The Absorption Spectrum of Chlorine Dioxide

The wave-lengths of 134 absorption bands of Cl${\mathrm{O}}_2$ have been measured with the Hilger E-1 and E-185 quartz instruments. The bands have been arranged in progressions and the energy levels deduced. Five vibrational levels of the normal electronic state have been found; the vibrational levels of the excited electronic state can be described with the use of two vibrational quantum numbers. Formulae for these bands and their isotope separations are given. The shapes of the molecule in the normal and excited electronic states have been calculated from Bjerrum's formulae for the vibrations assuming valency forces and all but one solution for each electronic state is excluded by the isotope effect or the intensity distribution of the bands. The central force formulae give no possible solutions. The normal modes of vibration for the molecule have been calculated. The intensity distribution is consistent with the extension of the Franck-Condon principle to polyatomic molecules. The predissociation limit agrees with a very approximate extrapolated value for the energy of dissociation through one mode of vibration of the excited electronic state into a ClO molecule and an O atom. Dissociation through a second mode of vibration of the excited electronic state gives only a very rough value. A discontinuity in the $\Delta E:{v_2}^{\prime }$ curve noted by Goodeve and Stein appears at about the energy required for an oscillation of the molecule through a straight line position and it is proposed that this is the cause of the discontinuity.