Reply to phosphorescence

[arcticpenguin]

Sorry, tried several times and could not reply to the phosphorescence thread.

This sounds a lot like image plates, refered to in this link as computed radiography. A carefully engineered material is exposed to radiation, usually X-rays, and it stores that image. The image is read out by scanning with a laser of a second wavelength, which causes the material to emit photons at a third wavelength. The plate can then be 'erased' by exposure with a bright light, typically UV.

It all has to do with electron orbitals. The specific choice of wavelengths for image acquisition, scanning, readout and erasing will vary with the material.

 

[MRC_Hans]

Mmm, I'll try to respond here, then :

OK, made the expriment last night.

Equipment:
One phosphorescent Stegosaurus skeleton model.
One 1mw red laser 630-660nm
One camara flash
One dark room

First, I placed the Stegosaurus in the dark room. Since it had just been taken out of a closet, it was emitting no visible light.

Then I shone the laser at part of it for about ten seconds. This resulted in a faint phosphorescense in those areas.

Then I irradiated the model with the photo flash. Result was a bright phosphorescense all over. Several consequtive flashes did not raise the level further.

I then again shone the laser at part of the model. After some seconds of laser irradiation, the phosphorescense was visibly reduced in the affected areas.

Conclusion:
1) The laser is only cabable of very weak exitation of the phosphorous material.

2) Irradiating with the laser "discharges" the strong phosphorescense resulting from the photoflash.

My thesis:
As several flashes whithin a short interval do not raise the charge perceptably, I must assume that irradiation not only charges some non-charged atoms, but also discharges some of those already charged. It is then possible that the low energy red light from the laser is cabable of triggering more discharges than charges.

Condition for this to be true: The energy required to trigger discharge is lower than the energy required to charge a phosphorous atom (mmm, or is it a molcule?). I do not know if this condition is true.

Hans

 

[repcon]

As AP suggests, maybe it's emitting light at a different frequency, IR possibly? Neat experiment anyway.

 

[JamesM]

I'm not a photochemist, so massive apologies in advance if I'm teaching anyone to suck eggs, but here's a simple model of phosphorescence:

1. Our molecule begins in the ground state, S0.

2. It then absorbs electromagnetic radiation (visible light in this case) and is in the excited state, S1.

3. There is then a "spin-forbidden" transition (called an "inter-system crossing") to an excited triplet state, T1.

4. Finally, there is a spin-forbidden transition from T1 back to the ground state, S0, which occurs with a loss of energy in the form of a photon - that's the light we see.

I can only suggest that the imposition of the ~600nm laser light excites either the S1 or T1 state to some other excited state (let's call it T2), which then decays back to S0 by either emission of a photon which isn't in the visible light range or that there is a whole series of smaller decays and/or internal conversions that dissipate the energy through, for example, heat to the surrounding molecules, which is why you don't see it (which is pretty much what AP and repcon have already suggested).

Without knowing what molecule is used in glow-in-the-dark products (maybe they use more than one), it would be hard to know which of these scenarios is more likely - there could well be a whole series of excitations, decays and non-radiative transfers occurring.

And, of course, there is the very likely scenario that my guess is completely and utterly wrong.

 

[Zombified]

I can only suggest that the imposition of the ~600nm laser light excites either the S1 or T1 state to some other excited state (let's call it T2), which then decays back to S0 by either emission of a photon which isn't in the visible light range or that there is a whole series of smaller decays and/or internal conversions that dissipate the energy through, for example, heat to the surrounding molecules, which is why you don't see it (which is pretty much what AP and repcon have already suggested).

Could be stimulated emission. You have an atom in an excited state, which produces a photon of a given frequency when it decays to a lower-energy state. If you shine a light on it of similar frequency, you increase the probability of the emission - the EM wave changes the orbit slightly, which results in some probability that the electron changes to a different state.

Stimulated emission is what is amplifying the coherent light in a laser, though any stimulated emissions in the material would be an entirely seperate process from what's going on in the laser diode.

Just a guess, I'm not a photochemist either.

 

[Walter Wayne]

With the usual I am not a physicist caveat ...

I believe phosphorescence has to do with bistable materials. A bistable material has several states which are stable. One may be more energetical favourable than the other, but both states remain stable because their is a high energy transitional state that an electron must go to to get from one stable state to the other.

One could visualize this as two valleys seperated by a peak. An object in the shallow valley has more energy than an object in the deep valley. However, it will not spontaneously fall into the deeper valley without first being pushed up onto the peak.

In phosphorescent material, one can put it in conditions which push many electrons into the higher of the two stable states (the shallow valley). I assume the laser in this case performs the function of pushing the electrons up to the transitional state (over the peak) so they can all get to the ground state (deep valley) at once. If no laser is used, the thermal energy that the electrons have will cause a slow discharge where occasionally an electron makes it to the transitional state and then emits a photon on the way down to the ground state resulting in a steady glow.

Hope I made sense. Also hope I was accurate.

Walt

 

[Zombified]

I think we're saying similar things. Although high-energy states can decay to low energy states, there are additional conditions that have to be fulfilled. Photons are spin-1 particles, so conservation of angular momentum rules out decays that don't change the atom's spin by 1 as well. If those are the only decay routes available, then a more complex process is required for the decay, and that generally means a much slower process.