While the photodissociation of methyl iodide (MeI) and its Rydberg state excitation energies in the Franck-Condon region have been extensively studied, mapping Rydberg states outside of the Franck-Condon region remains a challenge. In work previously undertaken by collaborators, ultrafast time-resolved photoelectron spectroscopy has been employed to map MeI Rydberg states. Higher-lying excited states of MeI were reached using an intermediate repulsive excited state and varying the probe pulse energy. As experimental data only contain information regarding electron binding energies and intensities, it was difficult to assign excited state potential energy surfaces to specific Rydberg states. Developing a full understanding of spectroscopic data, therefore, necessitates companion simulations. In this work, multireference configuration interaction (MRCI) ab initio calculations with/without spin-orbit coupling were therefore performed using the MOLPRO package on the valence and 6s and 6p Rydberg bands of the CH3I molecule as a function of the methyl-iodine distance with frozen C3v geometry. Spin-orbit coupling was required to generate the appropriate valence and Rydberg 6s and 6p bands, producing 44 spin-orbit coupled states and obtaining transition dipole moments between every pair of states. Results demonstrate that the valence band of MeI is repulsive while Rydberg states exhibit a PES curvature similar to that of the MeI+ ion. Previous experimental peaks were fit to a C-I distance using the calculated repulsive valence state surfaces and ion ground state surface, whose curvatures resemble those of the Rydberg states from the calculation. Transition dipole moments between repulsive valence states and Rydberg states show that the 6s1/2 and 6p1/2 Rydberg states possess significant transition moments with the 3Q0+ repulsive state. However, the adiabatic PESs of Rydberg spin-coupled states are far more complicated due to their “avoided crossings.” Thus, further comparisons between experiments and computations are needed for a more comprehensive explanation. This calculation provides a quantitative analysis of the PESs of MeI Rydberg states, facilitating the future interpretation of experimental results.

Ab initio study of CH3I Rydberg states with spin-orbit coupling

2019 Spring National Meeting

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