Rotational spectroscopy probes the lowest energy quantized molecular transitions. When assuming a perfect separation of rotational and vibrational energies, knowing the moment of inertia in three orthogonal axes is sufficient to determine the exact energy for each of these rotational transitions. At higher rotational energies, however, the vibration-rotation interactions contribute significantly to the observed energy levels of transitions, and so the Hamiltonian requires additional terms. Other deviations from the rigid-rotor assumption, such as large amplitude motion or hyperfine splitting require their own terms. The current state of the field for determination of these rotational constants is a manual process: a limited number of observed transitions are assigned by an expert researcher to sets of quantum numbers and least-square fits perturb values of rotational constants to align the assigned energies. This process requires significant expertise and time, along with costly ab initio calculations to create plausible starting values for each rotational constant.
There are programs which seek to automate this fitting process. As yet, however, all are designed for use in the sub 20 GHz region of the electromagnetic spectrum. At these low energies, distortions from the rigid-rotor approximation tend to be very small, and there are a limited number of transitions. When using these programs on spectra collected in the 100-900 GHz region, this approximation does not hold. The extant programs also operate with local techniques similar to the hand-fitting approach: small numbers of transitions are observed and assigned together. The vastly larger numbers of transitions which come from a larger spectral coverage makes this approach far more costly.
I present here a global rotational spectral fitter, RebelFit (Rotationally Excited Broadband Experimental Line Fitter), its development, validation on a previously assigned spectrum, and the plausible novel detection of two species within the previous dataset. I also present a brief primer on rotational spectroscopy and instrumental development on several rotational spectroscopy apparatus.