Surface Science Essay Example

  • Category:
    Physics
  • Document type:
    Essay
  • Level:
    Masters
  • Page:
    2
  • Words:
    865

Surface Science 11

Surface Science

Surface Science

Electrons are negatively charged particles which rotate in the orbitals. The electrons presence of electrons determines the chemical, physical and electrical properties of a material. Photoelectron spectrometers are instruments which are used to measure the energy of electrons emitted from any substances of matter by use of the photoelectric effect.

There are various approaches taken for electron detection in photoelectric spectrometers. The following essay takes an insight on three approaches which are in common use. The essay is being done as a assignment in Physics.

Auger Electron Detection – Auger Electron Spectroscopy

The approach is based on the principle that when an electron is upon ejection from the inner shell of an atom, a vacant position is left. The vacancy can then be occupies by an X ray which is radiative or Auger which is nonradiative. In auger electron spectroscopy, there is ionization of the atomic core levels. This ionization is done by the incident electron beams and ends up to the detection of the Auger electrons emitted by the spectroscope.

Construction

According to (Chourasia &Chopra, 2007) the Auger electron spectrometers constituted of

  • UHV environment

  • Electron gun

  • Electron energy analyzer electron detector

  • Data recording, processing and output system

In this essay focus is on the electro detector. The detector receives electrons which have passed through the analyzer hence the electrons have different energies.

The other type of spectroscopy in common use is X ray photoelectron spectroscopy (XPS)

Back scattered electron detection

The detection of back scattered electrons is done by the use of PN junctions. The junctions are normally situated at the lower side of the objective lens and are usually placed at a high take off angle (Wittke, 2008).

Construction

he following diagram illustrates typical back scattered electron detection.Student’s Name:T

Current flows when backscattered electrons hit the semiconductor. The current produced is determined by the amount of ions striking the PN junction.

Limitations

The resolution of backscattered electrons in space is low hence leading to clarity problems.

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Secondary detection approach

Secondary electrons are released when ions hit a surface. The electrons which are situated on the outermost region of an atom are released (Wittke,2008).

Construction

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Item function

  • Photomultiplier

  • Collector grid (charged)

  • Detects secondary electrons

  • Gathers low energy secondary electrons

In addition, the scintillator’s outer layer is coated with aluminum enables the electrons acceleration t the scintillator and also acts as a mirror, reflecting the photons produced.

Limitations

The approach can only be used for low energy electrons hence necessitating electron multiplication.

Cathodeluminiscene

The approach makes use of a photomultiplier tube which is used in the detection of cathodoluminiscent signals (Wittke,2008). Unlike some approaches, the amplification of photons is direct as opposed to initial conversion of electrons into photoelectrons.

The cathodoluminiscent portrays two kinds of information:

Intensity

Wavelength

Images of information display in cathodolumiscent detection.

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Some constructions in the approach enable the user get access to wavelength information. In order to be able to do this, a diffraction grating commonly known as monochromator is used.

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The intensities gotten in such a setup are then displayed on the computer in a manner similar to this.

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According to electron detectors and spectrometers (2011), some models of detectors made with respect to the various approaches include:

  • Gas filled detectors

Gas filled detectors

The electron fragments which make it through the spectrometer’s analyzer are directed to a mass spectrometer detector. The detector also acts as a multiplier hence it is sometimes referred to as a multiplier. The detector uses an approach known as ‘secondary electron emission’ (NASA, 2011). The electrons which are situated on the outermost region of an atom are released (secondary electrons) when ions hit a surface.

Construction

The amount of secondary electrons released is determined by the nature of particle, the sticking angle, nature of surface being stricken and the energy of the ions.

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The multiplier a type of channel electron multiplier since it is constituted of channels. Secondary electrons occur when the ions hit the walls of the inner surface. The electrons’ speed is increased in the tube by passing the electros via an electric field. The tube of this multiplier is also curved so as to ensure that there is no ion feedback which happens when the residual molecules are ionized inadvertently and afterwards accelerated hence resulting to ion formation.

A typical display in gas detectors is as shown in the figure below:

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Some of the gas filled detectors include:

Geiger Muller counters

Proportional counters

Ionization chambers

Gas filled detectors can either be gas flow or sealed detectors. The difference lies in that in gas flow, the gas is changed continuously while in sealed detectors the tube is sealed hence no gas flow.

Other detectors are:

  • Channeltrons

  • Semiconductor detectors

  • Electrostatic spectrometers

  • Magnetic detectors

  • Di-lepton detectors

  • Cherenkov detectors

References

Chourasia A. & Chopra, D. (2007). Auger electron spectroscopy, handbook of instrumental

techniques for analytic chemistry (1st edition) ,Texas A&M university – commerce

department of physics.

Electron detectors and spectrometers , (2011). Detectors, Electron detectors and spectrometers,

3(2), 16-27

Nasa (2011). Mass spectrometer – detector, atmospheric experiments laboratory Nasa, 2(6),11-

Photonisis (2011). Retrieved on 27th August 2010 from http://www.photonis.com/

Wittke, J. (2008). Wave length dispersive spectrometry (WDS), PHOTOELECTRICITY, 5(3),