The diff. between ionization and work?

[taqi]

hey,
these two measurements seem to mean the same thing to me. The ionization energy is the amount to release the most loosely bound electron, while the work function is the energy to release the electrons also. So is it the same? doesn't feel like it should be.

[Lasercookie]

hey,
these two measurements seem to mean the same thing to me. The ionization energy is the amount to release the most loosely bound electron, while the work function is the energy to release the electrons also. So is it the same? doesn't feel like it should be.

It is the same.

You might even find a trial exam question that gives you an energy level diagram and asks for the work function if you're lucky. I can't think of any examples of one off the top of my head. I think the textbook might have a question like it.

edit: I think there are some different stuff about ionisation energy when you go outside of vce physics.

[Lasercookie]

Hmm.. wait. I think what my teacher has told me is wrong. I'll talk to him about it Monday. They matched up with the diagrams he showed us (perhaps they were specific to the photoelectric effect) but looking at the ones they usually give you in exams, it doesn't match up. (I'm sure I've come across it in a question somewhere as well).

But I looked at the energies exams give you for ionisation and they're significantly larger than work functions (approximately double for a few of the atoms, I think a trial exam stated this once).

I've looked up a few explanations and haven't found any ones satisfactory. I found one that seems to make sense, but the specifics tend to be a bit different with different sources.

The general idea I'm getting is that photoelectric effect, the electron isn't knocked out as a far as it is if it were ionised. Something to do with the work function, it stays closer to the surface of the atom (seen the terms valence band and conduction band, not sure of the difference) and with the ionisation energy it goes off a lot farther (seen the term highest occupied molecular orbital). I'm not entirely sure of this, seen a few conflicting statements. It's possible that I'm confusing some photoelectric effect stuff with semiconductor stuff (apparently that has something to do with work function as well).

I'll confirm this with my teacher.

Damn, I'm developing a bad habit of getting things wrong, so close to the exam as well.

[taqi]

Hmm...i dont understand how an electron can be knocked out further than another one.
Ill wait for confirmation though, the internet (google) here is sadly lacking for explanation.

[Lasercookie]

Hmm...i dont understand how an electron can be knocked out further than another one.
Ill wait for confirmation though, the internet (google) here is sadly lacking for explanation.

It's something like that http://en.wikipedia.org/wiki/Conduction_band
I think I might be confusing that idea with a different kind of work function and not the photoelectric effect work function.

I'll stop speculating since I'm pretty confused here and just see what my teacher has to say. But I'm pretty sure there is some kind of difference that accounts for it. If you convert the 'first ionisation energies' from kJ/mol to eV, you find that they are the values used in the energy level diagrams.

edit: definitely looks like that band stuff doesn't apply to photoelectric metals. Looks like it applies to crystalline solids.

Also raises the question, does the metal in the photoelectric effect ionise (i.e. become charged) - since it's losing electrons, shouldn't it become positive? If it's more positive, would the negative electrons attract towards it? If the negative electrons are attracted to the positive metal, then wouldn't that suggest that with stronger intensity (more photons = more electrons being knocked off) the photoelectric effect would come to a halt? Isn't that what happens when you increase the retarding voltage (i.e. give the metal a more positive charge I think)

...damn I promised myself I'd stopped speculating.

[taqi]

I think i have an answer to that. If your talking about the photoelectric effect in the actual experiment, electrons are being continually replace as they are ejected, cause its hooked up to a battery. So there would be no real positivization of atoms, so all is well and the universe keeps running.

Also, people have told me, if you have the exact ionization energy, then the electron is technically still within its shell, so it actually drops back down releasing a photon. Which kinda defeats the purpose of ionization but oh well.

[Lasercookie]

I think i have an answer to that. If your talking about the photoelectric effect in the actual experiment, electrons are being continually replace as they are ejected, cause its hooked up to a battery. So there would be no real positivization of atoms, so all is well and the universe keeps running.

Ah, that makes sense.

[Lasercookie]

Also, people have told me, if you have the exact ionization energy, then the electron is technically still within its shell, so it actually drops back down releasing a photon. Which kinda defeats the purpose of ionization but oh well.

So I asked my teacher today, and he said that it's basically:

Work function - this is the exact energy to get an electron to the surface. If the energy of the light equals this, then the electron will drop back down like you described. However (if I understood what he said correctly) if you've got a positive bias on the other side (e.g. if the anode is given a more positive charge) the electron will attract towards that. Without that, it attracts back into the atom.

He said that if you have any energy greater than that, then it will be ionisation energy (and the extra energy is given to the electron as kinetic energy).

He briefly mentioned it's got something to do with how the metal is structured, the lattice etc. (I can't remember the exact context that he was talking about) About the electrons from the valence band moving into the conduction band, perhaps that was correct (I didn't ask him about that). I came across this:

Einstein reasoned that is a work threshold required for an electron to escape the metal. The particular amount of work is a complicated combination of the energy required for optical excitation of a valence band electron into the conduction band as well as the energy required in the diffusion of the photo-excited electron through the solid. Thus, the work is related to the electronic structure and lattice configuration of the metal among other things. As a result, this work function is extremely dependent on the metal.

[taqi]

Okay, that clears it up. Thanks good sir.