Analytical techniques

Question 1

What are the four techniques for elemental analysis?

Answer 1

X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spec. (UPS), Auger electron spec. (AES), Secondary ion mass spec. (SIMS)

Question 2

What is the process for XPS and UPS?

Answer 2

x-rays eject e- from the core of the atom

Question 3

How are XPS and UPS analysed?

Answer 3

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)

Question 4

what are core levels in XPS and UPS?

Answer 4

levels which don’t participate in bonding - aren’t affected much by the e- environ

Question 5

Axes of an XPS/UPS graph?

Answer 5

binding energy on x axis, no. of e- on y axis

Question 6

What is the process of AES?

Answer 6

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

Question 7

Axes of an Auger / AES graph?

Answer 7

energy (kinetic or e-) on the x axis, the differential of the electron intensity on the y axis

Question 8

How is Auger/AES analysed?

Answer 8

The differential of the electron intensity identifies both the atom and its local bonding environment

Question 9

What is a more accurate technique for elemental analysis than Auger and XPS?

Answer 9

SIMS

Question 10

what is the process of SIMS?

Answer 10

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

Question 11

Axes of a SIMS graph?

Answer 11

atomic mass units on the x axis, intensity on the y axis

Question 12

What are two types of x-ray absorbance techniques?

Answer 12

X-ray absorbance near edge structure (XANES), and extended X-ray absorbance fine structure (EXAFS)
or SEXAFS (“surface”…)

Question 13

how is x-ray absorbance measured?

Answer 13

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

Question 14

Axes on an x-ray absorbance graph?

Answer 14

energy on the x axis, absorbance on the y axis

Question 15

What is RAIRS?

Answer 15

reflection absorption infra-red spec.

Question 16

what are some reasons why a molecule adsorbed on a surface may not be observed in RAIRS?

Answer 16

• the molecule may not remain intact on the surface
• the molecule may have bonded parallel to the surface without a changing dipole moment

Question 17

What are the selection rules for RAIRS?

Answer 17

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

Question 18

Why is it that water is seen in RAIRS despite bonding parallel to the surface?

Answer 18

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

Question 19

what is a technique that doesn’t have issues with dipoles unlike RAIRS?

Answer 19

electron energy loss spec. (EELS)

Question 20

how does EELS work?

Answer 20

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

Question 21

axes for EELS?

Answer 21

energy on the x axis (meV), counts on the y axis

Question 22

what is TPD?

Answer 22

temperature programmed desorption

Question 23

How do you know if a molecule is intact from TPD?

Answer 23

if H2 is observed coming off, but only enough as from alcohol groups for example rather than from the hydrocarbons (if it were decomposed)

Question 24

What is the process of TPD?

Answer 24

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

Question 25

at what temp does h2 normally desorb from a surface?

Answer 25

room temp

Question 26

what is the process of surface analysis?

Answer 26

-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

Question 27

how does LEED indicate if a surface is clean?

Answer 27

sharper diffraction pattern

Question 28

what are issues with using light for imaging?

Answer 28

• 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

Question 29

how are the issues of using visible light overcome for analysis?

Answer 29

• when closer to the screen, interference is less noticeable
• when very close to the aperture, non-classical light is not affected by interference

Question 30

what technique uses visible light for analysis?

Answer 30

Scanning near-field optical microscope (SNOM)

Question 31

What are the features of a normal SNOM?

Answer 31

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)

Question 32

What types of samples is SNOM used for?

Answer 32

biological

Question 33

How does SNOM work?

Answer 33

the evanescent waves from the fibre optic cable scatter from the sample molecule and are detected on the other side of the glass

Question 34

why is SNOM scanning?

Answer 34

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

Question 35

what are the variations to SNOM apparatus?

Answer 35

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

Question 36

how do XPS and Auger give the amount of elements?

Answer 36

the signal strengths integrate to give an amount of elements

Question 37

in RAIRS, what is a 3000cm-1 stretch?

Answer 37

X-H stretch