Knowledge Bank

The electronic absorption spectrum of bromine at wavelengths above 5100 {\AA} consists of two band systems, B $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi ou+\leftarrow X1\Sigma g+$ and A $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi ou\leftarrow X1\Sigma g+$. The intense $B\leftarrow X$ system has been studied in $detail1,2,4$. For the weaker, `extreme-red’. $A\leftarrow X$ System, only vibrational analyses by Brown’ and $Darbyshire3$, and the analyses of a few bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">10-81</mrow>}Br2$ at high $resolution4$ have been reported. The vibrational numbering of the A $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi 1u$, state of bromine has therefore remained uncertain. We have analysed several bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">70</mrow>}Br2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">81</mrow>}Br2$ ($A\leftarrow$X) at high resolution, using a 6 m absorption cell. Accurate data for the rotational constants and band origins have been measured for both isotopes, and the vibrational numbering of the A $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi 1u$ state has been unequivocally determined. It is necessary to increase $Brown\prime s4$ provisional numbering for this state by 7 units.