International Journal of Mass Spectrometry 290 (2010) 133–141 Contents lists available at ScienceDirect International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms Multielectron dissociative ionization of CH 3 I clusters under moderate intensity ps laser irradiation G. Karras, C. Kosmidis ∗ Atomic and Molecular Physics Laboratory, Department of Physics, University of Ioannina, University Campus, GR-45110 Ioannina, Greece article info Article history: Received 3 December 2009 Received in revised form 23 December 2009 Accepted 4 January 2010 Available online 13 January 2010 Keywords: CH3I Cluster MEDI Negative ions abstract The interaction of 35 ps laser pulses with methyl iodide clusters in the intensity region of 10 12 –3 × 10 13 W/cm 2 is studied at  = 266, 532 and 1064 nm by means of time-of-flight mass spectrome- try. At  = 532 and 1064 nm the multielectron dissociative ionization (MEDI) of clusters results in highly charged atomic fragment (up to I 7+ ) ion production of high kinetic energy. The explosion of the multi- ple 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 35 ps laser pulses on CH 3 I monomers. For the observed MEDI of (CH 3 I) 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The interaction of strong laser beams with clusters is a topic of significant research activity over the last years. This interest is related to the fact that clusters can be thought of as an inter- mediate step between gas and condensed matter. Nevertheless, the laser–cluster interaction results in some experimental observa- tions that are interesting in their own right. The ejection of multiple charged ion fragments with high kinetic energy from molecular clusters has been repeatedly reported, while no such ions were observed, when the same molecular species (monomers) were irradiated under the same laser conditions [1–11]. Different mech- anisms have been proposed for the interpretation of the interaction of fs laser pulses’ with atomic and molecular clusters [12–20]. Due to the complexity of this kind of interaction the majority of the published work is dealing with atomic clusters and mainly those consisted of noble atoms. ∗ Corresponding author. Tel.: +30 26510 08537; fax: +30 26510 08695. E-mail addresses: kkosmid@uoi.gr, kkosmid@cc.uoi.gr (C. Kosmidis). Recently there have been some publications concerning the interaction of molecular clusters with ns laser pulses at relatively low intensities (10 9 –10 12 W/cm 2 ) which report the observation of multiple charged fragment ions ejected with high kinetic energies. At a first glance these observations seem unexpected, since the laser intensity is at least two orders of magnitude lower than that typi- cally needed for multiple molecular ionization leading to fragment ion production via Coulomb explosion. Multielectron dissociative ionization (MEDI) of clusters under relatively low laser intensities have been reported for species like NH 3 , CS 2 , CH 3 I and benzene [8–11,21,22]. Two main mechanisms have been proposed for their interpretation. The first one is that of Inverse Bremsstrahlung Scattering (IBS) which originally was proposed by Ditmire et al. for the case of atomic clusters under strong laser field irradiation [16]. Luo and co-workers [10] have attributed their experimental findings in the case of (CH 3 I) n and (NH 3 ) n clusters (at 10 10 –10 12 W/cm 2 ) to this mechanism. On the other hand, Sharma et al. [8] have reported on MEDI from (CH 3 I) n using an 8 ns laser at ∼5 × 10 9 W/cm 2 . They have sug- gested that the IBS model is inadequate and a plausible mechanism is that of «energy pooling» or excitation annihilation interactions. 1387-3806/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2010.01.003