Question

Task 3. Spectroscopy.

a) Why are flouroscence bands lower in energy than absorption bands?

b) If an electronic transition is symmetry forbidden and spin forbidden, list two ways of overcoming this to explain why the bands are still seen in the spectrum?

c) Define the coupling operator that sits between the excited state wave function and the ground state wave function in the transition moment integral.

Answer 1

a)

b) The transition may be forbidden via pure electronic symmetries; however, for an octahedral complex (for example) since it has a center of inversion, the transition is weakly allowed because of vibronic coupling. When the octahedra of a transition metal complex is completely symmetric (without vibrations), the transition cannot occur. However, when vibrations exist, they temporarily perturb the symmetry of the complex and allow the transition even if it forbidden electronically.

c) In an atom or molecule, an electromagnetic wave (for example, visible light) can induce an oscillating electric or magnetic moment. If the frequency of the induced electric or magnetic moment is the same as the energy difference between one eigenstate Ψ1 and another eigenstate Ψ2, the interaction between an atom or molecule and the electromagnetic field is resonant (which means these two have the same frequency). Typically, the amplitude of this (electric or magnetic) moment is called the transition moment. The coupling operator defines the coupling of the lower and upper electronic(or vibrational) states in the transition moment integral. In layman language, it signifies the happening of transition from one state to another.