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Using sub-Doppler laser induced fluorescence spectroscopy on a beam of metastable nitrogen N2(A 3∑+u), the predissociation of the N2(B 3Πg) state was studied quantitatively and under collision-free conditions. Predissociation rate constants γ in the range 0.1–200×106/s were measured (a) from the decreased fluorescence quantum yield and (b) from the increased linewidth. In method (a), coherent saturation effects were quantitatively accounted for. For the 3Πe0,3Πe,f1, and 3Πe,f2 levels, γ is generally high and independent of the molecular rotation in the range J=1–8, while for 3Πf0 levels γ is smaller and increases proportionally to J(J+1). This pattern identifies the N2(A′ 5∑+g) state uniquely as being responsible for the perturbation of N2(B). A pronounced dependence of γ on the vibrational level of N2(B) was also found in the range v′=13–18. It correlates quantitatively with calculated bound–free Franck–Condon factors between the B and A′ states. For the latter, a recent ab initio potential curve was used, with a slight (0.02 Å) radial adjustment. The B–A′ electronic coupling matrix element is on the order of 2 cm−1, much smaller than expected for a spin–orbit mechanism. This can be understood on the basis of the CI composition of the interacting states as calculated ab initio.

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From very high resolution (8 MHz FWHM) LIF measurements, the hyperfine coupling constants of N2(A3Z, +) and N2(B3Hg) have been obtained for three pairs of vibrational quantum numbers v', v'. The A-state constants are in very good agreement with accurate literature data. The vibrational dependence of some of the B-state constants is found to be much more pronounced. This is qualitatively explained in terms of the electronic structure of the two states.

Laser-induced fluorescence on a beam of metastable N& (A X"+) molecules has been used to study the hyperfine structure of the (10,6) band of the N~( 8-A) transition. The hyperfllne structure of the B Hp and Hi states was found to have an unusual, oscillatory J dependent:e. This was explained by perturbation theory, and hyperfine-structure constants for the B state were derived from a data fit. At the same time results from earlier, less-direct experiments on the A-state hyperfine structure were confirmed and extended. Using sub-Doppler laser spectroscopy, the hyperfine structure (hfs) of molecular transitions can be resolved. We have done hfs measurements on the (10,6) band of the Np ( B H g A X"+ ) system by laser-induced fluorescence (LIF), crossing a beam of metastable Nq (A) molecules at right angles with a dye laser beam. A very unusual hfs of the B 'Hp and 'Hl states was found, exhibiting an oscillatory dependence on J. This contrasts with the much more regular hfs of the A state, which is known from earlier, Rabitype experiments on a beam of Nt (A) (Refs. I and 2), but was determined in the present work for a much greater number of J levels. Nq (A) molecules were prepared by striking a dc discharge -2 A directly in an N~nozzle beam, using the nozzle itself as an anode. With a nozzle temperature of 720 K, nozzle diameter of 1 mm, and a backing pressure of 30 Torr, a rotational temperature of 315 K for N& (A, v=6) was measured by LIF. About 300 lines of the (10,6) band of Nq (B A) were record-ed on a strip chart and were identified using molecular constants from the literature. Using very slow ( -1 MHz/sec) laser scan speeds, the hfs of all lines was measured with a resolution of up to 15 MHz full width at half maximum (FWHM), and with very good S/N ratio. The hfs splittings were determined using an interferometer with a 62.S-MHz free spectral range.As an example, Fig. 1 shows the hfs pattern of the 033 ( 2 ) line; I I of the expected I 3 components are resolved. I All distinguishable hfs components of all measured lines were identified, on the basis of the AF=O, +1 selection rule and the approximate proportionality between line intensities and F values. hfs level splittings of both the Nt (B) and Nq (A) states were then derived from the measured splittings of the lines. Figures 3 and 3 show the results. The level separations, as a function of J, are plotted relative to the central component (having F= J), which is shown as a horizontal line.The estim;lted accuracy of the data is +2 MHz. In the Nq (A) state, Fig. 2, hfs splittings of the Fl and F3 finestructure terms (regular and inverted, respectively) tend to a constant limit with increasing J, while the Fq term shows a measurable hfs only for the lowest J. The hfs of N( B, Hp i q'.l Fig. 3, is very striking: For Hz, the splitting decreases ]monotonically, and in the same way for ortho-Nq and para-Nq. For Hp, levels with no detectable splitting alternate with levels which are split. For ortho-Nq, levels with even J are split; for para-Nq, those with odd J...