Raman transitions must currently be
specified using
spherical
tensor
notation, as commonly used for multiphoton transitions. Normal Raman
spectroscopy uses
two photons, and this has the same selection rules as two photon
transitions. For both of these you must specify a
Rank and a
Component, which
PGOPHER labels overall as
T(
Rank,
Component). For two photon and
Raman transitions possible values for Rank are 0, 1 (often weak) and 2;
in general the highest rank is the number of photons involved. For each
Rank,
k, there will be 2
k+1 components from -
k to +
k. It is common to need more than
one of these to simulate a given transition, as in the one photon case
where more than one cartesian component of the transition moment may be
required for asymmetric tops. Note also that for asymmetric tops the
different signs of the components have a special meaning and correspond
to different transition moments as defined
here and you will
need to choose the right one (or possibly both will be required).