A critical re-assignment of the Rydberg states of iodomethane based on new polarization data.

2- and 3-photon excitation of components of the lower Rydberg states of iodomethane (CH3I) using linearly and circularly polarized light, followed by ionization with one more photon, is used to determine their molecular term symbol, Ω, values as well as quantum defects. These Ω values, together with a detailed theoretical analysis, require a re-assignment of the 7s and 8s states to various components of the 5d and 6d states, but there is evidence of (n+2)s∕nd hybridization in the pairs of Ω = 1 states. Predissociation sets in for all Rydberg states beyond 6d based on the ground ((2)Π(3∕2)) state of the core, but sharp autoionizing resonances based on the (2)Π(1∕2) core state are assigned to the 9s, 7d, and 5f states. The dominant effect of the singlet∕triplet character of the Rydberg states on their accessibility from the ground state, seen in bromomethane and chloromethane, is again apparent and a concordant interpretation of the Rydberg spectra of CH3I can now be presented. Evidence for coupling of some Ω = 1 and Ω = 0(+) Rydberg states with a repulsive valence state and an ion-pair state, respectively, is also put forward.