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Photoelectron Spectroscopy

Photoelectron Spectroscopy. Lecture 5 – instrumental details General spectrometer design Vacuum generation and measurement. Electron Amplifier, Counter, and Recorder. Photon Source. Electron Detector. Electron Kinetic Energy Analyzer. Sample. Vacuum Pumps.

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Photoelectron Spectroscopy

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  1. Photoelectron Spectroscopy • Lecture 5 – instrumental details • General spectrometer design • Vacuum generation and measurement

  2. Electron Amplifier, Counter, and Recorder Photon Source Electron Detector Electron Kinetic Energy Analyzer Sample Vacuum Pumps 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 <≈ 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

  7. Examples of vacuum levels

  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 <<1

  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 > 10-2 torr McLeod gauge > 10-4 torr thermocouple gauge 760 - 10-3 torr ion gauge 10-3 - 10-10 torr

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