron emission fast?

Answer 5

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

Question 6

What is the photo electric effect?

Answer 6

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

Question 7

How does electron emission spectroscopy work?

Answer 7

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

Question 8

XPS for solids?

Answer 8

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

Question 9

Principles of photoemission spectroscopy?

Answer 9

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

Question 10

Important points about the photoemission spectroscopy spectra?

Answer 10

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)

Question 11

Depth profiling principle in photoemission spectroscopy?

Answer 11

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)

Question 12

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

Answer 12

There is no spin orbit coupling, not very strong

Question 13

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

Answer 13

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

Question 14

Why is there a step in background intensity?

Answer 14

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

Question 15

What are Auger electrons based on?

Answer 15

Secondary electron emission

Question 16

Chemical shift in XPS spectrum?

Answer 16

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

Question 17

Peak intensity in XPS spectrum?

Answer 17

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

Question 18

Electron density at C1s?

Answer 18

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

Question 19

What an important feature of XPS?

Answer 19

The possibility of permitting a chemical surface analysis

Question 20

What is qualitative XPS?

Answer 20

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

Question 21

What is quantitative XPS?

Answer 21

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

Question 22

Electron emission spectroscopy in the UV: UPS?

Answer 22

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

Question 23

Summary of XRF?

Answer 23

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)

Question 24

Summary of XPS?

Answer 24

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

Question 25

Summary of UPS?

Answer 25

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

Question 26

Summer of Auger electron spectroscopy?

Answer 26

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

Question 27

What does Koopmans theorem allow?

Answer 27

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

Question 28

Where do x ray photo electrons emit from?

Answer 28

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)

Question 29

How are charging effects removed?

Answer 29

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

Question 30

What information can be obtained?

Answer 30

Chemical information on oxidation states and neighbour atoms is obtained