Ultraviolet Resonance Spectrum of the Iodine Molecule

Abstract

The ultraviolet resonance spectrum of I₂ observed in electric discharges in the region 1830–2370 A has been photographed at high resolution with a 21‐ft vacuum grating spectrograph, and a thorough and complete analysis of the data in this region is given. The five doublet resonance series are shown to be excited from five rotational levels of vibrational level v″=0 of the ground state of I₂ to three vibrational levels ${\mathrm{(v}}_n^{\prime }{\mathrm{,v}}_{\text{n+0}}^{\prime }{\mathrm{,v}}_{\text{n+4}}^{\prime })$ of an upper electronic state by a single iodine atomic line 1830.4 A. The rotational constants for the ground state of I₂ may be represented: $$\begin{array}{c}{\text{(1)}}_v\text{=0.03734−0.0001208 (v+}\frac{1}{2}{\text{)+0.444×10}}^{\text{−6}}\mathrm{(v+}\frac{1}{2}{)}^2{\text{−1.839×10}}^{\text{−8}}\mathrm{(v+}\frac{1}{2}{)}^3{\text{−0.057×10}}^{\text{−10}}\mathrm{(v+}\frac{1}{2}{)}^4\\ {\text{(2)}}_v{\text{=1.99×10}}^{\text{−9}}{\text{+1.236×10}}^{\text{−10}}\mathrm{(v+}\frac{1}{2}\mathrm{).}\end{array}$$ The ground‐state $\mathrm{\Delta G(v+}\frac{1}{2})$ values have been obtained from v″=0 up to the dissociation limit. A precise value of the dissociation energy of I₂, has been found to be 12 452.5±1.5 cm^—1 as against the previous value of 12439±8 cm^—1. The potential energy curve of $X^1{\Sigma }_g^{+}$ has been computed by using the Rydberg‐Klein‐Rees method after modifying its final expressions, right up to the dissociation limit, and certain curious features in the neighbourhood of the dissociation limit are pointed out. The curve follows the van der Waals force law c/r⁶ between r=4.6 A and r=6.4 A with c=3.7×10⁶. Various possibilities for the nature of the upper electronic state are discussed.