Is fluorescence at shorter wavelength possible?

Question

Is it possible for a compound to absorb light of a certain wavelength and emit light of a shorter wavelength? Normally compounds emit light of a longer wavelength like UV to visible. I know shorter wavelengths have more energy so perhaps it has to absorb 2 photons for every 1 it emits.

Answer 1

Yes.
In searching around for examples of experiments demonstrating this, I came across a patent titled "Laser scanning fluorescence microscopy with compensation for spatial" which discusses this. The patent is described here:
http://www.patentstorm.us/patents/6449039-description.html

Note particularly this statement:
"Various fluorophores can absorb two or more photons of relatively long wavelengths simultaneously when sufficiently intense illumination light is applied to them, and will fluorescently emit a photon at a shorter wavelength than the wavelength of the incident light." This is precisely what you have described in your question.

Although a description in a patent isn't necessarily the same as a published study, I would imagine there are published experiments that would document this.

Answer 2

That is an interesting question. An energetic photon is captured by an electron orbiting an atom (or molecule) and is kicked up into a higher more energetic orbit. The electron will return to its rest orbit in one or more steps yielding one or more new photons with wavelengths consistent with the amount of excess energy released in each step. If an electron can capture two or more photons before returning to its rest state, and then returns in one step it should release all the excess energy in one step producing a highly energetic photon of shorter wavelength. X-rays are highly energetic photons produced by bombarding a metal crystal with electrons which move the metal's electrons to very energetic orbits.

Answer 3

Be careful of your terms. The emitted light is of a _lower_ frequency and _longer_ wavelength than the incident. "Shorter" frequency does not make sense, or at best, is confusing. The reason is because there is always energy lost in the process. Some of the incident energy is given back as the light that you see, but other energy is lost to other factors, such as changing momentum.