MAGNETIC ROTATION STUDY OF THE $A^{3}\Pi_{1 u}-X^{1}\Sigma_{g}^{+}$ SYSTEM OF $^{79}Br_{2}$

$^{a}$Present address: Unisearch Associates Inc., 96 Bradwick Drive, Concord, ON, Canada, L4K 1K8.Author Institution: Department of Physics and Astronomy, University of British ColumbiaThe magnetic rotation spectrum of $^{79}Br_{2}$ has been measured from 14,988 to $15,488 cm^{-1}$ using a tunable, cw dye laser (effective linewidth $\sim 2$ MHz) with DCM dye. The study was carried out using a cell filled to $\sim 6.5$ Torr with 99.4\% isotopically pure $^{79}Br_{2}$. Over 5000 transitions (with Doppler width of about 450 MHz) were measured, the majority of which $(> 4500)$ belong to the $A^{3}\Pi_{1 u}-X^{1}\Sigma_{g}^{+}$ 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 $B^{3}\Pi_{0+u}-X^{1}\Sigma_{g}^{-}$ system; the opposite is true for magnetic rotation, due to magnetic activity of the $A^{3}\Pi_{1 u}$ 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 $Br_{2}$, however, an interference effect between spin-uncoupling and magnetic mixing to other electronic states---primarily the $A^{13}\Pi_{2a}$ 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 $A^{\prime 3}\Pi_{2u}-X^{1}\Sigma_{g}^{+}$ system, the result of intensity-borrowing from near-resonant perturbations with the A state $v' = 27$ rotational levels $2 \leq J \leq 31$