[Physics] metastable energy levels


can someone give me a good definition of "metastable energy levels" in a helium plasma (glow discharge).

Am I right when I say : "by adding some neon in my helium plasma, I will obtain a molecular plasma of NeHe" ? I guess the metastable energy levels are the reason of forming NeHe, but I don't know why, can someone explain to me ?

If both my guesses are wrong, please give me some explanation, if you can.


Best Answer

A photon has a spin of 1, so if a photon is absorbed or emitted by a helium atom the spin of the atom must change by 1 to conserve orbital angular momentum. This is known as a selection rule. For example the ground state of helium is $1s^2$ and it can absorb a photon and be excited to the $1s^12p^1$ state, but it cannot be excited to $1s^12s^1$ because in that case the change in angular momentum is zero and this would violate conservation of angular momentum.

The converse of this is that while the state $1s^12p^1$ can quickly decay by emission of a photon, the $1s^12s^1$ state cannot because there is no lower energy state that differs by 1 unit of angular momentum. This makes the $1s^12s^1$ metastable i.e. it has a relatively long lifetime. It's only metastable not absolutely stable because the transition is not completely forbidden but instead has a low probability.

If atoms are not isolated they can exchange energy by collisions with other atoms. So for example a helium atom can absorb a photon and be excited to $1s^12p^1$, then it can exchange energy by collisions and decay to $1s^12s^1$. In this way metastable states can be prepared. This is common to many different types of system. For example phosphorescence occurs in solids where an excited triplet state decays to a long lived singlet state by interaction with the lattice. In such materials the metastable singlet state can live for many minutes.

To get back to your question, some Googling leads me to believe that the main metastable state in helium plasmas is indeed $1s^12s^1$, though this isn't my area and I'm not sure I claim this under oath. Metastable states are generally very reactive, and the Wikipedia article on helium compounds claims that metastable states (they don't say which) react with neon to form HeNe. However this is an unstable molecule and dissociates as soon as it decays to the ground state,

The He-Ne chemistry isn't involved in He-Ne lasers. Instead this works by the metastable excited helium atoms colliding with and transferring energy to the excited states of the neon atom. This creates a population inversion in the neon atoms that can then lase.

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