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Can a detuned laser can excite an atom? If so, how is this possible?

Waffle's Crazy Peanut
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dhmee
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2 Answers2

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Yes, a detuned laser can excite an atom. The result is that the atom undergoes Rabi oscillations with a different Rabi frequency, and an amplitude that gets smaller as the laser frequency moves away from resonance with the atomic transition frequency.

Because of the very large number of photons constituting the laser beam, one can think of it as an oscillating classical field. The effect of the field is to deform the charge distribution of the electron clouds surrounding the atom, so that the probability of finding the atom in the ground or excited states oscillates as a function of time.

You can think of this as like a classical oscillator with an external driving force. If you have a mass on a spring, you can make it oscillate at any frequency you like by shaking the end of the spring back and forth. However, the amplitude of the forced oscillations will get smaller as the frequency of your shaking moves away from the natural frequency of the mass-spring system.

Mark Mitchison
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Even one Photon can excite atom, not speaking about a flow of them (laser).

Being tuned or not, atom excitement happens because electron of the atom can absorb photons, and jump into further orbits, then we call the atom exited.

So even one photon can be absorbed by the electron to excite atom.

But please note that not any photon can be absorbed by a particular atom, it's energy should be exactly the energy difference between the two orbits that electron will jump between, and those orbits are discrete, not any orbit is allowed, due to Quantum mechanical effects.

TMS
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    This is missing the point of the OP's question. The effect of a laser cannot be understood solely in terms of the single-photon processes you describe. The laser does not need to be on resonance with the atomic transition in order to excite the atom; see my answer. – Mark Mitchison Mar 25 '13 at 01:49