Mass selective anion-ZEKE spectroscopy of the iodine-water cluster

Mass selected and high resolution anion-ZEKE spectra of I^-(H₂O) are presented. Linewidths as narrow as 3 cm^-1 have been observed for a resolution of the apparatus of 2 cm^-1 (maximum resolution 1.5 cm^-1). From a first tentative assignment we deduce electronic and vibrational information about the neutral ground and the lowest electronic excited states of the iodine-water cluster. The spectra exhibit the typical iodine spin-orbit splitting, two low frequency vibrational modes of 45 cm^-1 and 210 cm^-1 and a shift of the whole transition system of more than 3000 cm^-1. The latter is mainly due to the strong interaction of the iodine anion with the water molecule. Rotational excitation is discussed. In addition, mass spectra of anionic iodine-water clusters (0 < n < 18) are presented. There is no evidence for thermodynamically preferred cluster size.