Photoelectron Spectroscopy

1
Photoelectron Spectroscopy

• Lecture 5 instrumental details
• General spectrometer design
• Vacuum generation and measurement

2
Required spectrometer components
3
Spectrometer design considerations

• What type of samples are of interest?
• What resolution is required to gather the
  information of interest?
• Resolution of this experiment is always
  instrumental based.
• Experimental resolution is directly correlated
  with electron kinetic energy.
• What can be done to increase experimental
  sensitivity?
• Often a pay-off between resolution and
  sensitivity.
• What is it going to cost?
• Money for constructing spectrometer
• Pumping requirements
• Time for data collection

4
Sample Considerations

• Gas-Phase Photoelectron Spectroscopy
• Atoms, neutral Molecules, anions, clusters,
  etc.
• For neutral molecules, need a vapor pressure of
  10-4 torr in high vacuum at temperatures lt 500
  C
• Hot molecules will have more complicated
  vibrational/rotational contributions.
• Condensed-Phase Photoelectron Spectroscopy
• Film on conductive surface
• For valence spectroscopy, need uniform film
• (vapor deposition, SAMs, spin coating)

5
Why do we need vacuum?

• Low pressure is required for operation of
  electron detectors
• Pressure must be low enough to allow
  mean-free-path of electrons through the analyzer
• Pressure must be low enough that gas-phase
  samples are volatile
• Ultra-high vacuum is required to lower surface
  contamination for condensed-phase spectroscopy
• Vacuum pump a pump that removes gas molecules
  from a sealed volume in order to leave behind a
  partial vacuum

6
Vacuum Ranges
Atmospheric pressure 760 torr
Low vacuum 760 to 25 torr
Medium vacuum 25 to 10-3 torr
High vacuum 10-3 to 10-9 torr
Ultra high vacuum 10-9 to 10-12 torr
Extremely high vacuum lt10-12 torr
7
Examples of vacuum levels
Vacuum cleaner 600 torr
Liquid ring vacuum pump 24 torr
Freeze drying 1 to 0.1 torr
Rotary vane pump 1 to 10-3 torr
Incandescent light bulb 0.1 to 0.01 torr
Thermos bottle 10-2 to 10-3 torr
Near earth outer space 10-6 torr
Turbopumped vacuum chamber 10-6 to 10-9 torr
Cryopumped surface science chamber 10-9 to 10-11 torr
Pressure on the moon 10-11 torr
Interstellar space 10-17 torr
8
How Low Must Pressure be for a Surface to be
Clean?
If sticking coefficient S 1 And pressure
2.5 x 10-6 Torr A monolayer will form in 1 second
Lower pressure to 10-9 Torr A monolayer forms
in 1,000 seconds S is usually ltlt1
9
Methods for Vacuum Generation 1

• Positive displacement use a mechanism to
  repeatedly expand a cavity, allow gases to flow
  in from the chamber, seal off the cavity, and
  exhaust it to the atmosphere (rotary vane, scroll
  pump, roots blower)

10
Methods for Vacuum Generation 2

• Momentum transfer use high speed jets of fluid
  or rotating blades to knock gaseous molecules out
  of the chamber (diffusion, turbomolecular)

11
Methods for Vacuum Generation 3

• Entrapment capture gases in a solid or absorbed
  state (cryopumps, getters, ion pumps)

12
Vacuum measurement
Bourdon gauge gt 10-2 torr
McLeod gauge gt 10-4 torr
thermocouple gauge 760 - 10-3 torr
ion gauge 10-3 - 10-10 torr