[crosspolymer]

I'm reading a paper that talks about measuring "crystallinity index" of a sample of apatite (https://www.sciencedirect.com/science/article/abs/pii/001670379090230I) and it talks about measuring the "splitting of a triply degenerate antisymmetric bending vibration of orthophosphate". I am a second year undergrad and I've done a few lecture courses on spectroscopy, but how can a degenerate vibrational mode exhibit splitting in IR spectroscopy? How can the mode absorb at different wavelengths if it is degenerate?

Thanks

[mjc123]

It can't - it means that the degeneracy is removed.

An isolated PO₄^3- ion has T_d symmetry, and some of its vibrational modes will be triply degenerate. But if you put the ion into a crystal environment of lower symmetry, where you cannot have triple degeneracies, then this mode must split into an A and an E, or possibly three As. These will generally have similar, but not identical, energies, leading to a splitting of the band in the IR spectrum.

[crosspolymer]

Thank you mjc123. That makes sense.