Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Few questions about photoelectricity

  1. Feb 12, 2016 #1
    First - Basic photoelectric effect demonstration setup: A charged battery with a gap in the metallic junction between the poles placed relatively close from each other, but not close enough for measurable current to occur spontaneously. Emit photons of sufficient energy towards the gap surfaces - sparks start flying. Is this correct?

    Second - Is compton scattering classifiable as a similar kind of phenomenom taking place in a different kind of situation? I've understood that Compton scattering occurs in a situation where you emit a photon at convenient angle and sufficient energy and it hits an atom/molecule causing an electron emission from the atom/molecule, leaving the atom in an ionized state as an immediate effect - later obviously the electron could be replenished from some other source. If you do this experiment in a void with a large margin to any electron attracting sites, would the emitted electron just continue along its movement vector through the void?

    Bonus question - Electron density on the surface of a metal can be set into oscillating motion with photons. You emit photons, energies of which are below the work function of the metal, thus causing no photoelectron emission, but plasmonic 'ripples' on the metallic surface. Is this correct? I would assume that the occuring plasmonic waves are different at different photon energies, but does the angle of the colliding photons affect them in any distuingishable way? Does it cause maybe increased phononic effect at steeper angles, and a more pronounced plasmonic effect at more tangential angles?

    Thanks in advance! I will ask a crazy bonus question if my world doesn't get turned upside down by the grace of someone more versed in the topic.
  2. jcsd
  3. Feb 12, 2016 #2
    some additional information on photo emission;
    three step process;
    1. Inner photoelectric effect The hole left behind can give rise to augur effect which is visible even when the electron does not leave the material.
    2. In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy. The inner photoeffect has to be dipole allowed.The transition rules for atoms translate via the tight binding model onto the crystal.They are similar in geometry to plasma oscillations in that they have to be transversal.
    3. Ballistic transport of half of the electrons to the surface. Some electrons are scattered.Electrons escape from the material at the surface.
    In the three-step model, an electron can take multiple paths through these three steps.

    Since the energy of the photoelectrons emitted is exactly the energy of the incident photon minus the material's work function or binding energy, the work function of a sample can be determined by bombarding it with a monochromatic x-ray source or ultravoilet source, and measuring the kinetic energy distribution of the electrons emitted

    At the high photon energies comparable to the electron rest mass energy compton scattering can/ may take place. Above twice this energy a pair production is expected . Compton scattering and pair production are two other competing mechanisms.

    Indeed, even if the photoelectric effect is the favoured reaction for a particular single-photon bound-electron interaction, the result is also subject to statistical processes and is not guaranteed, as a photon has excited (usually K or L shell electrons ).
    The probability of the photoelectric effect occurring is measured by the cross section of interaction, σ.
    This has been found to be a function of the atomic number of the target atom and photon energy.
    A crude approximation, for photon energies above the highest atomic binding energy,
    sigma proportional to nth power of atomic number Z^n /E^3 ... n is of the order 4-5.
    (At lower photon energies a characteristic structure with edges appears, K edge, L edges, M edges, etc.) The obvious interpretation follows that the photoelectric effect rapidly decreases in significance, in the gamma ray region of the spectrum, with increasing photon energy, and that photoelectric effect increases steeply with atomic number.

  4. Feb 15, 2016 #3
    Okay thanks! It's been a long time I did any calculus on any of these kind of things (read: tried to really understand), but I take it that as a tourist approximation, I'm somewhere in the proximity of the right track.

    Extra question.

    http://imgur.com/i9jWmFE direct link if the pic is not working

    I leave further specifications out of this, but as far as I know electrons have a mass, albeit small. Imagine the crudely presented structure in 3-d. What do you reckon would start happening in this system? I have no idea if this is practically feasible, but the point I'm going for is, could it be possible for this kind of nanoscale structure to produce a steady flow of free electrons in the direction of the photons? Could this produce thrust? Minuscule thrust obviously, but a considerable thrust to mass ratio?

    And just FYI, the photon stream is supposed to pass close enough to the metallic interface in order to allow interaction.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Few questions about photoelectricity