Isotropic absorption and emission

  • Thread starter ArchieDave
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Something I've yet to understand: If a molecule has a dipole moment about a given access than absorption of a photon can readily occur. However, if it is possible to preferentially orient molecules by applying an electric field, would the rate of absorption be greatly increased or decreased? It seems like this would influence typical spectroscopic approaches when a field is present and I've never seen that to be the case. If this is true, is the affect mitigated in atoms where the the electrons can be "pushed around" by the laser field?
 

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DrClaude
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However, if it is possible to preferentially orient molecules by applying an electric field, would the rate of absorption be greatly increased or decreased?
Yes, absorption depends on the angle between the transition dipole and the polarization of the photon. When calculating absorption in a gas, one often starts from the maximum transition dipole moment (molecules aligned with the field), and then averages out over all possible angles.

It seems like this would influence typical spectroscopic approaches when a field is present and I've never seen that to be the case.
The fields needed to significantly orient molecules are very high. Ususally, to achieve orientation, an intense laser pulse is needed. Static fields are generally much too weak.

If this is true, is the affect mitigated in atoms where the the electrons can be "pushed around" by the laser field?
To see an effect in atoms, you would need a strong polarization to see an effect. Again, that would require laser-strength fields (or maybe processes such as optical pumping).
 

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