 So the Frank Condon principle is going to help us understand a phenomenon that you're already familiar with but may not know the details of, called fluorescence. So remember that we've learned that transitions, vibronic transitions between two different electronic states of a molecule from one vibrational state to a vibrational state in the other electronic state, those have to be vertical transitions. So if we're starting out in the ground vibrational state of this ground electronic state, that vertical transition could land us in several different choices for which vibrational state, but it's likely not going to land us in the ground vibrational state. So let's say the transition, let's say that we shine light on this molecule of the correct frequency to excite this particular transition. The difference in energy between these two states is the same as the energy of the photon and that's an allowable transition according to the Frank Condon principle. So we've excited the molecule from this state up to this state and then ask ourselves what happens next? So first of all, we've got that first step, we've got an excitation step, we've excited the molecule, lifted it up to a higher energy level. One thing that could have happened is it could fall right back down. If it falls back down and emits a photon of exactly the same energy, that process is called scattering, that's usually not terribly interesting. But the molecule certainly can emit a photon of the same energy that excited it. But because that emission process usually takes longer than another process, if it falls down a vibrational level, so from this vibrational level falls down to this vibrational level, perhaps down another one or two vibrational levels, that process is called vibrational relaxation and that actually happens more quickly than falling back down to the lower electronic state. So that vibrational relaxation is faster than the electronic relaxation. So let's go ahead and number these steps. Number one was excitation. These small down arrows between different vibrational states, those are the vibrational relaxation steps. As vibrational relaxation happens and I land back at the ground vibrational level in this upper electronic state, then there's no choice. From here there's no way to go down other than an electronic relaxation. There's no more vibrational energy left to lose because I'm sitting in the ground vibrational level of this upper electronic state. So from there there's no choice other than to wait around until the molecule undergoes electronic relaxation. And now remember that transition has to be vertical. It's not going to land back in the ground vibrational state of the lower electronic state. It might land in this state or this state. It's going to land in one of these vibrationally excited states of the ground electronic state because it has to make this vertical transition. So that step number three is the step we refer to as fluorescence. A molecule fluoresces after we excite it electronically. It relaxes vibrational down usually to the ground vibrational state and then falls down undergoes electronic relaxation. As it does that it's going to emit a photon. That loss of energy is going to be paid for by emitting a photon. That emission is guaranteed to have smaller energy than the excitation because of those intervening vibrational relaxation steps. So that fluorescence step is going to have lower energy or a longer wavelength than the excitation photon. That's one key fact about fluorescence is whatever energy, whatever wavelength of light we use to excite the molecule we're going to get back less energy or a longer wavelength of light when that molecule fluoresces. And then of course after it lands in this vibrational excited state relatively rapidly it's going to undergo more vibrational relaxation falling back down to the ground state because Boltzmann tells us if we let the system regain equilibrium almost all the population is going to be in that ground vibrational state. So step four after landing in this vibrational excited state is more vibrational relaxation. So both steps two and four involve vibrational relaxation. This process of fluorescence as I mentioned that's something you're familiar with. You've heard that word fluorescence or fluorescent objects before. In fact you're looking at fluorescence happening right now. The markers I'm riding with are fluorescent markers. The way that they work is the light that's illuminating this light board has a relatively high wavelength that involves the blueish end, white light, but it includes the blueish end of the spectrum. So these red or green or orange markers that I'm using they fluoresce and they emit light. So the reason it looks like they're glowing is because they literally are glowing. They're emitting light after being excited with white light and then emitting light of the specific colors of these fluorescent markers. More commonly you're used to fluorescent lights. Maybe the room you're in now or certainly a room you've been in not very long ago had fluorescent lights in the ceiling. Same thing inside those fluorescent tubes that you've seen that run those fluorescent lights. There's relatively high energy photons, ultraviolet energy photons that are exciting. The coating on the inside of the fluorescent light bulb and that causes the coating to fluoresce. It gives off white light in the visible region of the spectrum. So it's been excited with ultraviolet light and it's giving off lower energy visible light. Another example that you're likely familiar with is black lights or ultraviolet lights. If you've been to certain fancy roller coasters at amusement parks or a Spencer's gift store or a rave or something like that where you use a black light to cause your clothing to glow and fluoresce, the exact same thing is going on there. The black light is shining ultraviolet light that you can't see it because your eyes see visible light that causes certain molecules, certain dyes in some clothing, for example, to fluoresce and give off light in the visible portion of the spectrum. So all those phenomena are actually the same thing. They're all fluorescence. Excitation with one frequency followed by vibrational relaxation and then emission at a somewhat lower frequency is this process we call fluorescence. That is similar to but not quite the same as a different process called phosphorescence that you might also be familiar with. So we'll talk about that next.