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How to increase reaction rate using EM radiation?

  1. Aug 3, 2014 #1
    I want to pump energy into a chemical species to increase its reaction cross section with another.

    Typically these species react upon collision when their relative kinetic energy exceeds the activation energy. I want to find a way to increase the reaction rate for interactions at slower relative kinetic energies.

    I am imagining pumping the mixture with EM radiation at a frequency that is strongly absorbed by one of the species but I am not sure how to figure out what that frequency is, how much energy would be needed, or if this would even work!

    How should I go about investigating this? Is this possible? What things about the chemical species do I need to know to answer my question about frequency and energy?
  2. jcsd
  3. Aug 5, 2014 #2


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  4. Aug 6, 2014 #3
    They are actually two rarified gases in a chamber and I do have absorption spectra.

    I'll pose the question this way: when the molecules collide each has various kinetic, vibrational, and rotational energy. I want to know how determine how these energies "add" to total the activation energy. I wouldn't think it is a simple summation! I dont know the right words to google which is why I am asking here.

    My goal is to maybe make up in vibrational energy what a molecule lacks in kinetic energy and still maintain reaction rate.
  5. Aug 6, 2014 #4


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    I expect there will be an equilibrium between the kinetic, vibrational, and rotational energies. Collisions between gas molecules, and with the walls, will spread the energy throughout the available modes.

    Vibrational frequencies will be in the range of about 1 to 10 THz. Those frequencies are well above that of any RF power oscillator or gyrotron. You are not so much considering an EM radiation generator but illumination with IR. One way to generate that narrow band energy would be to use an IR laser or LED. Rather than pumping a laser, you may as well pump the reactant gasses.

    So I suspect that, no matter how you inject the energy it will all end up as heat anyway. You may as well just heat the walls of the chamber to increase the average kinetic energy of the molecules enclosed.
  6. Aug 6, 2014 #5
    Assume no equilibrium. Say I have 5 eV of rotational, 5 eV of one mode of vib and 3 eV of another, 10 eV of kinetic. Just throwing out numbers I have no feel for what an eV is. My question is does there exist an equation I can input all of these various forms of energy into and get reaction rate as an output?
  7. Aug 6, 2014 #6


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    Reaction rates are determined by temperature. Temperature is the average kinetic energy of the population of molecules. Denial of statistical equilibrium is not wise if you are considering the real world.

    I assume you are now talking about only one molecule. What is it going to react with?
    You could pump in more energy to disassociate that molecule.
  8. Aug 7, 2014 #7
    So reaction rates (and probability of reaction when two molecules encounter each other) is solely a function of kinetic energy?

    The amount of energy needed to disassociate is probably excessive for this application.
  9. Aug 7, 2014 #8


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    That is what the thermodynamic population statistics suggest.

    Single molecules can only be considered in the vicinity of absolute zero where vibrations and rotations can be investigated and manipulated. But that is another subject, totally separate to the reactions between your gaseous species.
  10. Aug 7, 2014 #9
    Let me ask it this way.

    Shortly before Species A encounters Species B, Species B absorbs a photon which excites one of its vibrational modes. Is it then just as likely to react with Species B as if it didn't absorb the photon at all, or does the added vibrational energy increases reaction probability and what equation would predict by how much?

    Again pretend there are no gas chamber walls to collide with. Obviously there are but humor me.
  11. Aug 7, 2014 #10


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    Molecules may change their modes of oscillation on collision with the humourless gas chamber walls. They also change their modes when in collision with other molecules. At the time of collision and possible reaction, you can consider the two reactant molecules to be partially disassociated, but all in one bag. Following the interaction, they may have exchanged components.

    The presence of photons with the right energies to excite molecular vibrational modes will increase the total available energy and so will increase the probability of reaction. The collision between two molecules will still be statistical.
  12. Aug 7, 2014 #11
    Is rate of reaction simply a function of a scalar "total available energy" or is it more nuanced i.e. certain energy modes contribute more than others to increasing reaction rate?

    If the latter then what equations would quantify the contribution of each energy mode to overall reaction rate?

    I dont even know what the subject would be called. "Quantum chemistry"?
  13. Aug 8, 2014 #12


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    All chemical reactions involve quantum effects. If you use the word “quantum” it becomes more Physics than Chemistry.

    I once suggested that the shrinking Physics Department put as sign outside the Chemistry Department stating “Physics Department, School of Molecular Physics”. They failed to act and are now a small division of the Mathematics Department.

    You must decide whether you are going to consider single molecules close to absolute zero, or the statistics of hot molecules. You cannot have it both ways.

    See; http://en.wikipedia.org/wiki/Molecular_vibration

    Then; http://en.wikipedia.org/wiki/Rate_of_reaction#Factors_influencing_rate_of_reaction
  14. Aug 10, 2014 #13
    I'm trying to have it both ways by assuming a very low pressure, a very large chamber, and a very long spontaneous emission time. Then I think I could maintain a non-equipartitioned state.

    Another way might be to assume that the other precursor (transparent to my pump energy) is present in larger concentration than the precursor I'm pumping energy into. Then my pumped precursor's first collision would likely cause a reaction, not a distribution of energy.

    The Rate of Reaction Wikipedia articles says:
    So creating excited intermediaries is the key here. Now I suppose that if I were to use Arrhenius' equation to predict my new reaction rate I'd need to have some equations that describe how my activation energy, temperature, and pre-exponential factor may change with the new intermediary. Any thoughts on what those equations might be?
  15. Aug 11, 2014 #14


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    I think the reaction mechanisms, their activation energies and the transition state(s) for your reactions need to be studied.

    At the single molecule level, I think accounting for energy by using a matrix representing all the possible energy storage modes will be needed. There is then also a scattering matrix that will specify energy transfer rates between those modes. Some of those modes may be non-linear.

    A 4 Megabyte file; http://highered.mheducation.com/sites/dl/free/0073402656/855958/Chapter_16.pdf
    So, do the vibrational energies add to the available molecular kinetic energy, or do they on average cancel, or present no orientation advantage.
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