Infared ray absorption

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Main Question or Discussion Point

If IR rays at the appropriate frequency are passed through a vaporized sample of an alkane, will the vibrations induced be converted into thermal energy? If yes, then will the process take a substantial amount of time (on the order of seconds)?
 

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GCT
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Read up on vibrational relaxation methods, the process takes shorter than typical electronic excitation
 
Gokul43201
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sid_galt said:
If IR rays at the appropriate frequency are passed through a vaporized sample of an alkane, will the vibrations induced be converted into thermal energy? If yes, then will the process take a substantial amount of time (on the order of seconds)?
Can't say I understand your question - vibrations are thermal energy.
 
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Gokul43201 said:
Can't say I understand your question - vibrations are thermal energy.
Yes but bond vibrations will not help for example in activating or accelerating a reaction until they are converted into translational motion.
Am I correct on this point?
 
Gokul43201
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No, that's not true. Vibrational energy is just as good as translational energy.
 
Astronuc
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Gokul43201 said:
Vibrational energy is just as good as translational energy.
In vibrational energy, the atoms in a molecule are transferring energy/momentum. Also remember, in a solid the atoms are vibrating, i.e. which is more or less translational but contrained to the lattice site.

A vibrating molecule can transfer energy/momenutm to other molecules, but the energy is transferred on the atomic rather than molecular level.
 
GCT
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As far as spectra goes, quantisized (spelling?) energy levels of a molecule refer to its electronic energies levels associated with molecular orbitals, vibrational energy levels are frequently superimposed on these electronic levels, furthermore if applicable, each of these vibrational energy levels are associated with rotational energy levels which have a smaller difference in quantisized energy.

An exicited electron to a particular vibrational energy level of a higher electronic state may undergo one of several different forms of nonradiative relaxation, vibrational relaxation occurs faster than the time frame for radiative emissions such as flourescence. Vibrational relaxation places the electron in the lowest vibrational level of a particular electronic state, it may occur for instance throough steps of collisions with particular molecules or solvent, the excess energy is given off as thermal energy.
I think this is what the OP was referring to.

Just trying to remember what I read in instrumental analysis, both of you (Gokul, Astronuc) are probably familiar with the topic.
 
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Thank you for the replies and the help.

So as I am understanding,

- if the alkane is reacting with another compound and infrared rays of the appropriate frequency are directed at the reacting gases, the infrared ray energy will be deposited into the vibrations of the alkane molecule which will consequently raise the temperature of the mixture and increase the reaction rate?

- if say 2 J of infrared rays are directed at the mixture of which 1.5 J energy is absorbed, the temperature rise will be the same if 1.5 J energy were supplied directly through a burner?
 
Gokul43201
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Yes ... if 1.5J went into exciting vibrational, translational and rotational modes rather than electronic states (and there is a difference between the former and latter). All three of the former modes contribute equally to the internal energy of the system (recall the equipartition theorem) - only different modes are excited at different characteristic energies. However, if you excited electronic states, you would only have fluorescence (or phosphorescence or photoionization, etc.) and the primary response would not create a raise in temperature. Typically though, electroninc excitations are in the UV range. IR is definitely more likely to excite "thermal" (in solids, phononic) modes.
 
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Thank you for the help.
 

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