The evaluation has a two-fold purpose, first, the selection of the best set of measured transition frequencies and, second, selection or calculation of consistent and reliable spectroscopic constants. Since measured or calculated uncertainties are the best indicators of the quality of the data, a substantial portion of the critical evaluation effort has gone into determining these uncertainties.
Generally, the selection of the most reliable transition frequencies posed few problems since there were relatively few cases where duplicate measurements have been reported for the same transition and most laboratories quote reliable uncertainties. In cases where problems did occur, the selection was based on both the overall consistency of the measurement in question with the other spectral data available, and on the reported uncertainty in the measurements. In nearly all cases the measurements with the smallest uncertainties reported by the authors were found to be the most reliable.
The determination of the most reliable molecular constants posed more severe difficulties. Occasionally inconsistencies arose in cases where data were reported by several independent workers who studied quite different regions of the spectrum, e.g., molecular beam measurements vs. microwave measurements or centimeter vs. millimeter-wave measurements. If all of the available data had not been analyzed simultaneously in these instances, a reanalysis was carried out to eliminate the discrepancies. These calculations also resulted in the detection of a number of misprints in the literature, which were not obvious through simple inspection of the reported assignments and transition frequencies. Since a question might arise concerning the correct value when a difference is noted between the present compilation and the reference cited, there are comments following the molecular constant tables in these cases.