 All right, so we've seen that the basic idea, in order to construct a laser, you need a few things. You need a way to generate a population inversion, and then once you've, one way in fact to generate that population inversion is to have a pumping transition, so pump molecules up to some excited state, which you'll maintain at a higher population than some lower state, and then those two states with the population inversion represent the lasing transition. So that's the bare bones of what you need in order to have amplification of light be a stimulated emission to generate a laser. So let's consider a specific case, a helium-neon laser. So the optical cavity of the laser is going to contain these two gases, helium and neon. So gas phase lasers are not the most common type of lasers these days, solid state lasers are much more common, but the helium-neon laser is a relatively simple one to understand what's going on. So draw some sketches of the different transitions involved, the pumping and lasing transitions for this particular laser. So helium atoms are normally in the ground state. So ground state for the electronic state for the atoms in a helium, as you know, is 1s2. We could, if we don't want to write the electron configuration, we want to be a little more accurate, we could call that a singlet S term symbol. So the singlet S, the ground state is a singlet S state. There's a high-lying energy state that you could either write as electron configuration 1s2, or you could write that also, its term symbol is also a singlet S, so the spins are paired essentially. So that transition turns out as the pumping transition that's made use of in the helium-neon laser. So if you can get molecules up into this upper state, so the electrons are in the upper state of this helium atom, then that energy needs to get transferred to one of the neon atoms in the same gas. So it just so happens that between the ground state of neon, so the ground state of neon as an electron configuration, you'd write that as 1s2, 2s2, 2p6. That is a singlet S term symbol for the ground state. So it just so happens that there's an excited state of neon that's pretty close to the same energy as this particular excited state of helium. So that excited state for neon, again as an electron configuration, you could write 1s2, 2s2, 2p5, and then the 6a10, 10th electron is in the 5s state. So it's 1s2, 2s2, 2p5, 5s1. The term symbol for that state is a triplet p state, triplet p1. And like I said, just coincidentally, this energy level for neon is about as far above the ground state as the 1s2s state in the helium is above its own ground state. So what that means is this pair of states is convenient to use to transfer energy from the helium to the neon. So essentially what happens is an excited, so I'll write the excited helium atom as a helium with a asterisk on it. In the gas, these two atoms can come near each other, collide with one another. And the electronic excitation in the helium atom can be transferred to the neon. So we'll write that chemical reaction as just a collision between a helium and a neon atom, the electronic excitation energy of the helium, the helium falls down, the neon gets excited. There was no radiation involved in that transfer, it didn't involve emission and absorption of a photon, it's just energy transferred due to the collision. So we call it a collisional energy transfer. So the net result is we end up with a neon atom in this excited electronic state. Is now the state that's maintaining a population inversion relative to some lower states in that same atom. So the excited triplet P state of the neon, that's a little bit above another triplet P, triplet P2 I believe, state of neon. That one could be described with an electron configuration if you want to think about it as 1S2, 2S2, 2P5 and then 3P. So instead of thinking of it as a Hartree product of electron wave functions, the excited single electron is excited from a 2P state up to a 3P state here or a 5S state up here. So this emission, so this excited neon will fall down spontaneously to one of any number of states in this neon atom. But if it happens to fall down to this particular 1S2, 2S2, 2P5, 3P1 state, then it will give off a photon. That particular photon, so this is our lazing transition. So what happens is that photon when it bumps into another molecule that's in the same excited state will spontaneously, not spontaneously stimulate the emission of another photon. Those photons will bounce back and forth in our optical cavity, amplifying the light and generating more and more coherent photons with the same wavelength. So there will be, after the molecule falls down here, it will eventually spontaneously fall back to the ground state here. So we have this continuous source of excited state neon atoms via pumping and collisional energy transfer, maintaining a population inversion, more atoms in this upper state than in this state, and so we can have amplification of the light via this lazing transition. So this is, in principle, the way I've drawn it, a three state system, but of course there's more than just these three states of the neon atom, and it also involves this interaction with the helium as well. So there's a lot of complications that are not drawn in this diagram. I haven't drawn, here's the 3P state. I haven't drawn the 4S or the 4P or any of the other excited states of this atom. I haven't acknowledged the fact that there's, in fact, multiple different spin states, multiple term symbols nearby. So there's a bunch of different, there's a whole manifold of different spin states of this particular 1S2, 2S2, 2P5, 3P state, all of which differ in energy slightly. The most common wavelength to which this helium-neon laser is tuned by adjusting the size of the optical cavity appropriately, these photons that are given off are 633 nanometers when we fall from this state down to this state. You can tune the laser differently. There's not only this excited state for helium, there's other, there's a triplet S state that's the unpaired spin state that's lower than this state. So there's all sorts of other, if I collisionally transfer energy over to one of the other states and fall from a different excited state to a different lower state as the lasing transition, I can get light of various different frequencies to be emitted by this helium-neon laser. But the most common wavelength of light emitted by a helium-neon laser is between this pair of states, the triplet P1 state here and the triplet P2 state here that we use as our lasing transition. So that illustrates a key feature about constructing lasers is that if you want to change the wavelength of light that a particular laser generates, you need to find a pair of states to use as your lasing transition and find a way to maintain a population inversion between that particular pair of states. And then if you construct your optical cavity differently, tune it for selecting particular wavelengths you can amplify that particular wavelength over all the other wavelengths of light that may be being generated in that optical cavity.