Photon Energy for Ionization
How do I know when the energy of a photon is enough to
ionize a particular atom?
Look up the "first ionization potential" of that atom.
The lowest is 3.9 eV for Cesium (Cs) in column-1 with sodium,
The highest is 24eV for Helium.
The photon needs to have at least that much energy to ionize the atom using only one photon.
The gentlest are near-UV: (1.25 eV.micron) / (3.9 eV) = ~0.3 micron.
Anything above about 6 eV is considered "vacuum-UV" (VUV):
it is absorbed by oxygen or air and needs a rough vacuum to be used well.
Besides, it is hard to find sources for anything shorter than about 0.2 microns.
So if you are trying it in the lab, you will really want to stay with the easiest metals.
Because far-VUV sources are rare,
2-photon and multi-photon ionization happens as often as 1-photon ionization, in practice.
Multi-photon ionization requires near-simultaneous encounters,
so it happens more often (relative to single-photon ionization)
as instantaneous intensity goes up.
Such as when you have a pulsed laser and the instant power is high,
and it is focused so the intensity is even higher.
In such conditions,
if you are lucky the first photon's energy matches an excited energy level of the atom,
and the second photon ionizes the excited atom, and the ionization rate can be pretty good.
The lifetime of the excited state is how close-together the two encounters must be for this.
Even if you are not so lucky,
two photons occasionally pass one atom so near simultaneously
that Heisenberg uncertainty broadens an energy state enough to include the first photon,
and again the photons' energy adds together to boost an electron off the atom.
In this case the ionization rate is lower, but still achievable with good lasers.
Multiphoton ionization is why a strong pulsed laser can make a spark in the air at it's focal point,
even though the photon's wavelength is visible or near-UV (not VUV).
Click here to return to the Physics Archives
Update: June 2012