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I've become interested in volumetric displays. In particular I'm looking at a pair of lasers scanning a volume of gas such that the intersection of the lasers would supply enough energy to cause the gas to emit visible light. Since this is the start of my research I would like to investigate a number of gases in order to determine a number of gases which might be suitable for this application. I have, however, forgotten how to determine the amount of energy needed to cause an outer shell electron to emit a photon, and then trying to back-calculate the required wavelength(s) and power requirements of the laser emitters.

I'm looking for some information on how to identify the required information for a given gas and how to calculate the energy required to cause visible light emission, and also information on how the wavelength of laser light impacts the ability of the gas to absorb the energy with the intended effect of emitting visible light. Links to introductory texts are always helpful, as are answers which help me get the terminology correct so that I can improve my searches for this information.

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Are you thinking about resonant light absorption i.e. using a laser frequency that matches an energy level in the gas atom? If so I'm not sure this will do what you want. The resonant absorption, and therefore emission, is just proportional to the light intensity so where the lasers cross the spot on only twice as bright as the rest of the beams. Your display would have a very poor contrast.

On the other hand you might be able to heat the gas and generate light. The beam crossing will only heat twice as much as the rest of the beam, but if you get the heating correct this might be enough to trigger emission only at the crossing point. However I suspect you're going to need a gas mixture e.g. one gas to absorb the laser and heat up and a different gas to respond to the high temperature by emitting light.

I did the obligatory google for "volumetric display", and I note that the Wikipedia article on it gives an example of a laser being used to heat air until it forms a plasma and glows. However that's done with a pulsed laser not crossed beams. Are you trying to build some already existing design of volumetric display, or are you thinking of inventing one?

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I was looking at the idea of using two laser beams to excite gas molecules to the point of emitting visible light. Individually the lasers would not have enough energy to do this but rather the combined energy input to the gas where they intersect being high enough to raise outer-shell electron energy to emit a photon. I read a few papers on using infrared lasers (which would be invisible to the eye) in order to help contrast, but before I even get that far I'm trying to figure out how to determine how much energy is needed and what the effect of the laser wavelength is. –  akohlsmith Oct 1 '12 at 18:27
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You can't excite an electron with two electrons of half the required energy. A photon with less than the required energy is just ignored. –  John Rennie Oct 1 '12 at 19:48
    
I am not sure if that's entirely correct. two-photon electron microscopy (en.wikipedia.org/wiki/Two-photon_excitation_microscopy) suggests that two lower-energy photons can occur, although I am NOT saying I understand the article completely. felix3d.com/web/download/paper_pw_03.pdf also mentions two-frequency, two-step upconversion which is where I got the idea in the first place. It specifically mentions optically-active atoms, molecules or ions as a requirement. –  akohlsmith Oct 1 '12 at 20:23
    
The key thing you need for this idea to work is a nonlinear interaction. This can be something like multiphoton excitation, or something more complicated, like one laser which pumps atoms to a dark triplet state and another which stimulates phosphorescence from that state. I'm sure it can be done, but this is not anything people haven't thought of before, and there are much easier/better methods of volumetric display. –  user2963 Oct 1 '12 at 22:15

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