Core-excited resonances in the dissociative recombination of CH+ and CD+ -: art. no. 052711

Storage-ring measurements of the dissociative recombination cross section of CD+ and CH+ molecular ions have revealed prominent resonances at low energy. They were assigned tentatively to capture into core-excited bound Rydberg states, electronically coupled both to the electronic initial continuum and to the dissociative final channel. We present here the result of calculations based on the multichannel quantum defect theory, using molecular data (quantum defects and electronic couplings) extracted from ab initio structure computations. in addition to the Rydberg states converging to the initial ground state X (1)Sigma (+) of the ion, we calculated Rydbeg series converging to the first two excited electronic states a (3)Pi and A (1)Pi. The cross section convoluted with the experimental anisotropic Maxwell distribution presents a resonant structure very close to the experimental one, when the ion is assumed to be initially in the lowest rovibrational level of the ground state. We can thus assign the resonances observed near 0.3 and 0.9 eV to low vibrational levels of the (2)Pi core-excited Rydberg states (a (3)Pi )5s sigma and (A (1)Pi )3p sigma, respectively. We also give a theoretical estimate of the branching ratio for dissociation to various asymptotic limits, obtained by solving coupled equations for the nuclear wave functions of interacting (2)Pi states of CH.