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

Storage-ring measurements of the dissociative recombination cross section o f CD+ and CH+ molecular ions have revealed prominent resonances at low ener gy. They were assigned tentatively to capture into core-excited bound Rydbe rg states, electronically coupled both to the electronic initial continuum and to the dissociative final channel. We present here the result of calcul ations based on the multichannel quantum defect theory, using molecular dat a (quantum defects and electronic couplings) extracted from ab initio struc ture computations. in addition to the Rydberg states converging to the init ial ground state X (1)Sigma (+) of the ion, we calculated Rydbeg series con verging to the first two excited electronic states a (3)Pi and A (1)Pi. The cross section convoluted with the experimental anisotropic Maxwell distrib ution presents a resonant structure very close to the experimental one, whe n the ion is assumed to be initially in the lowest rovibrational level of t he 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 theore tical estimate of the branching ratio for dissociation to various asymptoti c limits, obtained by solving coupled equations for the nuclear wave functi ons of interacting (2)Pi states of CH.