Electronic spectroscopy and electronic structure of copper acetylide, CuCCH

The optical spectrum of the linear CuCCH molecule has been investigated for the first time, using resonant two-photon ionization spectroscopy employing ArF (193 nm) or F2 (157 nm) excimer radiation for photoionization. Scans over the range 19 400-25 200 cm(-1) were conducted, leading to the observation of three electronic band systems. These are identified as the [20.2] ã1 ← X̃ (1)Σ(+), the [23.1] à (1)Σ(+) ← X̃ (1)Σ(+), and the [24.7] B̃ (1)Π ← X̃ (1)Σ(+) systems, although only the first two have been rotationally resolved. The ã1 state is tentatively assigned as having (3)Π1 symmetry, becoming optically accessible through spin-orbit interaction with the B̃ (1)Π state. Vibrational assignments have provided the frequency of the Cu-C stretching mode, ν3, in the ground and all three excited states, along with both bending modes, ν4 and ν5, in the à (1)Σ(+) and B̃ (1)Π states, and the Cu-C≡C bending mode, ν5, in the ground state. Comparisons are made to the known electronic states of CuF, CuCl, CuBr, and CuI, and it is argued that like these molecules, the CuCCH molecule is essentially ionic in both the ground and excited states, with the ground state correlating diabatically to Cu(+) (3d(10), (1)S) + CCH(-) (X̃ (1)Σ(+)) and the excited states correlating diabatically to Cu(+) (3d(9)4s(1), (1,3)D) + CCH(-) (X̃ (1)Σ(+)).