Hi, and welcome back to my series of videos for physical chemistry too. By now, you've had another exam in your class. I hope it went well for you. Today, I want to tell you a bit more about spectroscopy. As you might remember, spectroscopy is the study of how light and matter interact, and the ways we can use those interactions to learn about these structures and properties of molecules. As we'll see, spectroscopy involves both the absorption and emission of light, and both of these are useful when we want to study molecules in their properties. As I mentioned earlier, spectroscopy is the study of how light and matter interact. When a molecule absorbs a photon, the energy of the photon is transformed into kinetic and potential energy in the molecule. When the energy of the molecule increases, there are a few different things that could happen. First, one or more electrons in the molecule could move into a higher energy level. Also, the molecule could vibrate or rotate more quickly. It can be difficult to visualize how a photon could be transformed into those types of energy. The reason we might have trouble picturing this is because we usually fall into the habit of thinking of a photon as a particle, and this is actually a poor model if we want to understand how photons interact with matter. It's better to think of it as a wave, but even that can be misleading unless we keep in mind exactly what light waves are like. Remember, a light wave is an electromagnetic wave. That means it's made of both an oscillating electric field and an oscillating magnetic field. If we try to picture a light wave like an ordinary sine wave, we're actually only drawing one of those oscillating fields. The other field oscillates at a right angle to the first one, so a picture like this would be a bit more realistic. So, that's what the electric and magnetic fields of a photon are like. And as you can imagine, a molecule also has an electric and a magnetic field thanks to the electrons and protons in the molecule. When a photon and a molecule meet, it's their electric and magnetic fields that are actually interacting. For now, we're just going to talk about the electrical interactions. The magnetic interactions are important too, but we'll worry about those when we talk about NMR spectroscopy later on in the course. When we talk about UV vis spectroscopy, IR spectroscopy, or rotational spectroscopy, it's the electrical interactions that we're observing, and that's what we'll talk about for the next several videos. For most types of spectroscopy, we'll talk about the photons that are interacting with the electrons rather than the nuclei. That means it's the properties of the electrons that we need to worry about. As it turns out, the angular momentum of the electron is particularly important in spectroscopy, so here's an important thing to remember. Electrons have different types of angular momentum. There's orbital angular momentum, which we get the symbol of a script letter L, and there's spin angular momentum, which has the symbol S. It turns out that, just like electrons, photons also have a momentum and a spin. So when a photon and a molecule interact, the photons momentum is transferred to the electrons in the molecule. Since momentum and energy are related, absorbing a photons momentum results in an increase in the electrons energy, which means that the electrons energy level increases. Of course, an electrons energy level can also decrease if it emits a photon. So how can a photon transfer momentum to a molecule? To answer that question, it'll first be helpful to look at how we detect interactions between photons and molecules. We'll look at the absorption of photons first. In general, here's how we do it. We start with a polychromatic light source. In other words, a light source that emits a range of different wavelengths. We shine this light on a sample, but in many spectrometers, we don't hit the sample with all the wavelengths at once. Instead, we only want a small range of wavelengths to hit the sample. How do we accomplish that? Well, the first thing we do is place a monocrometer in the beam of light coming from the source. A monocrometer is a device that separates polychromatic light into its component wavelengths. The two most common types of monocrometer are prisms and diffraction greetings. You're probably already familiar with prisms. A prism is a piece of glass or plastic, usually in the shape of a triangle. The incoming light is refracted by the glass, and the angle of refraction depends on the wavelength of the light. As a result, the different wavelengths of light emerge from the prism in different directions. The other type of monocrometer is a diffraction greeting. A diffraction greeting is made of a reflective material that's been manufactured so that its surface has a series of closely spaced parallel ridges. Like a prism, a diffraction greeting can separate polychromatic light into its component wavelengths. If you've ever seen light reflecting from the surface of a CD or DVD, you're probably noticed that the light is separated into a rainbow of colors. That's because the surface of the DVD has a series of ridges similar to those on a diffraction greeting. Anyway, after passing through the monocrometer, the light in a spectrometer is separated into its component wavelengths. We can now select a wavelength of our choice by placing a card with a slit in it in the path of the light. We position the slit so that the wavelengths were interested in can pass through it and reach the sample, while the rest of the light is blocked. The light now passes through the sample and then reaches a detector, which can report the intensity of the light that made it through the sample, either as the prism transmittance or the absorbance, both of which we discussed in great detail in video 9. If we want to select a different wavelength, we just turn a knob on the spectrometer, which causes the prism or diffraction greeting to rotate so that a different part of the spectrum falls on the slit. If we repeat that adjustment for many different wavelengths, we can put all the absorbance or transmittance stated together to generate a spectrum. One thing that's important to realize is that if a solution absorbs a particular wavelength, that's not the color that the solution appears to be. Instead, the color of the solution is determined by the wavelengths that are transmitted through the solution. A rough idea of the color is given by the color wheel, which you may have seen before if you've taken a class in art, especially painting or design. The colors on opposite sides of the color wheel are called complementary colors. In general, if a solution strongly absorbs a particular color, the apparent color of the solution is the complementary color. So, for example, if the solution absorbs yellow wavelengths, the solution will appear to be a deep blue. It's not really necessary to memorize this wheel. You can create a rough version of it by drawing a circle and dividing it into six sections. These sections can be labeled red, orange, yellow, green, blue, and violet, similar to the Roy G. Bibv acronym you may be familiar with, but without the eye for indigo. Let's look at the general setup of a spectrometer again. Although this is a common spectrometer design, many spectrometers use a slightly different design. In this alternate arrangement, we don't place the monochrometer and slip before the sample. Instead, we allow all the light from the polychrometic light source to hit the sample. We then use the monochrometer and slip to select one of the wavelengths that the sample allows through. This light is then sent along to the detector. The benefit of this second design is that more light initially hits the sample, which might cause a photochemical response in the sample that might not occur with a lower intensity of light that we get with the first design. The second design is the one used in a spec 20, which is an instrument you might have used before in some of your lab courses. So far, we've been talking about absorption spectroscopy, but as we saw in video 9, another important interaction between molecules and photons is emission. As you might recall, emission occurs when we have electrons in a higher energy state and they lose energy to fall into a lower state. The lost energy is emitted as a photon whose wavelength depends on the difference between the energies of the two states. So, in order for emission to occur, we first need to raise the electrons to an excited electronic state. One common way of doing that is to heat the sample. That means we'll need a different design for our emission spectrometer. In an emission spectrometer, we first have the sample chamber, which often has a heating coil around it so that we can heat the sample to a high enough temperature that we can excite electrons in the sample to a higher state. The sample then emits light as the electrons return to the ground state. Just as we saw in the second absorption spectrometer design, we can pass the emitted light through a monochrometer and a slit so that light of only one wavelength will hit the detector. Well, that's enough new material for now. In the next video, we'll start looking at some interesting and important phenomena that can happen while the electrons are in an excited state, including things like phosphorescence and fluorescence. I hope you'll join me for that. But until next time, have a good week!