The Absorption Spectrum of Iodine Monochloride in the Near Infrared

Abstract

Previous analyses of the absorption spectrum of ICl vapour were confined to the region $\lambda 6940$ to $\lambda 5736\mathrm{A}$ where, at normal temperature, it consists chiefly of two $v^{\prime }$ progressions ($v^{\prime \prime }=0\mathrm{}$ and $v^{\prime \prime }=1\mathrm{}$). No bands of these progressions for which $v^{\prime }<7\mathrm{}$, $v^{\prime }<6\mathrm{}$, respectively, were measured, so that the determination of ${{\omega }_0}^{\prime }$ involved a fairly long extrapolation which, in view of the non-linearity of the ${{\omega }_v}^{\prime }$, $v^{\prime }$ relationship, was somewhat untrustworthy. Only a few bands of the $v^{\prime \prime }=2\mathrm{}$ progression were measured and consequently the values of ${{\omega }_0}^{\prime \prime }{x}^{\prime \prime }$ and ${{\omega }_0}^{\prime \prime }$ required further examination. By raising the temperature of ICl vapour to 400°C and suitably adjusting the pressure the absorption spectrum between $\lambda 6750$ and $\lambda 8770\mathrm{A}$ has been photographed. In this region the spectrum consists of portions of seven progressions for which $v^{\prime \prime }=1\mathrm{}$ to $v^{\prime \prime }=7\mathrm{}$, the $v^{\prime \prime }=0\mathrm{}$ progression not being appreciably developed. Values of ${{\omega }_v}^{\prime }$ from $v^{\prime }=2\mathrm{}$ to $v^{\prime }=9\mathrm{}$, and of ${{\omega }_v}^{\prime \prime }$ from $v^{\prime \prime }=1\mathrm{}$ to $v^{\prime \prime }=6\mathrm{}$, have been determined. Graphical extrapolation of two units of $v^{\prime }$ gave ${{\omega }_0}^{\prime }$ as 209.7 ${\mathrm{cm}}^{-1\mathrm{}}$ and assuming a linear ${{\omega }_v}^{\prime \prime }$, $v^{\prime \prime }$ law, ${{\omega }_0}^{\prime \prime }$ was found to be 383.0 ${\mathrm{cm}}^{-1\mathrm{}}$. The values of ${{\omega }_0}^{\prime }$ and ${{\omega }_0}^{\prime \prime }$ given by Patkowski and Curtis were 223.7 ${\mathrm{cm}}^{-1\mathrm{}}$ and 382.0 ${\mathrm{cm}}^{-1\mathrm{}}$ respectively. It appears that Wilson's value for the convergence limit of the $v^{\prime \prime }=0\mathrm{}$ progression is about 90 ${\mathrm{cm}}^{-1\mathrm{}}$ too high. The dissociation energy for the normal state as found by extrapolation from the lowest eight levels is some 5700 ${\mathrm{cm}}^{-1\mathrm{}}$ units greater than the energy required to dissociate the molecule by raising it to the higher state and increasing the energy in that state, so that there is a crossing of the potential energy curves. The possible significance of this with regard to the products of dissociation from the lower state is discussed. Bands of the $v^{\prime \prime }=3, 4, 5$ progressions of the I${\mathrm{Cl}}_{37}$ spectrum have been identified and for them the isotopic shifts are opposite in sign to that of the higher $v^{\prime }$ members of the $v^{\prime \prime }=0\mathrm{}$ progression previously recorded. Examination of the $G^{\prime }$, ${{\omega }_v}^{\prime }$ data showed that a third parabola, in addition to the two previously recorded by Curtis and Darbyshire, is now required to represent the observations. The revised values of vibrational and related constants for the ICl molecule are as follows (in ${\mathrm{cm}}^{-1\mathrm{}}$ units): ${{\omega }_0}^{\prime }=209.7, {{\omega }_e}^{\prime }=212.3, {{\omega }_0}^{\prime \prime }=383.0, {{\omega }_e}^{\prime \prime }=384.6,$ ${\nu }_{\mathrm{head}}^{00}=13655.3\left(1.686\mathrm{} \mathrm{Volt}\right), {\nu }^{\mathrm{el}}=13742\mathrm{}(1.695V),$ ${\nu }_{\mathrm{con}.\mathrm{}}^{00}=17343\left(2.139\mathrm{}\mathrm{V}\right), {D_0}^{\prime }=\int o^{v_c}{\omega }_v\mathrm{dv}=3690\mathrm{}(0.455V)\mathrm{}$ ${D_0}^{\prime \prime }(\mathrm{extrap}.\mathrm{})=23069\mathrm{}(2.847V), {D_0}^{\prime \prime }-({D_0}^{\prime }+{\nu }^{00})=5724\mathrm{}(0.706V).$