Chapter 11, Problem 11F.7AE

Interpretation Introduction

Interpretation:

The value of $J$ which corresponds to the location of the band head of a P branch of a transition has to be inferred.

Concept introduction:

An electronic state of energy has its own vibrational states.  The energy between the electronic states is large, followed by vibrational states and then rotational states.  During an electronic transition, electron from ground state moves straight to the excited state keeping the internuclear distance constant.  This is known as the Franck-Condon principle.  When the bond length in the electronically excited state ($\tilde{B}\text{'}$) is shorter than that in the ground state ($\tilde{B}"$): then $\tilde{B}\text{'}>\tilde{B}"$ and $\tilde{B}\text{'}-\tilde{B}"$ is positive.  In this case, the lines of the spectra appear in P branch.