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Pigments, EM radiation and quantum mechanics.

  1. Feb 5, 2015 #1
    An article in Wikipedia tries to explain pigments.
    One particular section has the following:

    "A wide variety of wavelengths (colors) encounter a pigment. This pigment absorbs red and green light, but reflects blue, creating the color blue."

    Questions arise... They may see stupid, but please, bear with me. I am aware of quantum mechanical weirdness and etc. I do have my own answers about it, but I would love to hear other people's insigths, because it really interests me..

    What is usually meant by "absorbs"? Not very clear.
    What does the light, that is absorbed actually do with the molecules of the pigment.

    If it heats it up, then it seems that the pigmentation phenomena is explained with quantum mechanics, and the heating up seems to be described with the help of electromagnetic idea of light.

    If the absorbed light heats up the pigment, then it still is in a way re-emitted... as infrared, right?
    Also it is not very clear how exactly does electromagnetic radiation make pigment molecules move faster:

    Doesn't electromagnetic radiation cause the dipolarization of a molecule: therefore change its shape? Does the shape change cause the molecules to collide and that in turn makes them speed up chaotically?

    If a pigment "reflects" a wavelength, isn't that kind of an absorption?
    Can this be also modeled as electron cloud being accelerated with a certain frequency?

    If there were only a few pigment molecules, very far from each other, not interacting almost. Do they still act the same way or is there a difference.

    Can pigment molecules be used to cause scattering, similar to Rayleigh scattering? If so, why, if not, why?
  2. jcsd
  3. Feb 5, 2015 #2

    Quantum Defect

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    When a pigment molecule absorbs a visible photon, that photon's energy excites the electrons in the molecule. A number of things can happen next. In plants, the chlorophyll pigment in this excited electronic state (electrons more weaklybound) is a reasonably good reducing agent -- i.e. it can reduce other species in a chemical reaction. This is exactly what it does. The energy that was absorbed by the chlorophyll goes on to drive a whole sequence of chemical processes, that ultimately produce glucose for the plant's chemical energy stores. [The energy also drives pretty much everything else in the plant.] When you take the chlorophyll out of the plant, , and dissolve it in say acetone, it is no longer planted next to the "reactors" that do all of this chemistry. When you excite the chlorophyll molecule with a photon in solution, you will see a new process taking place, the chlorophyll molecule will fluoresce -- it will emit a photon ( the lifetime in the excited state is quite short -- ca. 1 ns.) The photon will be lower energy than the one that was absorbed, with the difference going to vibrational energy of the chlorophyll molecule. Eventually, the vibrating molecule loses energy (possibly through the emission of an infrared photon) but more likely through collisions of the vibrating molecule with the surrounding solvent molecules. If there is a reduceable species present, you may still see reaction of the electronically excited chlorophyll molecule with the other species -- this reaction competes with the fluorescence, and you will see a decrease in the fluorescence intensity, compared with the case where no reducible species are present. [This is called fluorescence quenching.]

    If a pigment reflects light, there is no real interaction of the light with the pigment. The wavelength is not resonant with any electronic transition in the molecule, so electrons do not get excited to excited electronic states, as they do in the case of absorption. With very intense light sources, you can start to see inelastic scattering of non-resonant photons (Raman scattering). When this happens, the scattered photons come off with a reduction of energy (lower frequency). If you use a narrow-bandwidth laser to do this, the Raman spectrum cann tell you something about the chemical properties of the scatterer.

    When there are mutliple pigment molecuels present in close proximity -- i.e. like in a chloroplast, you may see some quantum wierdness effects. There was a report a few years ago that purported to show that plants have evolved to take advantage of these quantum effects to more efficiently harvest light. I remember some back and forth on this point, and I don't remember which way things turned out. If it is true, it is pretty amazing, I think, that evolution can take advantage of things that people have only really begun to understand in the last 100 years! I.e. plants were "doing" quantum mechanics while dinosaurs walked around, and our furry, little, rat-like ancestors were trying to stay out of their way!
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