International Journal of Mass Spectrometry 358 (2014) 36–42 Contents lists available at ScienceDirect International Journal of Mass Spectrometry jou rn al h om epage: www.elsevier.com/locate/ijms Generation of multiply charged tin and carbon ions in low intensity Coulomb explosion of tetramethyl tin clusters: Role of screening effects Purav M. Badani, Soumitra Das, Pramod Sharma, Rajesh K. Vatsa ∗ Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India a r t i c l e i n f o Article history: Received 24 July 2013 Received in revised form 10 November 2013 Accepted 10 November 2013 Available online 18 November 2013 Keywords: Coulomb explosion Molecular clusters Tetramethyl tin Time-of-flight mass spectrometry Screening effects a b s t r a c t Present work reports multiple ionization and subsequent Coulomb explosion of tetramethyl tin clusters induced by gigawatt intensity laser pulses. The time-of-flight mass spectra and charge density mea- surements revealed that the efficiency of laser–cluster interaction depends on ionization wavelength. Extent of energy absorbed from optical pulses by the cluster medium was found to increase with laser wavelength. Experimental results obtained in the present study have been rationalized on the basis of three stage cluster ionization model, i.e. multiphoton ionization ignited – inverse bremsstrahlung heating (IBS) – electron ionization. In addition to the experiments, theoretical calculations have been performed to account for screening effects in clusters. Our calculations suggest that the charged particles, generated upon initial multiphoton ionization of cluster constituents, significantly lower the Coulombic barrier of atoms/molecules that are present in its vicinity. This, in turn, decreases the ionization energy of cluster constituents, during subsequent steps of ionization, ultimately increasing the ionization level of clusters via the process of enhanced ionization due to ion shielding. Thus, above calculations predict dominant role of screening effects in evolution of higher charge state atomic ions during laser–cluster interaction. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Clusters undergo Coulomb explosion upon interacting with highly energetic electrons [1], particles [2] or photons [3,4]. This phenomenon occurs due to extensive stripping of electrons from constituent atoms/molecules of clusters resulting in buildup of excessive positive charge on cluster. When the repulsive Coulom- bic energy of clusters, arising from positive charges, overcomes the total cohesive energy, cluster disintegrates violently result- ing in generation of multiply charged atomic ions. Initial report on Coulomb explosion dates back to 1981 when Sattler et al. [1] observed doubly charged species upon electron impact ioniza- tion of metallic and vander Waal clusters. In 1994, Castleman and co-workers [5] observed extensive ionization and generation of multiply charged atomic species (I 17+ and Ar 8+ ) upon interaction of (HI) n (Ar) m clusters with intense femtosecond laser pulses. Sub- sequent studies on Coulomb explosion by other research groups [6–10] revealed the dependence of cluster size, laser wavelength, intensity, pulse duration, etc. on efficiency of energy transfer from optical field into the cluster medium. ∗ Corresponding author. Tel.: +91 2225592438. E-mail address: rkvatsa@barc.gov.in (R.K. Vatsa). Several ionization mechanisms such as Ionization Ignition Model (IIM) [11], Coherent Electron Motion Model (CEMM) [12,13], Charge Resonance Enhanced Ionization (CREI) [14], etc., have been proposed to account for occurrence of Coulomb explosion under high intensity laser fields (I ∼ 10 15 W/cm 2 ). However, these ion- ization models could not explain low intensity (I ∼ 10 9 W/cm 2 ) induced Coulomb explosion of clusters [15–20]. In order to ratio- nalize the phenomenon of Coulomb explosion at gigawatt laser intensity, a three stage cluster ionization model, i.e. multipho- ton ionization ignited – inverse bremsstrahlung heating (IBS) – electron ionization, has been proposed [18,19]. According to this model, Coulomb explosion is initiated via multiphoton ionization of atoms/molecules present in clusters. Subsequently, electrons released via inner ionization process are caged within the clus- ters. These quasi free electrons, under the influence of Coulombic field, extract energy from laser pulse via inverse bremmsstralung process. Once the quasi free electrons gain enough energy, further ionization takes place via electron ionization resulting in gener- ation of multiply ionized species within clusters. Finally, these charged clusters undergo Coulomb explosion due to strong elec- trostatic repulsion. Based on the above proposed three stage cluster ionization model, the quasi free electrons, generated upon initial MPI of clus- ter constituents, needs to acquire the energy equal to or greater than the ionization energy (I.E.) of neighbouring species, so as to 1387-3806/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2013.11.007