Where \ vec { d } is the dipole moment operator of the atom.
2.
*in quantum chemistry programs like MOLPRO or DALTON the second moment operator is a tensor defined as the sum of the tensor squares of the positions of a single electron.
3.
Both can be observed, in spite of the Laporte rule, because the actual transitions are coupled to vibrations that are anti-symmetric and have the same symmetry as the dipole moment operator.
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