The Halogen Molecules and Their Spectra.J−J-Like Coupling. Molecular Ionization Potentials

Abstract

The relations between molecular and atomic ionization potentials are discussed, and rules are formulated, differing for non-bonding, bonding, and anti-bonding electrons. Comparisons are made between ionization potential values predicted by these rules and observed values for ${\mathrm{N}}_{2}$, CO, ${\mathrm{X}}_{2}$, HX (X = halogen). Rather good agreement is found. Electron configurations of these molecules and ${\mathrm{O}}_{2}$ are discussed. The low-energy states of the halogen ions ${\mathrm{X}}_{2}^{+}$ and ${(\mathrm{XY})}^{+}$ are discussed, and some of their properties are predicted. With this as a background, low-energy excited states of ${\mathrm{X}}_{2}$ and XY are discussed. Many new low-energy states are predicted (Tables III and IV), some giving halogen atoms both in unexcited electron configurations on dissociation, others giving one atom with an excited electron. The foregoing predictions are used to give a reasonable interpretation of the observed absorption spectra of the XY and ${\mathrm{X}}_{2}$ molecules, extending into the vacuum ultraviolet. Suggestions are also made for interpreting observed fluorescence and emission spectra. The $D$ level of ${\mathrm{I}}_{2}$ is shown to be probably analogous to the $1s\ensuremath{\sigma}2p\ensuremath{\sigma}$, ${^{1}\ensuremath{\Sigma}^{+}}_{u}$ state of ${\mathrm{H}}_{2}$, with a wave function corresponding to ${\mathrm{I}}^{\ensuremath{-}}({\ensuremath{\sigma}}^{2}{\ensuremath{\pi}}^{4}){\mathrm{I}}^{+}({\ensuremath{\pi}}^{4})$. The vacuum ultraviolet bands of XY abserved by Cordes and Sponer in the regions mainly $\ensuremath{\lambda}\ensuremath{\lambda}1600\ensuremath{-}1950$ are of particular interest. The interpretation of an observed interval in these bands leads to the conclusion that the upper levels are in each case of the type $[{\mathrm{X}}^{+}\mathrm{Y}, ^{2}\ensuremath{\Pi}_{1\frac{1}{2}}]{\ensuremath{\sigma}}^{\ifmmode\times\else\texttimes\fi{}}$ and $[{\mathrm{X}}^{+}\mathrm{Y}, ^{2}\ensuremath{\Pi}_{\frac{1}{2}}]{\ensuremath{\sigma}}^{\ifmmode\times\else\texttimes\fi{}}$, with $\ensuremath{\Omega}\ensuremath{-}s$ ($J\ensuremath{-}J$-like) coupling between the ${\mathrm{X}}^{+}$Y core and the ${\ensuremath{\sigma}}^{\ifmmode\times\else\texttimes\fi{}}$ excited electron. Nearly $\ensuremath{\Omega}\ensuremath{-}s$ coupling is also present, according to the multiplet intervals, in the upper levels of the visible and infrared ${\mathrm{I}}_{2}$, ${\mathrm{Br}}_{2}$, ICl and IBr absorption bands. The possibility is discussed that part of the intensity in the continua associated with these bands belongs to an expected $^{1}\ensuremath{\Pi}\ensuremath{\leftarrow}^{1}\ensuremath{\Sigma}^{+}$ transition.