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Phil Davies Flashcards

1
Q

All spectroscopic methods Phil teaches

A

XPS (X-ray Photon spectroscopy)
TPD (Temperature programmed desorption)
RAIRS (Reflective absorption Infra-red Spec)
DRIFTS (Diffuse Reflectance IR fourier transfrom spec)
ATR (Attenuated Total Reflectance)
HREELS (High resolution electron energy loss spectroscopy)
XANES (X-ray absorption Near edge structure)
EXAFS (Extended X-ray absorption fine structure)
LEED (Low energy electron diffraction)
STM (Scanning Tunnelling Microscopy)
AFM (Atomic Force Microscopy)

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2
Q

XPS Info provided

A
  • Elemental composition
  • Empirical formula
  • Chemical state
  • Electronic state (oxidation state)
  • Also what elements bound to
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3
Q

XPS limitations

A
  • It operates at low pressure, only looks at vaccum reactions
  • Kinetically slow, cannot follow reactions faster than 30 minutes
  • Difficulty identifying bonding to surface
  • Buried interface means metal will not be seen in XPS
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4
Q

Developments of XPS

A
  • High pressure XPS being developed by use of rapid pumping analyser, close approach of analyser to surface and high photon flux to ensure good signal
  • Synchrotron radiation being used to give much high fluxes, much faster scan rates, much faster reactions
  • HAXPS systems being developed
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5
Q

How XPS is recorded

A
  • Spectra are obtained by irradiating a material with a beam of x-rays while simultaneously measuring the kinetic energy and number of electrons
  • requires UHV, due to electrons low mean free path
  • typically 1-20mins
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6
Q

How TPD works

A

-observes the desorbing molecules from a surface when the surface temperature is increased
-Shows a measure of binding energy
-Measure the molecules identity by mass spec
-Used under UHV (disadvantage)
-

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7
Q

How RAIRS works

A

Photon in-Photon out

Infra-red light induces vibrational transitions and by measuring the frequency of intensity of the absorbed light, following info achieved:

  • Chemical environment
  • Structure
  • Functional group identity

Used for single crystal samples

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8
Q

Advantages of RAIRS

A
  • High resolution

- Insensitive to ambient gas pressure (ideal for catalytic reaction)

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9
Q

Limitations of RAIRS

A
  • Signal very weak, small number of molecules
  • only dipole active modes
  • expensive
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10
Q

Surface selection rule (conducting surfaces)

A

Only vibrations perpendicular to the surface are IR active and give rise to and observable absorption band

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11
Q

Grazing Incidence

A

Gives the best sensitivity for IR measurements on metallic surfaces as the angle to surface is small

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12
Q

Best sensitivity for observing adsorption

A
  • p-polarised light
  • grazing angle
  • molecule with transition dipole arranged along surface normal
  • Molecule with large transition moment
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13
Q

Info on DRIFTS

A
  • Analyses powder samples
  • High pressure can be studied
  • We want a diffuse reflection

light signal bounces around the sample, occasionally passing through sample and stores absorption of sample

Signal intensity not necessarily directly proportional to conc

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14
Q

Info on ATR

A
  • Analyses powder samples
  • High pressure can be studied
  • Cheap
  • As long as theta is smaller than critical angle you get total reflectance
  • Some of the wave penetrates into the surface
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15
Q

info on HREELS

A
  • Need a FLAT single crystal surface
  • Electrons are inelastically scattered (loss of energy)
  • Surface technique as electrons low mean free path
  • Get two peaks; inelastically scattered and elastically scattered (X300 more intense)
  • Difference between peaks equates to vibration energy
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16
Q

Difficulties with HREELS

A
  • UHV (low pressure needed)
  • Very flat surface
  • Very difficult
17
Q

Electron scattering mechanisms

A

1) Dipole scattering
- Specular scattering (theta1=theta2)
- Long range order
- obeys surface selection rule
- Oscillating dipole

2) Impact scattering
- Scattered over the whole solid angle except specular
- short range
- Most sensitive to H modes (momentum)
- Does not obey the surface selection rules

18
Q

What is LEED

A
  • Determines the surface structure of a single crystalline material by bombardment with a collimated beam of low energy electrons
  • Observe a diffracted electron pattern as spots
19
Q

Disadvantages of LEED

A
  • Need 100A order
  • Doesnt tell you what the adsorption site is
  • Cannot tell you about inside the unit cell
  • cannot be rotated, unit cell can be translated over the surface
20
Q

Dynamical LEED

A
  • Understand that there is multiple scattering events not just one
  • This effects spot intensity
  • Measure intensity as a function of voltage accelerating electron IV LEED
  • Get info inside the unit cell, precise coordinates of atoms on the surface
  • Requires UHV and long range order
21
Q

What is STM?

A

When a conducting tip is brout very near to the surface, a bias (voltage difference) applied between the tow can allow electrons to tunnel through the vacuum between them

Needs:

  • Precision control over the tip position (< width of an atom)
  • Tip to end with a single atom
22
Q

Scanning modes in STM

A

Constant Height
Constant Current
Constant position (scanning tunelling spectroscopy)

23
Q

Advantages of STM

A
  • Real surfaces, not reciprocal lattice
  • Dont need long range order
  • Dont need vacuum
24
Q

Disadvantages of STM

A
  • Tip can effect the chemistry/what you see

- Need conducting samples

25
Q

What is AFM

A

-Relies upon electron-electron repulsion between tip and sample

26
Q

Scanning modes of AFM

A

1) Contact mode
2) Tapping mode
3) Phase imaging
4) Magnetic force imaging

27
Q

Whats is XANES and EXAFS?

A

XANES is the X-ray absorption spectra due to the photo-absorption that leads to electronic transitions from an atomic core level to final states in the energy region of 50–100 eV

EXAFS is the fine structure from interference phenomenon of the outgoing electron wave of the emitted photoelectron with parts of its own wave backscattered by the atoms in the direct vicinity.

Requires synchrotron radiation

28
Q

advantages of synchrotron radiation

A
  • Continuously variable wavelengths
  • High brilliance (lots of photons)
  • highly collimated
  • Pulse of photons
29
Q

Information from EXAFS

A
  • Identity of the nearest neighbour
  • number of nearest neighbours
  • seperation
30
Q

Information from XANES

A

-Differences to do with oxidation state

31
Q

Surface EXAFS

A
  • requires use of UHV as electrons only travel short distance
  • To make sure we are looking at the surface but also use high pressure we can make more surface (nanoparticles)

Advanced Spec (3 decks)

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