Talk:Phosphorescence/Archives/2021

Persistent luminescence

Both this page and persistent luminescence are trying to claim europium-doped strontium aluminate as belonging to their own class of glowing mechanism. The PL page seems to claim that this common use of "phosphorescence" in sources is merely an incorrect name for PL due to some non-triplet mechanism involved. Need some expert in quantum chemistry to help sort this out. Artoria2e5 🌉 11:36, 6 February 2021 (UTC)

Ok, the first thing to understand is that the terms "fluorescence" and "phosphorescence" are used differently in scientific contexts than they are to the general public. There is nothing wrong with that, and in fact it's very common. For example, this needed to be dealt with in the glass article. To the general public, the term "glass" refers to silicate glass exclusively, but in a scientific context, a "glass" is any material that exhibits a glass transition, including most plastics, porcelains, chocolates and candies, and even water or metals can become glasses if cooled correctly.

The thing to understand is that the general definition is a lot older than the latest scientific definition, and it's not within itself incorrect, just different. Behind the times maybe, although the two may never match up. That's just the way language works. It constantly changes in illogical and often unpredictable ways, and no encyclopedia or dictionary has ever been able to control the ever-changing language. We just have to work within the constraints the language provides us, and as a general encyclopedia, we have to try to explain both uses of the term.

To the general public, fluorescence is fast and phosphorescence is slow. If it glows under a blacklight, it's fluorescence, and if it glows in the dark, it's phosphorescence. That's why a fluorescent lamp is called that, even though the coating of phosphors is technically phosphorescent.

In the scientific sense, more recent studies have shown that we can further divide this phenomenon into different atomic/molecular transitions, and categorize them by the typical timescales those transitions fall into. In this case, the fluorescence decay time (not to be confused with the fluorescence lifetime, or the time between absorption and emission) the fluorescence decay time can be counted on the order of tens to hundreds of nanoseconds, while true phosphorescence is is on the order of a few microseconds to 1 second.

Anything beyond 1 second is scientifically classed as persistent phosphorescence (aka: persistent luminescence). This is probably a better term. This is basically phosphorescence slowed way down due to defects in the crystal structure (ie: a missing atom, an interstitial or substitutional atom (see Alloy for diagram) etc... True, this is a related but different mechanism from true phosphorescence, and is the mechanism of anything we commonly think of as glow-in-the-dark, but to the general public they're both the same thing.

That's why I feel that this is the place to explain it. This should really be the "parent article", in which the general concept is laid out, and then the two different mechanisms are explained, and then from here a "main article" link should go to the more technical, in-depth article. Zaereth (talk) 21:07, 6 February 2021 (UTC)

I did a little more reading on the subject, and I'm going to use a simple metaphor here on talk, partly for you but partly for myself, to help formulate in my head how o best phrase it in the article. Solely for the purposes of this discussion, I'm going to use the metaphor of a flywheel. A flywheel is an object that stores energy in the form of a spin. A flywheel is what keeps your car engine running between the spark plugs firing. Some of the rotational energy of the engine is stroed in the spinning flywheel, and that energy is used to complete things like compression strokes even when no cylinder is firing to produce the energy.

You can use this same metaphor to describe inductance in an electrical circuit. Inductance creates eddy currents in things like wires, and the energy is stored in the spin of those eddies.

In the case of atoms and molecules, the terms "spin" refers more to some mathematical quality than anything we can pin down as being a property of actual rotation, but nevertheless, I think a flywheel analogy will still work in this case ... a least, here on the talk page (probably not for mainspace).

In fluorescence, a high-energy photon strikes the electron of, say, a carbon atom in a dye molecule. The photon is absorbed, and that electron is literally thrown out into a higher orbit, meaning it has higher energy. The electron "wants" to be at its lowest energy state, so it does the inverse of absorption, and it emits a photon, rapidly releasing its excess energy, and dropping into its lower orbit.

With triplet phosphorescence, That high-energy photon strikes the carbon atom, knocking it's electron out of orbit, but then that electron strikes its neighboring atom, and knocks one of its electrons loose. Now, instead of storing the energy in one flywheel, it's split between two flywheels. And this may happen a third time or more, depending on how large and complex the molecule is. Either way, it takes more time to bring all of the energy of the different "spins" back together so they can emit a photon and o back to normal state.

Now, let's say you have a crystalline substance with a lot of missing atoms in the crystal lattice, called "vacancies". These vacancies can actually trap electrons. When a high-energy photon comes along, it knocks an electron loose from an atom, essentially ionizing that atom. If the atom is, say, a zinc atom in a zinc-oxide compound, yet it's neighboring oxygen atom is missing due to a vacancy defect, well this vacancy can actually trap the loose electron. It still acts like a flywheel, spinning around and around the trap, but it just can't get out until the thermal energy sort of knocks it out of the trap and back into orbit around the atom. Only when that happens does that atom emit light, and depending on how "deep" the trap is (how much electron-volts it exerts), it can be anywhere from a few seconds to a several hours after excitation. It is highly dependent on the temperature of the material; a substance that will glow in the dark at room temperature likely will not in very cold temps, and visa versa. Zaereth (talk) 11:38, 7 February 2021 (UTC)

The more I read about this; the more it strikes me that this seems to be an entropy-driven process. Electrons get trapped like in a pitfall, which requires energy, and that energy is stored until a random thermal-spike comes along to give that electron a kick in the pants and help boost it up out of the trap. That energy can be stored and released in this way is no mystery, but it's that thermal kick in the pants that is needed to trigger its release, and this is a totally random process. It's a good example that things we think are in thermal equilibrium are in reality in a constant state of totally-random thermal flux (ie: its why your coffee steams even though the thermometer says it's below the boiling point; random thermal-fluctuations produce atoms that may be much, much hotter and in other areas much much cooler than the average temperature at totally random intervals). Zaereth (talk) 22:41, 15 February 2021 (UTC)