Give a reason for selecting helium and neon, He-Ne, as the elements of the active medium of a laser.
If we have two different energy levels — we’ll call them 𝐸 one and 𝐸 two — we know that, in general, a laser works by excited atoms in this raised-up energy level having a photon collide with that excited atom, sending it down to a lower energy level. During this transition, the atom emits a second photon of the same wavelength as the one that hit it in the first place.
Now there are two photons, both of the same wavelength and both in phase, that can move on to hit other excited atoms and repeat this process. In the helium-neon laser, it’s the neon that does this photon emission. Neon atoms in laser medium are initially in their ground-state energy level. Then when an excited helium atom collides with this ground-state neon atom, it transfers its energy to the neon.
As a result, the neon atom is raised up in its energy level to what’s called a metastable state. This is a state above the ground-state energy level which is fairly long-lasting. Helium is able to transfer the neon up to this metastable state because its excited energy level and the excited energy level of the neon in its metastable state are nearly the same. This correlation between helium and neon is what allows for a population inversion to occur in the He-Ne laser, which then allows lasing.
Thanks then to the atomic structure of helium and neon, they’re a good pair for the active medium of a laser, and we can write this as a sentence. We can say that helium and neon have metastable excited states with energy levels almost equal to each other. This is one reason helium and neon are selected as the elements of the active medium of a laser.