Why do molecules have rotational Raman spectra?

[QMrocks]

Hi all,

i am studying the physics of Raman Spectra. Was able to understand the origin of Vibrational Raman Spectra... which was an effect due to the change of polarizability with internuclear distance due to molecular vibration. But, i still yet to figure out why rotation of molecules can result in Raman spectra too? I'm reading the book 'molecular physics and elements of quantum chemistry' by H. Haken. They mention something about the polarizability of the molecule as a tensor and that its their tensor anisotropy that contribute to this effect. But i was not able to appreciate in essence why this is so... Can someone pls direct me to better resource on this?

Thanks.

 

[dgiznya]

rotational raman

An easier way to understand it might be to state that for a molecule to be rotationally Raman active, the polarizability perpendicular to the rotational axis should be anisotropic, or the polarizability should be different in different directions. This results in the polarizability being described by a tensor.

dgiznya

 

[QMrocks]

An easier way to understand it might be to state that for a molecule to be rotationally Raman active, the polarizability perpendicular to the rotational axis should be anisotropic, or the polarizability should be different in different directions. This results in the polarizability being described by a tensor.

dgiznya

This is exactly the point which i do not understand. Why must the polarizability be anisotropic for it to be Raman active?

 

[dgiznya]

rot raman

If the polarizability is not anisotropic the polarizability will not change as the molecule rotates and you need this changing polarizability for the molecule to be active.

This is similar to vibrational raman where the polarizability of the molecule must change as the molecule vibrates for it to be raman active.

dgiznya

 

[QMrocks]

If the polarizability is not anisotropic the polarizability will not change as the molecule rotates and you need this changing polarizability for the molecule to be active.

This is similar to vibrational raman where the polarizability of the molecule must change as the molecule vibrates for it to be raman active.

dgiznya

You mean you need this anisotropy to see the Raman shifted lines? These discrete lines are however QM effects, which is due to the transistion to an excited virtual state to a rotational state or ground state (depending if Stokes or anti-Stokes). How would one able to bring the anisotropic effect into this QM picture? It seems to me even if the polarizability is isotropic, these transitions can still take place...