Observation and analysis of rotational and nuclear hyperfine structure in bands of the red system of copper dichloride

Abstract

Part of the electronic spectrum of copper dichloride, CuCl₂, between 590 and 600 nm, has been recorded in the gas phase using a sample prepared at low temperatures (ca. 10 K) in a free jet expansion. Under these conditions it is possible to resolve vibrational, rotational and even hyperfine structure. Nine bands in this region have been analysed in detail. Six involve the dominant isotopomer, Cu³⁵Cl₂ and the other three the mixed form, Cu³⁵Cl³⁷Cl (it is not possible at this stage to identify the copper isotope). All the bands show parallel rotational structure of a linear molecule, consistent with an electronic assignment to a ²Π_u–²Π_g transition suggested by ab initio calculations. Seven of the bands are assigned as ²Π_3/2–²Π_3/2, one as ²Π_1/2–²Π_1/2 and the other as ²Δ_5/2–²Δ_5/2. The majority of the bands are thought to involve the molecule in its ground vibrational state but the last of these presumably involves excited vibrational levels in the bending vibration ν₂. The Cu—Cl bond length determined from the lower-state rotational constant for Cu³⁵Cl₂ is r^″₀= 0.203 614(26) nm. The greater occurrence of transitions which involve the ²Π_3/2 spin component suggests that this spin component is lower in energy than the ²Π_1/2 component, contrary to theoretical expectations. The vibrational structure of the spectrum in the wave-length region studied is very complicated and no detailed vibrational assignments have been made. It is thought that the (0–0) band of the electronic transition lies at significantly longer wavelengths than those covered in this study.