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If a gas is illuminated with a laser of a frequency less than (but more than half of) the work function of the atoms in the gas, the atoms will be excited into Rydberg atoms.

Is there a finite chance that the Rydberg atoms themselves will be hit by a second photon and thus be ionised?

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    $\begingroup$ I talk about multi-photon processes in an earlier answer. At high enough intensitites there isn't even the need for a intermediate orbital. $\endgroup$ Dec 21, 2015 at 14:02
  • $\begingroup$ In a gas of Rydberg atoms, Rydberg-Rydberg collisions can also cause ionization through transfer of energy from one Rydberg atom to another. $\endgroup$
    – tmwilson26
    Dec 21, 2015 at 14:03
  • $\begingroup$ Search around for "passive Q-switch" and "saturable absorber" for examples of this sort of thing happening, although not necessarily for gases. $\endgroup$ Dec 21, 2015 at 16:43

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It is also possible for the ionization to take place via a nonlinear optical two-photon absorption (two photons being absorbed simultaneously). These events are more improbable than the single photon absorption, but their probability scales with the intensity of the light source so high power laser irradiation is certainly capable of producing them.

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  • $\begingroup$ Two-photon absorption has even become a lively research and trcnology topic these days. Neural researchers use it to select and activate individual neurons, and if you search the web, you can find amazing micro-sculptures created by using two-photon transitions. $\endgroup$ Dec 22, 2015 at 2:20
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Yes, an excited state can absorb more energy. So multiple photons can excite an electron through multiple levels, over time (assuming the orbital doesn't lose its energy in the mean time). Of course, no single photon can excite more than its energy.

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