Doubly Excited States of Cb, C;, and C;+

The stability of strongly bound excited states of Cat Cz and Cz’ resulting from double excitations of the type aE+MO’ relative to the ground-state configuration (with MO2 standing for 317:, 3a&r” or lx: ) was investigated using a multi-reference MRD-CI method. For Cz, the doubly excited configuration 30~1~: corresponds to the experimentally observed E’C: (3’Z: ) state, which exhibits a heavy mixing with the less bonding configurations 20:1x: (X’C,’ ) and 2a~3a~lx~ (2’Z: ). This mixing is reflected in two ways: the E’,Y,’ state does not show the features expected for a strongly bound state (i.e. short R, and high o,), but despite its mainly two-electron 3a,lrr,+2a~ character it allows the emission band E’C: +AlZZ, to be observed with a moderate intensity by one-photon spectroscopy. The states l*.Zz (2cr,lxi) and 2*x,’ (30,ln~ ) of C: as well as 23,‘fl,, (3r4la~ ) and 3r,?Y: (1~: ) of Cg+ exhibit a definite enhanced bond strength relative to the ground state. Radiative decay from all these doubly excited states into lower-lying, singly excited 217, states should be suitable for experimental study because of favorable Franck-Condon envelopes and the one-electron character of the transition.