ESR video

SPEAKER 0

ESR or electron spin resonance is a kind of magnetic spectroscopy that can probe the structure of molecules, especially if they have a single unpaired electron. Its diverse uses show up in fields like, uh, material physics, chemistry, geology and even archaeology. The sample that we'll be probing in this experiment is Diphen or pic hydrazone or DPP H. The main thing you need to know about this substance is that one of the atoms in its molecules has a single unpaired electron, and it responds to magnetic fields just like a free electron. In other words, its spin responds to the external magnetic field that we'll apply. Recall that the energy of magnetic dipole in a magnetic field depends on the orientation of the dipole. With respect to that field, it has minimum energy if it's aligned with the field and maximum energy if it's anti aligned. In the case of a single spin, the energy is quantized. Since the component of the magnetic dipole in the direction of the field can only take two possible values, one corresponding to spin up and one to spin down. The energy difference between the two states is proportional to the strength of the magnetic field. If we subject the sample to electromagnetic radiation, the spin can flip from one state to the other by absorbing or emitting photons. A transition is likely if the photon energy proportional to its frequency matches the energy difference between the two states. This is the resonance condition that gives electron spin resonance its name for the samples we probe with ESR. It's safe to assume that they're in thermal equilibrium, which means they're most likely in the lower state to begin with the lower energy state during the ESR experiment. The photons are thus absorbed as the spin flips to the higher energy state. So the essential ingredients we need for ESR are an external magnetic field to create an energy difference between the two spin states an RF, or radio frequency electromagnetic field to allow us to go between the two spin states and an ability to monitor the magnetic response of the sample as the spin changes. To do this, we have a pair of helmholtz coils to generate a magnetic field. We have an RF oscillator circuit. Both of these are plugged into an ESR control box, and the output waveform is monitored on the oscilloscope. Let's have a look at each of these. In turn, we can create a region of almost uniform magnetic field with the use of two current carrying coils arranged in this configuration called the Helmholtz Configuration. In this configuration, the coils are aligned along a common axis, and the radius of the coils should be equal to the the distance between them. Now it's important that the coils are oriented so that their magnetic fields add together in the middle. This is the way to create a space of uniform field. If you happen to connect them up wrongly or put them the wrong way around, you'll get the opposite effect of what you want. You'll get a a regional, steeply changing magnetic field to check the directions of your mag magnetic fields. You can use a compass, and if you want a quantitative measurement of the magnitude, you can use this gauss metre with its probe. The oscillator circuit consists of an inductor coil, which you can see on the front and inside the box where you can't see it is a variable capacitor, and you can control the capacitance by turning the knob on the top at the back there is an on an off switch. If you want to vary the, uh, frequency of the circuit more than what ch changing the capacitance will do for you, you can swap out one of the inductor coils and replace it with another. And we have a choice of three inductor coils for you to choose from. The three inductor coils together with the variable capacitor means you have a frequency range from 15 megahertz up to 100 and 30 megahertz. The oscillator circuit serves two purposes. Firstly, it generates the radiofrequency electromagnetic field that excites the electrons from spin down to spin up in the sample, the DPP H sample is inserted directly into the inductor coil of the oscillator circuit. Notice then that the sample is subject to two different magnetic fields. One is the field generated by the coils, which will be along this axis, and the other is the field within the inductor created by the oscillator circuit, and that will be in this direction. So they're at right angles to each other, and that's important for creating the resonance condition. Not only do the photons of the electromagnetic field have to have the right frequency to give the energy needed to to flip the spins. But we also need to supply angular momentum to the spin to change it. And the photons arranged in that direction have the right angular momentum. The photons will be linearly polarised. But linear polarisation is just a superposition of left and right circularly, polarised photons. And it's one of these kinds of circularly polarised photons that will have the right angular momentum both the direction and the magnitude to flip the spin. The second purpose of the oscillator circuit is to detect the change in the magnetization of the sample as the spin flips from lower to higher. It does this by detecting the change in the inductance of the coil. The material inside the inductor coil affects the inductance of the coil in particular as the the spins, the magnetic dipoles flip around. That will change the magnetic response of the material to the magnetic field of the inductor. And this is what we detect and is measured signal on our oscilloscope. The ESR control unit, uh, is where all aspects of the experiment come together. You can see this by all of the cables that feed into it. So working from left to right, we have, uh, the signal from the oscillator circuit feeds via a probe into the box. We have, uh, the ability to generate current and the current that goes through these cables is what drives the magnetic field, creates the magnetic field through the helmholtz coils. The signal that's received by the box and the, uh, the waveform of the current going