The microwave spectrum of SiF+

The first detection of the silicon monofluoride cation by spectroscopic means has been achieved at millimeter and submillimeter wavelengths. Frequencies of rotational transitions spanning a range of J values from 1 to 14 and all vibrational states from v=0 to 15 were precisely measured. Lines of 29 SiF+ up to v=4 and 30 SiF+ up to v=3 were also included in this study. These data were all well fit by a standard Dunham expansion with eight terms, with no requirement for a Watson type ΔSi01 parameter, describing breakdown of the Born–Oppenheimer approximation, to explain the isotope dependence. The parameters Be (or re ), ωe , and the Dunham potential constants a1 –a5 were well determined from this analysis and showed very satisfying agreement with the results of our recent large basis set MP4SDQ and CI calculations, which we had used to determine the search range for locating the spectrum of SiF+ . The final results are re =1.526 495 0(2) Å and ωe =1050.7(2) cm−1.