In a helium-neon laser, which of the following processes causes excited helium atoms to lose energy and return to their ground-state energy level? a) Collisions with unexcited helium atoms, b) collisions with unexcited neon atoms, c) emission of photons by spontaneous emission, d) emission of photons by stimulated emission.
If we consider the operation of a helium-neon laser, the laser has a medium consisting of helium and neon gas particles, which both start out in their unexcited ground-state energy level. By electrifying this medium, the helium atoms are given energy and rise up to a higher energy level than their ground state they initially occupied.
At this point, we have a population inversion of excited helium atoms in a metastable state. Since the laser is designed to move helium atoms from the ground state up to this metastable state. That means there are very few helium atoms remaining in the ground state. And therefore, they’re not available to receive collisions from the excited helium atoms. This tells us that answer option a isn’t the one that we want to choose.
Furthermore, because this energy level occupied by the excited helium atoms is a metastable state, that means that these atoms are unlikely to decay spontaneously back down to a lower energy level. This tells us that option c for our answer isn’t the one we want to choose either. Spontaneous emission is a very unlikely, a rare process for atoms in a metastable state.
At this point in a helium-neon laser process, no neon atoms have yet been excited. And it’s the excitation that leading to the decay of neon atoms, which creates the photons which the laser ultimately releases. This means that photons aren’t yet widely available for colliding with these excited helium atoms. If they did, that might lead to stimulated emission. But because of the absence of photons, we won’t want to choose option d for our answer either.
More common to the helium neon lasing process is for these excited helium atoms to encounter ground-state neon atoms. When excited helium atoms collide with ground-state or unexcited neon atoms, energy is transferred and the neon atoms are raised up to their own metastable state.
With the energy transferring from helium to neon, the helium atoms then return to their ground-state energy level. So choice b collisions with unexcited neon atoms is the process which causes helium atoms to lose energy.