What to see and what not to see in three-photon absorption: (3+1) REMPI ofHBr

Honl-London type approximation expressions are derived for transition stren gths of the Omega'=0,1,2,3, <--Omega"=0 (Sigma, Pi, Delta and Phi <--Sigma) 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 spect ra. The forms are used to simulate room temperature (3+1)REMPI spectra of H Br, for different electronic transitions. The analysis as well as compariso n with (2+1)REMPI spectra is used to demonstrate the usefulness of three-ph oton absorption spectroscopy to identify excited states and to derive spect roscopic parameters. A Rydberg state, not observed in single or two-photon absorption, with band origin 82 837 cm(-1) was identified and analyzed for the first time. It was assigned as the L(1)Phi(3) ((sigma(2)pi(3))5d delta) ) state, (0,0) band. (C) 2000 American Institute of Physics. [S0021-9606(00 )00929-6].