Photoelectron Spectroscopy
• Lecture 5 – instrumental details
– General spectrometer design
– Vacuum generation and measurement
Required spectrometer components
Photon
Source
Sample
Electron Amplifier,
Counter, and Recorder
Electron Kinetic
Energy Analyzer
Vacuum
Pumps
Electron
Detector
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
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 <≈ 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)
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
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
<10-12 torr
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
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 <<1
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)
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)
Methods for Vacuum Generation: 3
• Entrapment: capture gases in a solid or absorbed state
(cryopumps, getters, ion pumps)
Vacuum measurement
Bourdon gauge
> 10-2 torr
McLeod gauge
> 10-4 torr
thermocouple gauge
760 - 10-3 torr
ion gauge
10-3 - 10-10 torr