Module 3 : Molecular physics and molecular structure
Lecture 32 : Rotational structure of particular electronic transition
Case-III
Let us first consider the case where <
In this case, from equation-32.9
it can be seen that is negative quantity. So for m negative i.e P-branch, the separation will go on increasing. On the other hand, for m positive, the sign of the second and the third term will be opposite. Hence for R-branch the spacing between the lines will decrease and slowly the lines will merge. This forms the band head R-branch which is the vertex of the parabola as shown in figure-32.4.
Figure- 32.4
If we see this kind of pattern it means that the equilibrium internuclear distances for the upper state is larger than that of the lower electronic state. The diatomic molecule does change the molecular conformation going from lower electronic state to higher electronic state.
<
is negative quantity. So for m negative i.e P-branch, the separation will go on increasing. On the other hand, for m positive, the sign of the second and the third term will be opposite. Hence for R-branch the spacing between the lines will decrease and slowly the lines will merge. This forms the band head R-branch which is the vertex of the parabola as shown in figure-32.4. 