## Abstract

Rotational analysis of band contours of the 0-0 band at 3280 $\overset{\circ}{\mathrm A}$ and a 1-0 band at 3230 $\overset{\circ}{\mathrm A}$ of the asymmetric top 2,1,3-benzothiadiazole have been carried out. The method used is that of computer simulation of the observed contour with the band type, i.e. rotational selection rules, and excited state rotational constants A', B', and C' as input data. It is shown that the 0-0 band is type B and therefore that the electronic assignment is $^1$B$_2$-$^1$A$_1$. The 1-0 band at 3230 $\overset{\circ}{\mathrm A}$ is shown to be a type A band from which it follows that the vibration active in this band must be of symmetry species b$_2$. The excited state rotational constants for the $^1$B$_2$ electronic state are: $A' = 0\cdot1309 \pm 0\cdot0003 cm^{-1},\, B' = 0\cdot0405 \pm 0\cdot0001 cm^{-1},\, C' = 0\cdot0309 \pm 0\cdot0001 cm^{-1}.$ The quoted uncertamties are those of the changes of the rotational constants and do not mclude those of the ground state. The excited state was assumed to be planar and the results support this assumption. One feature of the rotational constants is a slight decrease of I$_A$. This, together with information from a vibrational analysis of the system, is consistent with an increase of the C$_5$C$_4$C$_9$ angle in the excited state. The origin of the 0-0 band is at 30410$\cdot$5 $\pm$ 0$\cdot$2 cm$^{-1}$.