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Introduction to spectroscopy

Spectroscopy is the study of the interaction of light and matter. Many types of spectroscopy rely on the ability of atoms and molecules to absorb or emit electromagnetic (EM) radiation. The absorption or emission of different forms of EM radiation is related to different types of transitions. Microwave radiation is associated with molecular rotational transitions, infrared radiation is associated with molecular vibrational transitions, and UV/visible radiation is associated with electronic transitions. Created by Sal Khan.

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  • blobby green style avatar for user thejuliansalas
    What absorbs the radiation?
    The electron, proton, neutron, any of them?

    And despite which one absorbs it, the reaction will be the same?
    (4 votes)
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    • leaf red style avatar for user Richard
      It depends on what type of spectroscopy you're observing. Something like absorption spectroscopy deals with the electrons and their transitions between an atom's shells. While something like nuclear magnetic resonance spectroscopy, which is used in MRI, involves electrometric absorption by the nuclei of atoms.

      The reaction is determined by the wavelength (and hence the frequency and energy) of the light absorbed.

      Hope that helps.
      (9 votes)
  • boggle blue style avatar for user Schäfer Kleinert
    Does the radiation bounce off the atoms in the compounds, turning into certain spectrums based on what it bounced off, or is the radiation absorbed by the atom, which at the same time emits a separate photon of the wavelength?
    (2 votes)
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    • leaf red style avatar for user Richard
      Atoms both absorb and reflect photons of light at different wavelengths. Atoms are selective and prefer only certain wavelengths of light, while rejecting all others. If we consider the light striking an atom to be a continuous spectrum, then the atom will select certain wavelengths to absorb and remove from the spectrum, while reflecting all others. The resulting reflected spectrum is the original continuous spectrum minus the wavelengths which the atom absorbed.

      Hope that helps.
      (5 votes)
  • sneak peak green style avatar for user yonsuissa
    I am playing around with the PHET interactive simulator and have a question.

    When the torch is emitting visible light, and the molecule I've set it to is water, nothing seems to happen. The water molecule doesn't seem to absorb any of the photons. But then when Googling, I can see that water molecules do absorb light.

    I'm confused why nothing is happening in the simulator. Similarly, not all molecules seem to absorb UV photons. Please can someone explain what's happening?

    Is it because water molecules' characteristic absorption peaks are not in the visible light range? But if so, how can they be affected by both microwaves and infrared?
    (1 vote)
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Video transcript

- In this video, we're gonna talk about spectroscopy, which is all about the interactions between light and matter. And when we're talking about light, we're not just talking about visible light, We're talking about electromagnetic radiation in general. And so what I'm going to do to give us an intuition here is use the PhET simulator by the University of Colorado. I encourage you to go to this URL and try it out for yourself. But you can see what the simulator does is it allows us to essentially see how different wavelengths of electromagnetic radiation can interact with matter, in this case various molecules. And just to get our bearings, we can click on this light spectrum diagram, and we can see that on this diagram what people would normally consider radio waves. These are some of the lowest frequencies and longest wavelengths of light. And then when you get to higher frequencies, you get to microwave, and the higher the frequency, there's also the higher the energy per photon. And then you get higher frequencies in that and higher energy, that's infrared, and then higher frequency and energy, that's visible light. That's what our eyes can sense. And then you get even higher frequency, and more energy. You get to ultraviolet. Then X-ray and then gamma rays. And this isn't a linear scale. You can see that this is a logarithmic scale here. This is in powers of 10. So we see some pretty dramatic increases in frequency and energy as we go from the left to the right. But in this video, we're gonna focus in particular, on microwave, infrared, visible and ultraviolet wavelengths of electromagnetic light, or electromagnetic waves, and think about how they interact with molecules. So if we start with microwave radiation, and here we have a water molecule, I've picked that right over there and I can get my simulation going. You can see what it's doing is, when it gets absorbed, it causes a rotational transition in the water molecule. It makes the water molecule rotate in a different way than it was before. And then the water molecule can also emit the radiation and then rotate differently. And so you can see it doesn't always do that. There's a little bit of a probability involved, but this is actually the basis of how microwaves work, your microwave oven, is it causes the water molecules to get agitated in a rotational way, which increases the heat in that system. Now we could also look at infrared light, which is once again, we have to remember, gets us into higher frequencies, and see what that does to molecules. So based on this simulation, it looks like the infrared light is when it gets absorbed, it causes this water molecule to start to vibrate. So microwave radiation caused it to rotate or to have a change in state of its rotation, while infrared makes it vibrate. And we could see that with other molecules as well. Let's try carbon monoxide. Once again, it's not rotating it, it's causing it to vibrate. Now what about visible light? Well, visible light will have different interactions with different types of molecules, but let's try it out with nitrogen dioxide. So there's certain situations where nitrogen dioxide will absorb, that's when you saw it glowing and what you see when it's glowing, what it's really doing is it's putting electrons into a higher energy state, or into a higher orbital and then when it stops glowing, it means that those electrons are going back to a lower energy state. They are re-emitting radiation. So there, you can see it. You can see that just now, it's remitting visible light, in this case a different direction. And when it did that, the electron that was excited, went to a lower energy state. Now what about, let's think about ultraviolet light, which has even higher energy than visible light. What can that do? Well, here, we can see that it takes, in certain cases, electrons, and it's able to excite them so much that it's able to break that bond itself. And so let me keep resetting it. So you can actually break bonds. Let's see what it can do to some ozone? Same thing, it excites it so much that it can actually break the bond. It's exciting electrons so much. I can keep resetting it. So the big picture here, the big takeaway. You could have microwave radiation, which tends to change the rotational motion of a molecule. We saw that with the water molecules. You have infrared radiation, which is higher energy and higher frequency, which tends to lead to a change in vibrational motion. And then you have visible light, which can excite electrons, take them to a higher energy state, and then be readmitted when the electron goes back to its base state. And then you can have ultraviolet light, that's so powerful, it can excite electrons so that in some cases it can even break covalent bonds.