What is the difference between Raman and IR in terms of polarization? I know IR uses the dipol moment while Raman is based on polarization. What is the difference between dipol moment and polarization?
dipole moment is a separation of positive and negative charges. when this separation changes during the vibrational motion of a molecule it is IR active. if there is no permanent dipole moment, there is no IR activity (e.g. N2, O2, H2).
polarizability is the relative tendency of a charge distribution, like the electron cloud of a molecule, to be distorted from its normal shape by an external electric field, like the one in an electromagnetic wave. if the polarizability changes during the vibrational motion of a molecule it is Raman active.
in Raman it is polarizability rather than polarization. does this already answer your question?
dipole moment is a separation of positive and negative charges. when this separation changes during the vibrational motion of a molecule it is IR active. if there is no permanent dipole moment, there is no IR activity (e.g. N2, O2, H2).
polarizability is the relative tendency of a charge distribution, like the electron cloud of a molecule, to be distorted from its normal shape by an external electric field, like the one in an electromagnetic wave. if the polarizability changes during the vibrational motion of a molecule it is Raman active.
In general, polarization or polarizability is the process of creating dipoles. Dipole moment is the product of magnitude of charge and distance between them.
Induced dipole moment which comes from polarizability is completely distinct from the permanent dipole moment. Some non polar materials (not have permanent dipole moment) highly polarizable.
Your question is the core difference between Raman and IR. As mentioned by Johannes, dipole moment is a separation of positive and negative charges in a molecular structure, such as H2O.
polarizability is the relative tendency of a charge distribution, in other words, the more changes in the electron cloud geometry, the greater the polarizability.
Lets look at H2O to know whether it has polarizability or not. H2O has a very high permanent dipole moment, so its electron cloud geometry may not be reshaped due to its highly rigid electron cloud shape.
On the other hand CO2 possesses a high polarizability and its electron cloud can be reshaped.
IR activity is due to "change in dipole moment (induced dipole moment)" of CO2 which is caused by bending and asymmetrical stretching in its bonds. Therefore, for the IR activity of a molecule, there is not always need for a permanent dipole moment.
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