Spin uncoupling in the 6s Rydberg states of methyl iodide. Rotational subband structure in two-photon resonant absorption

Abstract

Rotational envelopes of some of the lowest-energy (R6s) Rydberg transitions [i.e., the two electronically degenerate B3E(1) and C1E(1) band systems and the totally symmetric A1(0+) band system] of methyl iodide are calculated for a two-photon absorption process and compared to the experimental spectra obtained using the technique of two-photon resonant multiphoton ionization (MPI). The rotational band shapes, which were computed with molecular constants extracted from a simulation of high-resolution, one-photon absorption data, reproduce all significant features of the experimental MPI spectra, including temperature and polarization dependencies. Since one- and two-photon processes are governed by different selection rules, the present results are an independent verification of spin uncoupling in these R6s states. These results, therefore substantiate the conclusion that the R6s electron uncouples from the molecular frame during the evolution of the Rydberg state and that, consequently, the molecular constants extracted from experimental band shape analysis pertain to the freely rotating, electronically degenerate, molecular–ion core.