Photoelectric Effect: Understanding Kinetic Energy of Electrons

[Esfand Yar Ali]

It is explained in a book that to know the kinetic energy of the photo-electrons ,we reverse the polarity so that the minimum energy can be known.but I want to know that if we are incidenting same frequency of light on the metal surface and we also know that energy is directly proportional to freaquency then the kinetic energy of the ejecting photo-electrons should be same,but it was written in that book that kinetic energy of the ejecting electrons are diffrent.please explain this phenomenon

 

[UltrafastPED]

There are several ways to find the work function of a metal:

Here is a nice description of the thermionic method:
http://avs.org/AVS/files/96/96788451-2031-48b4-a768-73b94102cd5e.pdf

When you analyze the lab technique used with photo emission, you will find some similarities with the thermionic method: http://demoweb.physics.ucla.edu/node/118

 

[Esfand Yar Ali]

I need some explanations about this phenomenon of 'diffrent velocities' of the ejected electrons,the links you've given have no explanation on this

 

[ZapperZ]

This has more to do with the nature of the material, and thus, requires an understanding of solid state physics.

Photoelectric effect are typically done on metals, which has a conduction band consisting of "free" electrons. This is an energy band (google it), the highest of it (in a relative term) are the electrons at the Fermi energy (at T=0K).

When a photon hits a metal, that photon has the probability of exciting electrons with a range of energies, not just the ones right at the Fermi energy. It can excite those below those electrons as well. If the electrons that are excited below the Fermi energy will acquire a different energy than the one excited from the Fermi energy, because these electrons are "deeper" in the energy "well". So they will end up with a different kinetic energy.

Since this band is continuous, you can expect, with bombardment of many photons, electrons being emitted with a range of energies.

Zz.

 

[Esfand Yar Ali]

I think this explanation is good Zapper