What is the difference between photoelectrons and thermionic electrons?

Question

The photo electric effect tells me that light is a particle and electrons are dislodged by direct collisions between photons and electrons.

However, why is it that thermionic electrons can be build up by gradual accumulation of energy?

Answer 1

In the means of production. Both come from a piece of material, but photoelectrons are produced when you shine light on a metal, whereas thermionic electrons are emitted when you heat up a metal (usually by passing a current through it). Because they are produced in different ways, the electrons come out with different energy distributions.

Edit:
Photoproduction of electrons usually occurs by the absorption of a single photon, which has a well-defined energy. If the photon has too low an energy (ie too low a frequency, too long a wavelength) then it doesn't kick out an electron. Odds are the electron which absorbed it won't absorb another--its just statistically unlikely unles the light is extremely intense. So this electron doesn't leave the metal and the current of photoelectrons is basically zero. It will remain so until you change the color of the light.

On the other hand, heat is distributed among all the parts of the metal, and if the metal is sufficiently thermally insulated, stays around, jiggling the atoms more and more. In a usual thermal ionization experiment, you add much more energy to the metal than you do with light. The more you add, the more is around for an electron to get, and the energy is not "monochromatic" the way the light energy is: it comes in lumps of many sizes. The average size of the lump depends on the total amount of energy you've put into the metal. So you can build up a store of energy until the lumps are big enough to kick out electrons.

Notice that it's all just energy, and if you heat the metal up with enough light you can get both thermionic emission and enhanced photoemission of the pre-heated electrons, as well as double-absorption photoemission.

Answer 2

A photon is a quantum of power. An electron is a charged subatomic particle. A photoelectron is an electron it really is knocked out of its orbit via soaking up a photon of sufficient power to split it from its unique atom.

Answer 3

http://en.wikipedia.org/wiki/Thermionic_emission

try this link