The 1650-1350 Å absorption spectrum of nitrogen dioxide

An analysis of the 1650-1350 $\overset{\circ}{\mathrm A}$ band system of nitrogen dioxide has been carried out. A pattern of band spacings and intensities is found that is complex but regular. It is shown that this pattern is qualitatively, and to a large extent quantitatively, just what would be expected for a transition in which the shape of the molecule changes from bent to linear. The transition is a parallel one and the upper state has $^2\Sigma^+_u$ symmetry. The symmetrical stretching frequency is increased from its ground-state value to ca. 1420 cm$^{-1}$ in the upper state. The upper-state bending frequency is ca. 600 cm$^{-1}$. The N-O length is decreased from its ground-state value, probably to 1.1(3) $\overset{\circ}{\mathrm A}$. The upper state resembles closely the ground state of NO$^+_2$. The transition is to be classed as one of the Rydberg transitions leading to the first ionization potential of NO$_2$; and the orbital to which the odd electron is transferred in the transition is (p$\sigma$) in type. The anharmonic constant g$_{22}$ for the linear upper state is found to be 2$\cdot$(3) cm$^{-1}$. Other Rydberg transitions may well be present in the region, but have not been definitely identified.