15min:

C. D. BOONE, A. CHAND, F. W. DALBY AND I. OZIER, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, Canada, V6T 1Z1.

The magnetic rotation spectrum of ⁷⁹Br₂ has been measured from 14,988 to 15,488 cm^-1 using a tunable, cw dye laser (effective linewidth 2 MHz) with DCM dye. The study was carried out using a cell filled to 6.5 Torr with 99.4% isotopically pure ⁷⁹Br₂. Over 5000 transitions (with Doppler width of about 450 MHz) were measured, the majority of which (> 4500) belong to the A³ _1u--X^{1 +}_g system of the molecule. The range of upper-state vibrational levels observed (v' = 13 to 38) extended very close to the dissociation limit of the A state. With more standard techniques such as absorption or fluorescence, transitions in the A--X system are typically difficult to detect because of much stronger transitions in the same region from the B³ _0⁺u--X^{1 +}_g system; the opposite is true for magnetic rotation, due to magnetic activity of the A³ _1u electronic state.

Magnetic rotation signals normally get weaker with increasing J, since the effective g-factor, g_J, decreases as J increases. As a result, magnetic rotation spectra usually give strong signals only for low-J transitions. For the A--X system of Br₂, however, an interference effect between spin-uncoupling and magnetic mixing to other electronic states---primarily the A'³ _2u state---led to the measurement of rotational transitions right up to the breaking-off point (e.g. J = 82 for v' = 25). These high-J transitions were comparable in strength with those at low-J. The interference effect manifests itself as the addition to g_J of a term that increases with J.

Structure observed in low-J transitions has been attributed to hyperfine effects in the A state. Transitions to quasi-bound levels above the dissociation limit of the A state appear to suffer from strong, systematic perturbations, likely due to interaction with a dissociative state. Perhaps the most interesting feature of the spectrum is a series of extra lines tentatively identified as belonging to the A'³ _2u--X^{1 +}_g system, the result of intensity-borrowing from near-resonant perturbations with the A state v' = 27 rotational levels 2 \le J \le 31.

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\noindent ^a Present address: Unisearch Associates Inc., 96 Bradwick Drive, Concord, ON, Canada, L4K 1K8.