I^-(CH₃I)₂ photoexcitation: The influence of dipole bound states on detachment and fragmentation

We present the results of a photoelectron imaging study of the I ^-(CH₃I)₂ cluster anion over excitation wavelengths 355-260 nm. The resulting spectra and photoelectron angular distributions (PADs) suggest extensive electron-molecule interaction following photoexcitation. Fragmentation channels are observed subsequent to excitation between 355 and 330 nm. The origin of these features, which begin 200 meV and peak 70 meV below the X band direct detachment threshold, is described in terms of a predissociative dipole bound state. The nature of the fragments detected and the energetics of the channel opening argue strongly in favor of an asymmetric, head to tail cluster anion geometry posited by Dessent Acc. Chem. Res. 31, 527 (1998)10.1021/ar950061f. Above the direct detachment threshold, PADs display evidence of phenomena akin to electron-molecule scattering. The fragment anions disappear above the X band threshold but reappear some distance below the second (A) direct detachment band. At these energies there is also rapid variation of the X band PAD, an observation attributed to autodetachment via spin-orbit relaxation of the iodine core of the cluster.