Hyperfine structure of N2 (B 3? g andA 3? u + ) from LIF measurements on a beam of metastable N2 molecules

Abstract

Crossing an intense beam of nitrogen molecules in the metastable N2(A) state with the beam from a CW dye laser, laser-induced fluorescence was observed in the first positive system of N2,B3Πg−A3Σu+. About 300 lines of the (10, 6) band were studied at sub-Doppler resolution (15 MHz FWHM). From the well-resolved hyperfine structure of the lines, the hyperfine splittings of both the upper and the lower state were derived for a range of rotational quantum numbers up toJ=12. Using multiple independent determinations of each splitting via lines belonging to different branches, the hfs could be measured with an accuracy of about 2 MHz. Fitting known theoretical expressions for the hyperfine energies to the data, the following nuclear coupling constants were obtained (in MHz): For theA state,v=6: α=12.86, β=−11.40,e2q0Q=−2.5. For theB state,v=10:K11=86.46,D11=12.67,D1−1=−44.64,G11=69.18,Q11=0.64,Q1−1=1.38. The hfs is mostly due to nuclear magnetic dipole interactions. For theA state the results are essentially in agreement with hfs constants derived from RF resonance experiments, but are superior as regards the data fit over the entireJ range covered. For theB state, the results are new and are interpreted in terms of a simple LCAO model. The Fermi contact coupling constant is in good agreement with unpublished SCF results by V. Staemmler. The striking dependences of the hfs splitting on the fine structure levels, Λ sublevels and onJ are explained both quantitatively and in terms of vector models.