Hyperfine structure in the B3Πg-A3Σ+u band of N2

We have recently measured hyperfine splittings in selected rotational lines of the (1–0) vibrational band of the B 3Πg  A 3Σu transition of 14N2 in a low pressure discharge. These measurements were performed primarily to develop the synthesis and spectroscopic analysis methods for very high resolution spectroscopy of other transient species that can be created in an electric discharge. Significant improvements have been implemented in the frequency control and measurement of the laser compared to our prior sub-Doppler saturation work with the Ti:sapphire lasers.  These scans were recorded by stepping a radio-frequency synthesizer between 50 and 500 MHz, to control the offset of the laser frequency from a stable optical reference point. Frequency modulated saturation dip spectroscopy produced hyperfine resolved sub-Doppler lines with typical full-width at half-maximum line widths of 7 MHz. Hyperfine transitions within 27 rotational lines in the R33, Q33, Q21, Q22, R11, and Q11 branches were recorded for J between 2 and 12, and fit with the Hamiltonian used previously by Ottinger and co-workers. [H. Geisen, D. Neuschäfer, and C. Ottinger, Z. Phys. D, 4, 263-290 (1987); 17, 137-144 (1990)]

 
FM-detected sub-Doppler saturation spectrum of N2: hyperfine structure in the R33(10) rotational line of the (1-0) band of B 3Πg – A 3Σu+ near 11331cm-1.  The solid (red) line is a simultaneous fit to observed (blue points) absorption and dispersion features

    Multiple spectra like those shown above provide a secure determination of hyperfine splittings in both A and B states, which can be reduced to a small number of hyperfine constants.  The resulting parameters are related to the distribution of the unpaired electron density around the nuclei in the A 3Σu+ and B 3Πg states, as probed by the expectation of the electron spin and orbital angular momenta.  In combination with previously published results from higher vibrational levels, we were able to characterize the vibrational level dependence of the hyperfine parameters B 3Πg states.  Only the Fermi contact interaction showed strong changes on vibrational excitation and it was found to have a significant quadratic component to its vibrational dependence. Changes with bond length in the small contribution of atomic s character to the half-filled 3σg orbital, which is primarily 3pz in character, are likely responsible.

D. Forthomme, C.P. McRaven, G.E. Hall, and T.J. Sears, J. Molec. Spect., 282, 50-55 (2012).

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