Rotational analysis and assignment of the green band system of FeH to the e ⁶II-a ⁶Δ; transition

The technique of laser excitation spectroscopy at Doppler resolution has been used to record bands in the electronic spectrum of FeH around 532 nm. A number of lines were also recorded at sub-Doppler resolution using intermodulated fluorescence spectroscopy. Dispersed fluorescence studies were used extensively to aid the assignment of a total of 153 lines in this region of the spectrum to either the (0,0) or the (0,1) band of the e ⁶II-a ⁶Δ; transition. All the assignments from the excitation spectrum obey the selection rule ΔΩ=-1, giving subbands which involve the lowest three spin components in each state; transitions with ΔΩ=0 were seen only in dispersed fluorescence. Term values have been determined for the three components in both the ground vibrational level of the e ⁶II electronic state and the first excited vibrational level of the a ⁶Δ electronic state (those for the υ=0 level of the a ⁶Δ state have been determined previously). This study provides the first characterization of the spin components and the rotational levels of the e ⁶II state of FeH. This state is heavily perturbed and exhibits pronounced lambda-type doubling, making it very difficult to model the energy levels using an effective Hamiltonian approach.