Photoelectron Spectroscopy

Photoelectron Spectroscopy

• Lecture 7 – instrumental details– Photon sources– Experimental resolution and sensitivity– Electron kinetic energy and resolution– Electron kinetic energy analyzers

Laboratory Photon Sources

• Gas discharge VUV sources: ~ 0.005 eV resolution (40 cm-1)– He I: 21.2 eV (most common for UPS)– He II: 40.8 eV– Ne I: 16.7 eV

1s

2s

2p

3s

3p

HV

He Ih = 21.2 eV

He Ih = 23.1eV

Related (sort of): Metastable Atoms• Rare gas in high voltage can also form a metastable state

– He* 23S: 19.8 eV, lifetime ~ 10 sec

– M + He* M + He + e-

– Transition probability depends on spatial overlap

– Penning Ionization Electron Spectroscopy (PIES)

or Metastable Atom Electron Spectroscopy (MAES)

1s

2s

2p

HV

E = 19.8 eV

(C5H5)2Fe

He I PES

He* (23S) PIES

e2g

a1g

e1u

e1g

e2ga1g

e1u

e1g

10 11 12 13

78910IP/eV

Ek/eV

Laboratory Photon Sources

• X-ray guns, ~ 1 eV resolution– Most used are: Mg K (1253.6 eV); Al K (1486.6 eV)– other sources from 100 – 8000 eV available

Laboratory Photon Sources

• Laser sources, ~ 8 eV max, very high resolution and intensity– pulsed source; not continuous flux of photons– photoelectron spectroscopy of negative ions

• Two or more photon ionization– Using powerful laser source, even these very low probability

events can be observed.– Complete separate field of study is multi-photon ionization (MPI)

spectroscopy.– Advantage: extremely high resolution.– We will discuss these in last lecture if we have time.

Synchrotron Radiation Source

• range of resolutions with various monochromators• continuous range of photon energies• additional cross section, resonance, polarization information

The Advanced Photon Source, Argonne National Lab

Why does the photon source chosen matter?

• We know that we need to select a photon source with sufficient energy to cause ionizations of interest to occur.

• Choice of photon source “sets” the kinetic energy of the photoelectrons of interest.

• Now we need to consider how to measure the kinetic energy of these electrons.

Electron Kinetic Energy Analyzers

• A few important concepts:

– Throughput: What % of photoelectrons produced are detected

– Resolution: How close in kinetic energy can two electrons be, and still be separated by the analyzer

• Resolving Power: E/E

• higher kinetic energy, lower resolution

– electrons with higher kinetic energy are faster than electrons with lower kinetic energy

Deflection (Electrostatic) Analyzers

• Electrons can be separated, focused by kinetic energy using an electric field

• Most common is the hemispherical analyzer

• Resolving power E/E >1,000

Throughput of Deflection Analyzers

Analyzer Entrance

steradian: solid angle subtendedby a circular surface

A sphere subtends 4 steradians

More about kinetic energy and deflection analyzers:

• Resolving power: E/E – This means resolution is dependent upon kinetic energy– Scanning through kinetic energy range to collect spectrum:

different working resolutions for different portions of the spectrum

• Measured photoelectron count rate (intensity)– Also dependent upon kinetic energy

• How do get around these difficulties?– Slow down electrons before they get to analyzer

• Rather than scanning through electron kinetic energies with a deflection analyzer:

• Use an electron-optics lens to slow electrons to a “pass energy”

• Gain better resolution, but lose sensitivity

Hemispherical Analyzer with Electron Optics

Time-of-Flight Analyzers• Resolving power ~100• Need to have “packets” of electrons• Hence useful with lasers: low photon energy (therefore low kinetic

energy), pulsed source

• Magnetic Bottle: Magnetic field in ionization region allows a large solid angle of photoelectrons to be collected, increasing spectrometer sensitivity.

• In principle, 2 steradians of photoelectrons can be collected.