Resonant Auger decay of dissociating CH₃I near the I 4d threshold

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH3I in the region just below the I 4d(-1) ionization threshold. The resonances include the Rydberg series converging to the five spin-orbit and ligand-field split CH3I (I 4d(-1)) thresholds, as well as resonances corresponding to excitation from the I 4d(5/2,3/2) orbitals into the sigma* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH3I+, in particular, the X E-2(3/2) and E-2(1/2) states, and on spectator decay that populates the lowest-lying (CH3I2+)sigma* states of CH3I+. The CH3I (I 4d(-1))sigma* resonances are broad, and dissociation to CH3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v(3)(+) similar to 25) in the C-I stretching mode of the X E-2(3/2) and E-2(1/2) states, providing insights into the shape of the dissociative core-excited surface. The observed spectator decay processes indicate that CH3I+ is formed on the repulsive wall of the lower-lying (CH3I2+)sigma* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d(-1))sigma* and (CH3I2+)sigma* potential surfaces. Data are also presented for the spectator decay of higher lying CH3I (I 4d(-1))nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal.