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Electronic ground state (S 0 )

Electronic excited state (S 1 )

Nuclear displacement

m m

v=0

hn

DH

Re(ground)

v=1

v=2

v=3

v=4

v=5

E

Fig. 12.3Energy diagram for a diatomic molecule exhibiting rotation, vibration as well as an electronic structure.

The distance between two massesm 1 andm 2 (nuclear displacement) is described as a Lennard–Jones potential

curve with different equilibrium distances (Re) for each electronic state. Energetically lower states always have

lower equilibrium distances. The vibrational levels (horizontal lines) are superimposed on the electronic levels.

Rotational levels are superimposed on the vibrational levels and not shown for reasons of clarity.

480 Spectroscopic techniques: I Photometric techniques