Electron attachment to the interhalogens ClF, ICl, and IBr.

Electron attachment rate coefficients have been measured for the interhalogens ClF, ICl, and IBr over the range 300-900 K using a flowing-afterglow Langmuir-probe apparatus. The ClF case was also studied theoretically. ClF was found to attach electrons somewhat inefficiently with a rate coefficient of 7.5x10$^{\mathrm{-9}}$ cm$^{\mathrm{3}}$/s at 300 K, doubling by 700 K. Even so, attachment to ClF is more efficient than seen earlier for F$_{\mathrm{2}}$ and Cl$_{\mathrm{2}}$, which brings up the interesting distinction that attachment to F$_{\mathrm{2}}$ and Cl$_{\mathrm{2}}$ is known to have $p$-wave threshold behavior, while in ClF the inversion symmetry is broken, allowing an $s$-wave component. The increase in the rate coefficient for attachment to ClF with temperature was found to be less pronounced than with F$_{\mathrm{2}}$ and Cl$_{\mathrm{2}}$. \textit{Ab initio} potential energy curves were calculated for ClF and ClF$^{\mathrm{-}}$, and R-matrix theory was used to obtain the resonance widths and energies for the ground state curve crossing, which takes place near the equilibrium internuclear separation in ClF. A local complex potential model was used to calculate attachment cross sections and thermal rate coefficients. There is reasonable agreement between theory and experiment within the estimated 25{\%} uncertainties in the data. Cl$^{\mathrm{-}}$ is the only product ion from thermal electron attachment to ClF. Attachment to ICl is even less efficient by almost an order of magnitude than to ClF, namely, 9.5x10$^{\mathrm{-10}}$ cm$^{\mathrm{3}}$/s at 300 K. Attachment to IBr is small enough that we place an upper limit of \textless 10$^{\mathrm{-10}}$ cm$^{\mathrm{3}}$/s at 300 K.