Effects of solvation on dissociative electron attachment to methyl iodide clusters

Using laser photoelectron attachment to methyl iodide clusters in a differe ntially-pumped seeded supersonic helium beam and mass spectrometric ion det ection, we have measured the rate coefficients for formation of (CH3I)q.I- (q = 0-2) ions over the electron energy range 0-100 meV with an effective e nergy width of about 2.5 meV. Whereas a prominent vibrational Feshbach reso nance just below the onset for the C-I stretch vibration (v(3) = 1) is obse rved for dissociative attachment to monomers (yielding I- ions), only weak and broad structure, shifted to lower energies, is detected for formation o f (CH3I). I- ions and essentially no structure is left in the attachment sp ectrum for (CH3I)2 . I-. These observations are interpreted by model R-matr ix calculations which successfully describe the DA cross-section for the mo nomer and qualitatively recover the trend observed for cluster ion formatio n. For the clusters, the effects of increased electron-target long-range in teraction and of solvation as well as coupling to soft vibrational modes le ad to strong broadening and shifting of the vibrational Feshbach resonance and, ultimately, to its disappearance.