Analytical techniques Flashcards
What are the four techniques for elemental analysis?
X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spec. (UPS), Auger electron spec. (AES), Secondary ion mass spec. (SIMS)
What is the process for XPS and UPS?
x-rays eject e- from the core of the atom
How are XPS and UPS analysed?
1) elements each have unique core e- energies
2) the energy of adsorption = the core level energy
3) multiple peaks per element bc of the different core levels
4) the intensity of the peaks gives how much of the element is present
5) diff charges of ions have diff absorption peaks in XPS (ONLY)
what are core levels in XPS and UPS?
levels which don’t participate in bonding - aren’t affected much by the e- environ
Axes of an XPS/UPS graph?
binding energy on x axis, no. of e- on y axis
What is the process of AES?
1) electron beam fired at sample
2) e- no.1 is ejected on collision with the e- from the beam
3) e- no.2 fills the empty core state left fro =m e- 1, WITHOUT emitting a photon
4) using the energy from e- 2, e- no.3 is completely ejected to balance the energy
the ejection of e- 3 is the energy measured
Axes of an Auger / AES graph?
energy (kinetic or e-) on the x axis, the differential of the electron intensity on the y axis
How is Auger/AES analysed?
The differential of the electron intensity identifies both the atom and its local bonding environment
What is a more accurate technique for elemental analysis than Auger and XPS?
SIMS
what is the process of SIMS?
1) the surface is bombarded with Ar+ from a sputter gun (cleaning the surface)
2) the sputtered species removed from the surface layer are analysed using a TOF mass spec
Axes of a SIMS graph?
atomic mass units on the x axis, intensity on the y axis
What are two types of x-ray absorbance techniques?
X-ray absorbance near edge structure (XANES), and extended X-ray absorbance fine structure (EXAFS)
or SEXAFS (“surface”…)
how is x-ray absorbance measured?
by the intensity of emitted fluorescence photons and secondary electrons
XANES is on the first line of the graph at the absorption edge, EXAFS is after the absorption edge
Axes on an x-ray absorbance graph?
energy on the x axis, absorbance on the y axis
What is RAIRS?
reflection absorption infra-red spec.
what are some reasons why a molecule adsorbed on a surface may not be observed in RAIRS?
- the molecule may not remain intact on the surface
- the molecule may have bonded parallel to the surface without a changing dipole moment
What are the selection rules for RAIRS?
A charge above a metal surface causes the e- to rearrange and an image charge is seen
If bonded parallel to the surface, no net dipole is observed so no vibrational stretching modes are seen
If bonded perpendicular to the surface the dipole is enhanced (doubled if vertical) so is seen in RAIRS
Why is it that water is seen in RAIRS despite bonding parallel to the surface?
the O-H can wag up and down as long as NOT in the gas phase, this gives a dipole moment so the vibrations are seen at a low wavenumber
what is a technique that doesn’t have issues with dipoles unlike RAIRS?
electron energy loss spec. (EELS)
how does EELS work?
1) uses the loss in energy of scattered electrons to work out the molecular modes
2) shows the structure at v small energy losses using low energy modes or lattice vibrations
axes for EELS?
energy on the x axis (meV), counts on the y axis
what is TPD?
temperature programmed desorption
How do you know if a molecule is intact from TPD?
if H2 is observed coming off, but only enough as from alcohol groups for example rather than from the hydrocarbons (if it were decomposed)
What is the process of TPD?
1) molecule deposited at low temp then slowly heated - any loss of atoms detected by mass spec (e.g. H2 will be observed from alcohol groups)
2) continued heating above 400 completely decomposes the molecule
at what temp does h2 normally desorb from a surface?
room temp
what is the process of surface analysis?
-cleaning by repeated sputter and anneal
- LEED checks if its clean
- species on surface checked (XPS, Auger)
- cooling and molecule deposited on surface
- temp increase, monitoring w/ LEED
- checking for desorption (h2, o2)
- heating above room temp to order (LEED)
- analysis at room temp
how does LEED indicate if a surface is clean?
sharper diffraction pattern
what are issues with using light for imaging?
- there is interference (diffraction) from the different path lengths from two or more light sources
- an object smaller than the wavelength of light can’t be seen unless v close to the aperture
how are the issues of using visible light overcome for analysis?
- when closer to the screen, interference is less noticeable
- when very close to the aperture, non-classical light is not affected by interference
what technique uses visible light for analysis?
Scanning near-field optical microscope (SNOM)
What are the features of a normal SNOM?
1) wavelength around 500nm
2) light source is a metal coated fibre optic
3) aperture <100nm (smaller than visible light)
4) aperture is v close to the sample (10nm)
5) detector is far from the sample (through glass)
What types of samples is SNOM used for?
biological
How does SNOM work?
the evanescent waves from the fibre optic cable scatter from the sample molecule and are detected on the other side of the glass
why is SNOM scanning?
bc of the small aperture the probe must be scanned across the surface or the sample shifted, to build a complete image - CAN’T resolve individual atoms
what are the variations to SNOM apparatus?
1) non-transparent substrate, fibre optic is the detector, light going into the aperture from the sample
2) the fibre optic are both the source and the detector - light from the sample in and out of the aperture simultaneously
how do XPS and Auger give the amount of elements?
the signal strengths integrate to give an amount of elements
in RAIRS, what is a 3000cm-1 stretch?
X-H stretch
