VIBRATIONAL NUMBERING OF THE $A^{2}\Pi(l_{u})$ STATE OF BROMINE

Abstract

The electronic absorption spectrum of bromine at wavelengths above 5100 {\AA} consists of two band systems, B $^{2}\Pi_{o^{+}_{u}}\leftarrow X^{1}\Sigma^{+}_{g}$ and A $^{2}\Pi_{o_{u}}\leftarrow X^{1}\Sigma^{+}_{g}$. The intense $B\leftarrow X$ system has been studied in $detail^{1, 2, 4}$. For the weaker, `extreme-red’. $A\leftarrow X$ System, only vibrational analyses by Brown’ and $Darbyshire^{3}$, and the analyses of a few bands of $^{10-81} Br_{2}$ at high $resolution^{4}$ have been reported. The vibrational numbering of the A $^{3}\Pi_{1u}$, state of bromine has therefore remained uncertain. We have analysed several bands of $^{70} Br_{2}$ and $^{81} Br_{2}$ ($A\leftarrow$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 $^{3}\Pi_{1_{u}}$ state has been unequivocally determined. It is necessary to increase $Brown’s^{4}$ provisional numbering for this state by 7 units.