Electronic Molecular Spectra - significance of band origin?

[DanAbnormal]

Can anybody tell me the significance of the band origin for a P(Q)R structure in the ultraviolet region of a molecules spectrum?
In standard vibrational spectra, I understand that its roughly the harmonic frequency of the oscillator, and that the rotational constant can also be found from separations.

Now say I am looking at vibration-rotation transitions between the ground state and the excited state. What is the significance of the band origin and rotational constant?

If the internuclear separations of the ground state and excited state are equal, then the spectral structure should be like that for ordinary vib-rotational spectra, is that right? Therefore the ground state and first excited states have the same rotational constant B (becuase it depends on R, internuclear distance) but what doe the band origin signify, because obviously its positioned somewhere in the UV, whereas the band origin for normal vibration spectra is centred in the infrared...

Can someone enlighten me please?!

Cheers

 

[BigJDubs]

The band origin for a P(Q)R structure is the energy of the lowest energy state in a given vibrational transition. This is the same as the zero-point energy for an ordinary vibrational spectra. The rotational constant represents the energy associated with the rotation of the molecule, and the position of the band origin can provide information about the strength of the intermolecular interaction between the ground state and the excited state.