The ground state and excited d-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 1Σ+ in symmetry, deriving from the 3d10Cu5d10Auσ2 molecular configuration. Its bond length has been determined as r0=2.3302±0.0006 Å (1σ error limits). The first excited state is the a 3Σ+ state, which derives from the 3d10Cu5d10Auσ1σ*1 molecular configuration. This state possesses a nominal bond order of zero, but is nevertheless bound by D0=1157±150 cm−1 and has a bond length of 2.428±0.028 Å. It is observed through the a 3Σ+–X 1Σ+ transition which is very weak, with f≊5×10−5. Seven higher energy band systems are also observed, most of which correlate asymptotically to d-hole states described by the Cu 3d104s1, 2S+Au 5d96s2, 2D5/2, Cu 3d94s2, 2D5/2+Au 5d106s1, 2S, and Cu 3d94s2, 2D3/2+Au 5d106s2, 2S separated atom limits. The D 0+ 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.