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 1-SIGMA+ in symmetry, deriving from the 3d(Cu)(10)5d(Au)10-sigma-2 molecular configuration. Its bond length has been determined as r0 = 2.3302 +/- 0.0006 angstrom (1-sigma error limits). The first excited state is the a 3-SIGMA+ state, which derives from the 3d(Cu)(10)5d(Au)10-sigma-1-sigma*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 angstrom. It is observed through the a 3-SIGMA+ -X 1-SIGMA+ transition which is very weak, with f almost-equal-to 5 x 10(-5). Seven higher band systems are also observed, most of which correlate asymptotically to d-hole states described by the Cu 3d(10)4s1,2S + Au 5d(9)6s2, 2D5/2, Cu 3d(9)4s2, 2D5/2 + Au 5d(10)6s1, 2S, and Cu 3d(9)4s2, 2D3/2 + Au 5d(10)6s2, 2S separated atom limits. The D 0+ excited state, however, displays a very large oscillator strength in transitions with the ground state (f almost-equal-to 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.