Work Function Measurements

Question 1

vibrating capacitor Kelvin probe

Answer 1

oscillating metallic probe brought to surface, sample and tip levels unbalanced, Fermi levels balance and generate contact potential difference, apply external pressure to balance energy levels

Question 2

scanning tunnelling microscopy

Answer 2

sharp metal tip brought to few angstroms of surface, measures current that flows across vacuum gap due to tunnelling, tunnelling due to orbital overlap, investigates how e- density varies at surface

Question 3

ultraviolet photoelectron spectroscopy

Answer 3

directly probes density of electronic states near surface, no of e- detected reflects e- density at particular energy, gives a raw spectrum with a background due to scattering of low energy electrons which can be corrected

Question 4

Why distribution of energies of electrons when using UV photoelectron spectroscopy

Answer 4

photons emitted can be from range of states within valence bond of material, higher energy of incoming e- = deeper penetration into valence band and wider range of accessible energy states

Question 5

distribution of KE of e- ejected using UV photoelectron spectroscopy

Answer 5

max for photons exciting from higher occupied states in valence band and min at 0 corresponding from exciting e just over Evac

Question 6

uses of multi-photon photoelectron spectroscopy

Answer 6

can see empty orbitals

Question 7

vibrational spectroscopy used

Answer 7

IR/Raman, attenuated total reflection spectroscopy (ATRS) and reflection-adsorption IR spec (RAIRS)

Question 8

ATRS

Answer 8

IR radiation injected into crystal = total internal reflection at surface, evanescent IR wave from bounce off each crystal and can measure adsorbance

Question 9

RAIRS

Answer 9

identifies environment of adsorbate using shifts in wavelengths depending on ads-ads interaction giving repulsion and dipole-dipole coupling in array of oscillating dipoles

Question 10

Inelastic e- scattering

Answer 10

e- scatter and lose energy by exciting vibrational motions in surface or gain energy by accepting from already thermally excited surface, measure change in energy

Question 11

mechanisms where e- lose gain/lose energy in inelastic scattering

Answer 11

dipole scattering, impact scattering and -ve ion resonance scattering

Question 12

dipole scattering

Answer 12

e- scattered at long range due to Coulombic oscillating field at surface, e- momentum perpendicular to surface conserved, energy lost by small angle deflections around specular direction giving dipole scattering lobe,

Question 13

impact scattering

Answer 13

e- kinematically scattered at short range off-specular scattering over wide range of angles and in/out of plane of incidence