The ground state and excited ?-hole states of CuAu

Resonant two-photon ionization spectroscopy has been applied to jet-cooled diatomic CuAu. Eight band systems have been observed, rotationally resolved, and analyzed. The ground state is X??+ in symmetry, deriving from the 3d??Cu 5d??Au, ?? molecular configuration. Its bond length has been determined as ro = 2.3302 ? 0.0006 ?. (l? error limits). The first excited state is the a??+ state, which derives from the 3d??Cu5d??Au???*? molecular configuration. This state possesses nominal bond order of zero, but is nevertheless bound by Do = 1157?150c m-? and has a bond length of 2.428 ? 0.028 ?. It is observed through the a ??+-X??+ transition which is very weak, with f~ 5X10-?. Seven higher energy band systems are also observed, most of which correlate asymptotically to d-hole states described by the Cu 3d ??4s?, ?S + Au 5d? 6s?, ?D5/2, Cu 3d ?4s? ?D5/2 + Au 5d ??6s?, ?S, and Cu 3d ?4s? ?D3/2 + Au 5d??69, ?S separated atom limits. The D0 + excited state, however, displays a very large oscillator strength in transitions with the ground state(f~0.11), and it is argued that this state corresponds to the ion pair state Cu + Au -. Future experiments are planned to test this hypothesis by measuring the permanent electric dipole moment of this state.