Raman transitions must currently be specified using Spherical tensor
notation (like multiphoton transitions). Normal Raman spectroscopy uses
two photons, and can give rise to Rank 0, 1 (often weak) and 2
contributions to the overall transition moment. You may need more than
one of these to simulate a transition, and for each Rank, k, there will be 2k+1 components from -k to +k. PGOPHER labels these as T(Rank,
Component). For a linear
molecule the following components might be expected to contribute: