PHOTOELECTRON SPECTROSCOPY OF $N_{2}O^{-}, CO^{-}_{2}, (N_{2}O)^{-}_{2}$ and $(CO_{2})^{-}_{2}$

We have recorded the negative ion photoelectron (photodetachment) spectra of the gas-phase negative ions $N_{2}O^{-}, CO^{-}_{2}, (N_{2}O)^{-}_{2}$ and $(CO_{2})^{-}_{2}$. Nitrous oxide and carbon dioxide are isoelectronic molecules whose negative ions have bent geometries. Due to the large structural differences between these ions and their linear neutrals, appreciable Franck-Condon overlap is not expected during photodetachment transitions between the lowest lying levels of these negative ions and their neutrals. The photoelectron spectra which we obtain for $N_{2}O^{-}$ and $CO^{-}_{2}$ are consistent with this expecttion. Both spectra contain a single broad peak which we interpret primarily as a progression in the bending mode of the neutral. Some structure is visible on this peak in both spectra, it being more pronounced in the $CO^{-}_{2}$ spectrum. The maxima of these spectra correspond to electron binding energies of $\\sim 1.5 eV$ for $N_{2}O^{-}$ and $\\sim 1.4 eV$ for $CO^{-}_{2}$. These values are interpreted as the vertical detachment energies of $N_{2}O^{-}$ and $CO^{-}_{2}$. In contrast to the similarities between the photoelectron spectra of $N_{2}O^{-}$ and $CO^{-}_{2}$, we find substantial differences between the spectra of $(N_{2}O)^{-}_{2}$ and $(CO_{2})^{-}_{2}$. The photoelectron spectrum of $(N_{2}O)^{-}_{2}$ is similar to that of $N_{2}O^{-}$ in that it consists of a single broad peak. Its maximum, however, is shifted by $\\sim 0.2 eV$ to lower electron kinetic energies relative to the maximum in the $N_{2}O^{-}$ spectrum, and this represents an increase in the electron binding energy by a corresponding amount. We interpret the $(N_{2}O)^{-}_{2}$ spectrum as arising from the photodetachment of an ionic species which is best described as a bent $N_{2}O^{-}$ ion solvated by a neutral linear $N_{2}O$ molecule, i. e. $N_{2}O^{-}(N_{2}O)_{1}$. The 0.2 eV shift is a rough measure of the ion-solvent dissociation energy of $N_{2}O^{-}(N_{2}O)_{1}$ into $N_{2}O^{-}$ and $N_{2}O$. By contrast, the maximum in the $(CO_{2})^{-}_{2}$ photoelectron spectrum has been shifted out of our spectral range. This indicates that the electron binding energy of any maximum in the $(CO_{2})^{-}_{2}$ spectrum is larger than that for the maximum in the $CO^{-}_{2}$ spectrum by at least 1 eV, and that there are substantial differences between $(CO_{2})^{-}_{2}$ and $(N_{2}O)^{-}_{2}$. While these observations do not definitively distinguish between a simple solvated-ion vs. a more chemical ``oxalate-like" bonding picture for $(CO_{2})^{-}_{2}$, they do suggest that if $(CO_{2})^{-}_{2}$ is a solvated ion, the interaction energy between $CO^{-}_{2}$ and $CO_{2}$ is $>1 eV$.