to the coils is fed to the oscilloscope through these output cables here. So there's two outputs one tells you about the magnetic field that's generated, and the other tells you about the signal that's read from the, uh, oscillator circuit. The display at the top of the screen can show three different quantities selected by pressing this button here. When the light at the top is showing, we have the frequency of the oscillator circuit showing and recall that this is controlled by varying the capacitance through the knob on the top or by swapping in different inductor coils. If we press this button here, we move to the second choice of reading, and this is now telling us the current that the box is supplying to the coils. In particular, it's telling us about the DC current that's being supplied. We can adjust the DC amount of current by turning the knob here. Finally, if we go to the lower setting, this tells us about the AC component of the current, and we have the option by controlling with this knob here of going from purely DC. So there's zero AC modulation through to quite a large modulation in time. Now, why do we want to modulate the current? Well, To detect the ESR resonance, we can either change the frequency of the photons or we can adjust the magnetic field. In this experiment, we're going to adjust the magnetic field, and we're going to oscillate that in time to scan backwards and forwards across the resonance. And we do this with the AC component in order to simplify what we observe on the oscilloscope. This knob here gives us a phase delay between the two signals. The default position should be all the way around to the right. But if the signal is looking a bit complex, then maybe pull it back a little until the waveform simplifies. The oscilloscope receives two signals from the ESR box one. We plug into the X, uh, spot and the other into the Y input. Now we can show those each of those or both of those as a function of time on the oscilloscope display by changing the setting from XY format to YT format. For most of the measurements you will do, it's, uh, more useful to switch to XY mode, and you can see the resonance as a dip on the screen. If the waveform doesn't fit nicely onto the screen, you can essentially zoom in and out by adjusting the scale of the X and Y axis independently using these controls here. If when you zoom in or out, you find the signal goes off the screen, you can put an offset in either direction. Using these controls here, I'll set the oscilloscope to show the signals as a function of time. The yellow signal here is the X input, and that's the magnetic field or the current that generates the magnetic field in the coils. Recall that we can adjust the DC component and the AC component, uh, independently, and if I do that, you can see the change in that yellow waveform. So I'm adjusting the DC component. So it's going up. The whole waveform is going up and down on the screen, and I can adjust the size of that modulation. So small modulation or large modulation. Now, as I change the DC component, you might notice that the other signal that's this blue signal, which is the, uh, output from the ESR device. The magnetic response as the the spins flip from low to high. Um, that signal comes in and out of resonance. So if, uh, at this setting, for example, the response is basically a straight line, we're far away from resonance. If I change the magnetic field, then, depending on the frequency of the oscillator, there's a magnetic field at which we get a nice pattern of of dips. And this is essentially the resonance condition. If we don't have that phase delay, let me adjust that here. If I turn the phase delay a little, you can see the two patterns move with respect to each other. Let's look at the response in a bit more detail in the XY mode, so now we have the magnetic field control on the X axis And if I adjust the modulation, the scanning backwards and forwards, you can see that the range over which we see the signal increases or decreases. And if I adjust the DC component, this moves us away from resonance or onto resonance. So exactly on resonance is when this dip is nicely centred and I can clean up the wave form a little bit by adjusting this phase delay. So now that we have our resonance shown on the oscilloscope, what can we do with that? What do we measure and why? By how much you have to change the current as you change the frequency tells us about how much the magnetic field has to change to get to resonance again. And you can use this to map out the energy difference between the two spin states as a function of magnetic field. If you do this, you can check that the proportionality factor is the same as an electron. That's, uh, that's free a free electron model. Because our electrons are in a material sample bound to a molecule, it's not quite the same, and the small difference can tell us something about the material properties that the electrons are embedded in. So that's one purpose. You'll also see that the resonance is not an all or nothing affair. Um, there's a certain width to this pattern that we see here. This is telling us that we can get the spins to flip, even if we don't have exactly the right magnetic field. In other words, we don't have to quite make the resonance condition. The resonance condition just gives us the maximum effect. The width of this resonance is determined by two effects. One is that we're not dealing with isolated electrons, but they're embedded in molecules in, uh, material lattice. And the electrons can give energy to the lattice or take away from it so that we don't have to quite match the exact energy with our photons. So this gives us hints about, uh, what the material is made of, that the electrons are embedded in the electrons from different molecules. The spins of these electrons can also interact with each other, and again, this gives us more information about, um, about the material. Because of these two effects, the width of the resonance tells us about the material properties that the electrons are embedded in