Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free-jet expansion: Determination of the structure and electronic ground state of nickel dichloride

By use of a free-jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, 58Ni35Cl2, 60Ni35Cl2, and 58Ni35Cl37Cl in natural abundance. Spin components with O values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ?1, ?2, and ?3 have been determined for nickel dichloride in the upper state and for the bending wave number ?2 in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is 3S-g in character. Evidence is presented from the nickel isotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically 3?u in character; it probably derives from an electronic 3?g state. The zero-point averaged bond lengths are determined for both states as r0'=0.209?435(13) nm and r0?=0.205?317(14) nm. The fine structure parameters for the math?3S-g state are interpreted in terms of low lying 1S+g and 3?g states, which are shown to lie a few thousand reciprocal centimeters above the ground state.