|MadSci Network: Physics|
Dear Simon, First, I want to confirm your reasoning and your conclusions. Indeed, if you can “pump” an intense beam of specific, low-energy photons into a material, you will be able to electronically excite it and induce photoemission. This is the basis for such techniques as “Infrared Multiphoton Ionization” (IRMPI) and “Resonance-Enhanced Multiphoton Ionization” (REMPI). The specificity of the photon energy depends on the energy transitions you are exciting within the material, and thus depends on the electronic make-up of the material. It also depends on the “free carrier lifetime” (how long the electrons stay energized before decaying) and give up their energy to the crystal lattice in the form of heat, etc. This research into photon-matter interaction has exploded since the advent of laser technologies into the physical sciences in the last century. Second, I want to answer your specific question….In the photoelectric where DO electrons come within the metal? They come from the highest occupied electronic state within the metal. This electronic state, named the “Fermi level”, is near the middle of a series of closely-packed electronic states called a “band”. That part of the band below the Fermi level are states full of electrons and that part above are states empty of electrons ( defined at absolute zero temperature). But since this is a band of nearly continuous states, electrons - in a spontaneous manner – are “free” to roam from the Fermi level upto the empty or “unoccupied” states above the Fermi level. This process is called “conduction”, and the band as a whole is called the conduction band. Thus, the photoelectrons may also be thought of as “free electrons”, but this is not exactly correct. So what is the nature of the “photoelectric work function”? The work function for a metal in Einstein’s Nobel-winning equation is the difference in energy from the top, unoccupied level, of the conduction band and the Fermi level – from which photoelectrons arise. The key to all of this is to realize that the discrete atomic levels of all the metal atoms are highly interactive in a crystal lattice of that metal. This leads to a “continuum” of states – the conduction band. For an understanding of the photoelectric effect in semiconductors and insulators a direct quantum mechanical analysis is necessary. This necessity arises from the fact that in semiconductors and insulators, the highest occupied electronic state is energetically separated from the unoccupied levels. This is called a “band gap”. Two separate bands are formed. The occupied levels are within the lower “valence band” and the unoccupied levels -as before - are within the upper “conduction band”. But now, electrons cannot spontaneously roam free and conduction cannot occur. But this is leads us into another, interesting subject area. The key physical concept which Einstein illustrated in his work on the photoelectric effect is this: light is made up of discrete particles, quanta, we call photons. In the 19th Century, Newton’s great insight that light was made up of photons was discarded in favor of a “wave theory about light”. Einstein’s worked helped correct this fallacy. You may be taught in the educational process you’re enduring that there is a thing called the “wave-particle duality” about light…that light sometimes acts like a wave; sometimes like a particle. Understand what your teachers explain about the empirical phenomena and the calculations related to interference, diffraction, etc. but ignore the physical concept that they might say explains it – the so-called “wave theory of light”. Light, as we measure it, ALWAYS acts like particles - like photons - but in statistical manner. Archaic traditions in science sometimes die hard. I’ll end with a quote from Richard Feynman from his book “QED” (Chapter 1). He won the Nobel Prize in 1964 for his explanation of quantum electron dynamics: “I want to emphasize that light comes in this form – particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves. I’m telling you the way it DOES behave – like particles.” ---* Dr. Ken Beck
Try the links in the MadSci Library for more information on Physics.