Vibrational Analysis of the Absorption System of Sulphur Dioxide atλ3400−2600

Photographs of the bands of sulphur dioxide in the region of $\ensuremath{\lambda}3400\ensuremath{-}2600$ have been taken under low, medium and high dispersion, both at room temperature and at 200\ifmmode^\circ\else\textdegree\fi{}K; the pressure of the absorbing gas was varied from 0.3 mm to 480 mm. Thirty bands can be represented by the formula $\ensuremath{\nu}=29622+770{{v}^{\ensuremath{'}}}_{1}+320{{v}^{\ensuremath{'}}}_{2}+813{{v}^{\ensuremath{'}}}_{3}\ensuremath{-}6{{v}^{\ensuremath{'}2}}_{1}\ensuremath{-}2.5{{v}^{\ensuremath{'}2}}_{2}\ensuremath{-}20{{v}^{\ensuremath{'}}}_{1}{{v}^{\ensuremath{'}}}_{2}\ensuremath{-}25{{v}^{\ensuremath{'}}}_{2}{{v}^{\ensuremath{'}}}_{3}\ensuremath{-}15{{v}^{\ensuremath{'}}}_{1}{{v}^{\ensuremath{'}}}_{3},$ where ${{v}^{\ensuremath{'}}}_{1}$, ${{v}^{\ensuremath{'}}}_{2}$, ${{v}^{\ensuremath{'}}}_{3}$, are the quantum numbers of the symmetrical valence, the deformation and the antisymmetrical vibrations respectively. The three fundamental frequencies for infinitesimal vibrations in the upper electronic state are ${{v}^{\ensuremath{'}}}_{1}=794$, ${{v}^{\ensuremath{'}}}_{2}=345$ and ${{v}^{\ensuremath{'}}}_{3}=833$ ${\mathrm{cm}}^{\ensuremath{-}1}$. In addition, twelve bands have been identified that correspond to transitions from excited vibrational levels in the normal state. The relatively long ${{v}^{\ensuremath{'}}}_{1}$ and ${{v}^{\ensuremath{'}}}_{2}$ progressions indicate that both the bond distance and the angle have changed considerably in the transition to the excited electronic state. The vibrationless transition at 29622 ${\mathrm{cm}}^{\ensuremath{-}1}$ is weak, as one would have expected from considerations of the Franck-Condon principle. Substituting the three fundamental frequencies in the formula based on a valence force model, one obtains a value of 100\ifmmode^\circ\else\textdegree\fi{} for the apex angle in the excited state, as compared with 120\ifmmode^\circ\else\textdegree\fi{} in the normal state. The absence of any regularity in the rotational structure supports the resulting conclusion that the molecule in its upper state has become a more asymmetrical top.