Video: Eg17S1-Physics-Q14



Video Transcript

What is the role of the metastable state of neon atoms in the helium-neon laser?

Let’s start on our answer by considering what a metastable state of an atom is in the first place. Atoms we know have electrons. And electrons can possess different levels of energy. For example, here, we’ve shown some energy levels for a made-up atom. Let’s call this atom “strawberrium,” because why not. Electrons in an atom of strawberrium can occupy one of five energy levels, with the first energy level being its ground state. Since physical objects tend to occupy the lowest possible energy level allowed, we know that the preference of electrons, so to speak, would be to occupy this energy level, 𝐸 sub zero, the lowest state allowed.

But, what if we do something to energize that electron, to bump it up to another energy level. Say we bump it up to energy level 𝐸 three. This happens by the way at an energy cost. Some energy must come in and be transmitted to the electron. So we can make this transition. And now that our electron is at this elevated or excited energy level, a few different things can happen.

One fairly common thing to happen is for this electron to drop down spontaneously to a lower energy level. Remember, it’s constantly seeking to get back to its lowest possible allowed energy state. It’s sort of like that very comfy overstuffed chair you may have in your room. Could you sit up straight in the chair with great posture? Sure, but would you want to? In a similar way, the electron tends to go back to 𝐸 sub zero, the ground state.

But anyway, let’s say that our electron has made this transition to 𝐸 two. Interestingly, not all energy levels are created equal. For some, where electrons occupy those levels, they very rapidly decay down to a lower energy level. When that happens, we say that the lifetime of that energy level is fairly short. But other energy levels, and let’s 𝐸 two is one of them, are different. Electrons that end up there have a relatively long lifetime before they might spontaneously decay down to another lower level. Energy levels like this, whose electrons have a very long lifetime at that level, are called metastable states, which is one way of saying that an electron that ends up here is fairly stable. Even though eventually, yes, it will decay back down, for quite some time it tends to stay where it is. This brings us to our topic of the helium-neon laser.

It turns out that metastable states are very important for lasing capability. That’s because, without them, we could never achieve a large population of equally excited atoms of any particular type. Even if we were constantly pumping our atom with energy in order to bump the electrons up to higher and higher levels, the electrons wouldn’t stay there if there were no metastable states for them to end up in. If we have not just one but a collection of atoms, with electrons in an energy level above their ground state, then we have achieved what’s called a population inversion. Like we said, normally, the electrons would seek out the ground state. And the fact that they’re not there, and not there as a population, is a very impressive feed. It’s so unusual that, like we said, it’s called an inversion of the population.

With a setup like this, if one of the electrons were eventually to decay either spontaneously or through stimulation down to a lower energy level, then the particular wavelength of the photon emitted during that process would be a match for the rest of these excited electrons too. In other words, if it interacted with them, they too could decay down to the same energy level and emit an essentially identical photon. This process could occur many times over until there’s a significant collection of these coherent photons. It’s these photons and mass which constitute laser radiation.

So far, we’ve talked about all this in the context of an imaginary element we’ve called strawberrium. But the principle we’ve talked of of metastable states and electron transitions can happen for any atom, neon included in the helium-neon laser. Let’s write out then just what is the role of the metastable state of neon atoms in this laser. We can say that the role of metastable states is that it allows the accumulation of excited neon atoms due to its long lifetime so that the atoms can reach the condition of population inversion. That is, the great majority of its atoms are in an excited state. This describes the role of the state in neon atoms in a helium-neon laser.

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