Photoelectron Spectroscopy

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

PhotonSource

Sample Electron KineticEnergy Analyzer

ElectronDetector

Electron Amplifier,Counter, and Recorder

VacuumPumps

Required spectrometer components

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

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)

Sample Considerations

• 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

Why do we need vacuum?

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

Vacuum Ranges

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

Examples of vacuum levels

How Low Must Pressure be for a Surface to be “Clean”?

If “sticking coefficient” S = 1And pressure = 2.5 x 10-6 TorrA monolayer will form in 1 second

Lower pressure to ~ 10-9 TorrA monolayer forms in 1,000 secondsS 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

McLeod gauge> 10-4 torr

Bourdon gauge> 10-2 torr

thermocouple gauge760 - 10-3 torr

ion gauge10-3 - 10-10 torr