# I About standing waves and reasonance

1. May 16, 2016

### KFC

Hi there,
I am reading a book regarding fundamental atomic physics, in which it introduces one kind of electronic scattering called Kapitsa–Dirac effect. I read the some introduction in wiki https://en.wikipedia.org/wiki/Kapitsa–Dirac_effect, and it states that the effect was first observed in diffraction of electrons from a standing wave of light. I understand most of the statements in the wiki page. But in other materials, they refer to the similar stuffs with "resonant standing wave" of light. This is confusing to me on the term "resonant". To my understanding, in the text of fundamental physics, standing wave is associated with resonance because it will be formed when the frequency of the "input oscillation" satisfied to some condition. But in other reference on Kapitsa–Dirac effect or in the above wiki page, I saw a term "near resonant standing wave laser field". I don't understand why it said "near resonant", what does it really mean?

Thanks.

2. May 17, 2016

### Staff: Mentor

The effect in atoms involves the exchange of momentum between the atom and the light field through absorption and stimulated emission. The effect is greater closer to an electronic transition in the atom (this is where the probability of the coherent scattering process, simultaneous absorption and emission of photons, is highest). But for light that is resonant, the two-step process becomes important, where the atom first absorbs a photon, and then some time later emits a photon through stimulated or spontaneous emission. The latter introduces noise. So the best situation is to be near-resonant: close enough to the transition such that photon scattering is high, but far enough that absorption followed by spontaneous emission is low.

3. May 17, 2016

### KFC

Thanks. But how close to the resonant frequency is said to be "near"?

4. May 17, 2016

### Staff: Mentor

In the paper found at http://dx.doi.org.proxy.ub.umu.se/10.1103/PhysRevLett.56.827, they work with sodium, using the 3s 2P3/2 state as the excited state, with detunings of the order of 100 to 1000 MHz. That excited state of sodium has a linewidth of about 9.79 MHz, so the detuning is 10-100 Γ.