# Conservation of energy and momentum in photoelectric effect

Sometimes it is shown that in a Compton scattering it is not possible that the photon transfers all it's momentum and energy to the electron, see for example here:

If one assumes complete energy and momentum transfer the corresponding energy equation is:

$$h\nu + m_0c^2 = 0 + \sqrt{m_0^2 c^4 + p^2c^2}$$

where $\nu$ is the frequency of the incoming photon and $m_0$ the rest mass of the electron and $p$ the momentum of the electron after the scattering.

The momentum equation is:

$$\frac{h\nu}{c} + 0 = 0 + p$$

Substituting for this for $pc$ in the energy relation gives:

$$h\nu + m_0 c^2 = \sqrt{m_0^2 c^4 + h^2\nu^2}$$

Squaring both sides results by the binomial formula in $2h\nu m_0 c^2 = 0$ which is a contradiction since $\nu \neq 0$ and $m_0c^2 \neq 0$.

In the photoelectric effect however the photon seems to give all it's momentum and energy to the electron? I guess that the corresponding conservation laws are nevertheless valid because the atom or complete metal will get some energy any momentum, but how can one see this and the difference to the compton effect in formulas?

In the photoelectric effect however the photon seems to give all it's momentum and energy to the electron?

No, the photoelectrons are emitted with a range of energies. The well known expression:

$$E = h\nu - \phi$$

gives the maximum energy, but photoelectrons are emitted with energies ranging from zero to this maximum value.

The emission of photoelectrons is a messy business. The incident photon creates a photoelectron with a high quantum yield, but this electron has its momentum pointing in the same direction as the incident photon i.e. down into the metal. Most of the time the electron just transfers energy to the lattice, but around $1$ time in $10^5$ to $10^6$ the electron will either ricochet out, or it will hit other electrons and eject them from the metal surface. That's when we observe a photoelectron, and given the random nature of the process it's no surprise that the ejected electrons have a spread of energies.

• Ok, so the $\phi$ represents that the solid takes some part of the energy to fulfill energy conservation. But how would be the equation for the momentum conservation? Feb 4, 2015 at 17:39
• The initial electron scatters off phonons and other electrons within the metal, so tracking the momentum changes is basically impossible. I suppose the chunk of metal would change momentum to balance the momentum of the ejected electron. Feb 4, 2015 at 19:08

First your statement "In the photoelectric effect however the photon seems to give all it's momentum and energy to the electron" is not correct. In Photoelectric, the photon gives 'all' its energy to the electron, but most photon momentum goes to the more massive atom, the bind force between the electron and the atom serves as a transient of momentum. Due to the large mass of an atom, or solid, compared to the electron, the system can absorb a large amount of momentum without acquiring a significant amount of energy.

The equation of conservation of energy would look like:

$h\nu + m_0c^2 + w = \sqrt{m_0^2 c^4 + p^2c^2}$

where $w$ is the work function: it is the work required to remove the electron from the metal, in other words it the work to be done to overcome the binding force between the electron and the atom. In case of Free electron the work function $w=0$, in case of bounded electron $w$ would have a value.

For the conservation of momentum: $\frac{h\nu}{c} + 0 = p_1 + p$

where $p_1$ is the momentum acquired by the atom, and $p$ is the momentum acquired by the ejected electron and of course $hv/c$ is the momentum of the incident photon. I hope that helped.