The Structure of the Ultraviolet Absorption Spectrum of Formaldehyde. I

Abstract

The ultraviolet absorption spectrum of formaldehyde was photographed under high dispersion so that the rotational structure could be analyzed. The present paper gives details of six bands at 3530, 3430, 3390, 3370, 3295 and 3260A. The bands are due to transitions in which the electric moment is in the plane of the molecule and perpendicular to the CO-axis. As the asymmetry of the molecule is very slight, the structure of the bands is very nearly the structure of vertical bands due to a symmetrical top. The position and intensity of the lines is in very good agreement with this except if the component $K$ of the angular momentum along the CO-axis is smaller than four. In this case, the effect of the asymmetry makes itself felt in a shift and doubling of the lines in agreement with the theoretical expectations. A rigorous check of all the regularities is furnished by combination relations. The distortion of the molecule by the rotation can be studied in the deviations of the observed energy values from those of a rigid top. A value for the force between the two C - H bonds results from this. In the bands 3430 and 3295 there are strong perturbations which affect the dependence of the rotational energy on the quantum number $K$ only, not that on the total angular momentum $J$. The molecular constants for the normal state of the molecule and six vibrational states of the excited electronic level can be obtained with good accuracy (see Table V and §10).