ABSTRACT
 

Hönl-London type approximation expressions are derived for transition strengths of the W´=0,1,2,3, <- W"=0 (S,P,D and F <- S) three photon transitions for diatomic molecules belonging to Hund´s case (a) and intermediate (a)-(b) coupling schemes. These are used to demonstrate what may be seen and what may not be seen in three photon absorption spectra. The forms are used to simulate room temperature (3+1)REMPI spectra of HBr, for different electronic transitions. The analysis as well as comparison with (2+1)REMPI spectra is used to demonstrate the usefulness of three photon absorption spectroscopy to identify excited states and to derive spectroscopic parameters.  A Rydberg state, not observed  in single or two photon absorption, with band origin  82837 cm^-1 was identified and analysed for the first time. It was assigned as the L¹F(3) ((s²p³)5dd)) state, (0,0) band.