91Ó°ÊÓ

1. Figure 40-21 show partial energy-level diagrams for the helium and neon atoms that are involved in the operation of the helium-neon laser is given.
2. The energy of the helium state $E_3(20.61\mathrm{eV})$is very close to the energy of the neon state $E_3(20.66\mathrm{eV})$.

Due to the laser action, there occurs a potential difference that gives rise to the current passing through the helium-neon gas mixture serving—through collisions between helium atoms and electrons of the current—to raise many helium atoms to state E₃, which is metastable with a mean life of 1 microsec. (The neon atoms are too massive to be excited by collisions with the (low-mass) electrons.)

According to the given data and concept, when a metastable$\left(E_3\right)$ helium atom and a ground state$\left(E_0\right)$ neon atom collide, the excitation energy of the helium atom is often transferred to the neon atom, which then moves to the state $E_2$. In this manner, neon level $E_2$(with a mean life of 170 ns) can become more heavily populated than neon levelrole="math" localid="1661494432656" $E_1$ (which, with a mean life of only 10 ns, is almost empty).

This population inversion is relatively easy to set up because (1) initially, there are essentially no neon atoms in the state $E_1$, and (2) the long mean life of helium level$E_3$ means that there is always a good chance that collisions will excite neon atoms to their level,$E_2$ and (3) once those neon atoms undergo stimulated emission and fall to their $E_1$level, they almost immediately fall down to their ground state (via intermediate levels not shown) and are then ready to be re-excited by collisions.

Hence, the energy transfer occurs even if the energy levels of helium and neon are not the same.