Determination of rovibronic term values of a diatomic molecule from experimental data on the wavenumbers of spectral lines

It has been shown that, if experimental data are available on the lines of the rovibronic spectrum of a diatomic molecule that pairwise couple three or more vibronic states, then there is a relation between the wavenumbers measured for the lines and the absolute values of all corresponding rovibronic term values. A method is proposed for determining the optimal set of rovibronic term values with error estimations by varying the desired values of the rovibronic term values so as to minimize the weighted standard deviation of the wavenumbers that are calculated according to the Rydberg-Ritz combination principle from the wavenumbers measured for a sufficiently large number of rovibronic spectral lines of various systems of bands. It is substantial that this method does not require any assumptions on the molecular structure. The new method is applied to determine the rovibronic term values of 12 singlet electronic states of the isotopomer ¹¹B¹H of the boron hydride molecule, which have already been studied experimentally, with the use of all available experimental data for 15 systems of bands, which are obtained in 10 works. Thus, the energy spectrum of the diatomic molecule has been determined from the experimental data on the wavenumbers of rovibronic spectral lines without any assumption on its structure.