Multielectron dissociative ionization of CH3I clusters under moderate intensity ps laser irradiation

Abstract: The interaction of 35ps laser pulses with methyl iodide clusters in the intensity region of 1012–3×1013 W/cm2 is studied at λ =266, 532 and 1064nm by means of time-of-flight mass spectrometry. At λ =532 and 1064nm the multielectron dissociative ionization (MEDI) of clusters results in highly charged atomic fragment (up to I7+) ion production of high kinetic energy. The explosion of the multiple charged cluster ions is found to be isotropic, while an asymmetric charge distribution prior to their fragmentation is observed. Moreover, negative ion formation is observed. The MEDI of clusters exhibits a dependence on laser polarization and the intensity thresholds for the observation of the fragment ions are determined for linear and circular laser polarization. These intensity thresholds are found to be about three orders of magnitude lower than those reported by experiments with 35ps laser pulses on CH3I monomers. For the observed MEDI of (CH3I) n clusters a mechanism is proposed. According to this mechanism, the clusters are initially single ionized by multiphoton absorption and an internal electric field is created within the cluster, which distorts the potential barriers. This distortion increases the probability for electron tunneling even at relatively low laser intensities, giving thus rise to the formation of negatively charged moieties within the clusters, which prevent the cluster elongation and facile further the tunneling process. This procedure results in a higher distortion of the internal barriers and an increased ionization of the clusters, which, due to increased repulsive forces, finally fragment leading to multiple charged high kinetic energy fragment ion production. [Copyright &y& Elsevier]