MASS SPECTROMETRIC AND CHARGE DENSITY STUDIES OF ORGANOMETALLIC CLUSTERS PHOTOIONIZED BY GIGAWATT LASER PULSES Purav Badani, 1 Soumitra Das, 1 Pramod Sharma, 1 and Rajesh Kumar Vatsa 1 * 1 Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India 2 Department of Chemistry, University of Mumbai, Mumbai 400098, India Received 9 September 2014; revised 29 January 2015; accepted 17 February 2015 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/mas.21469 Clusters on exposure to nanosecond laser pulses of gigawatt intensity exhibit a variety of photo-chemical processes such as fragmentation, intracluster reaction, ionization, Coulomb explo- sion, etc. Present article summarizes the experimental results obtained in our laboratory utilizing time-of-flight mass spec- trometer which deal with one such aspect of cluster photochem- istry related to generation of multiply charged atomic ions upon excessive ionization of cluster constituents (Coulomb explosion) at low intensity laser field (10 9 W/cm 2 ). To understand the mechanism of laser–cluster interaction, laser as well as cluster parameters were varied. Mass spectrometric studies were carried out at different laser wavelength as well as varying the nature of cluster constituents, backup pressure, nozzle diameter, etc. In addition, charge density measurements were also preformed to get information about the total number of ions generated upon laser–cluster interaction as a function of laser wavelength. In case of pure molecular clusters, the charge state of atomic ions as well as charge density was observed to enhance with increasing laser wavelength, signifying efficient coupling of the cluster medium with nanosecond laser pulse at longer wavelength. While in case of clusters doped with species having comparatively lower ionization energy, the efficiency of laser–cluster interaction was less, in contrast to studies carried out using femtosecond lasers. Results obtained in the present work have been rationalized on the basis of proposed three- stage cluster ionization mechanism, that is, multiphoton ioniza- tion ignited-inverse Bremsstrahlung heating and electron ioni- zation. # 2015 Wiley Periodicals, Inc. Rapid Commun. Mass Spectrom. 9999: XX–XX, 2015 Keywords: Mass Spectrometry; Charge Density; Organometal- lic Clusters; Laser-Cluster interaction; Coulomb explosion I. INTRODUCTION Laser–matter interaction has been a topic of vide interest for last few decades (Leith & Upatnieks, 1962; Letokhov, 1977; Christensen, 1982; Britnell et al., 2013). When low intensity light interacts with matter, it induces a linear optical response in terms of vibration, rotation and/or electronic excitation. Induc- ing non-linear responses such as multi photon absorption/ ionization and optical field ionization (tunneling ionization and barrier suppression ionization) have become possible due to advent of high intensity laser light (Peticolas, 1967; DiMauro, Freeman, & Kulander, 2000). Mode of ionization during such interaction processes essentially depends on the intensity of laser pulses and the electric field associated with it. Multiphoton ionization (MPI) dominates at laser intensity <10 13 W/cm 2 , while at intensities >10 13 W/cm 2 , optical field ionization (OFI) begins to play important role (Nakashima et al., 2000). The boundary between the tunneling and multiphoton ionization regimes is parameterized by the dimensionless Keldysh non- adiabaticity parameter (g) (Keldysh, 1965) which is given by g ¼ v L v tun ¼ ffiffiffiffiffiffiffiffiffi E 0 2U P r ð1Þ where v L is the angular frequency of the optical field, v tun the tunneling rate at the peak of the optical field, E 0 the field-free binding energy of the electron, and U p is the ponderomotive potential. The boundary between the tunneling and multiphoton regimes is generally considered to be g  1. For, g > 1, multiphoton ionization dominates while for g < 1, tunneling ionization is predominant (Sheehy, 2001; Krishnan et al., 2014). In addition to the laser intensity, temporal profile of laser pulse and wavelength also play a crucial role in determining fragments ions formed during such interaction processes (Rajeev et al., 2013). Apart from the laser conditions, the form of matter being irradiated also plays a crucial role in determining the extent of ionization during such interaction process. Ideally, solids with high local electron density provide an efficient medium for coupling of optical energy with the matter (Mathur & Rajgara, 2010; Mathur et al., 2010). Alternatively, gas phase clusters which are aggregates of atoms/molecules are used as medium for laser–matter interaction studies. These clusters possess solid/ liquid like density and allow the experimentalist to simulate the isolated nanoscale condensed phase conditions using sophisti- cated experimental tools that are accessible for gas phase experiments (McCarter et al., 1999; Feigerle, Bililign, & Miller, 2000). In this regards, time-of-flight mass spectrometer has been widely used to probe different aspects of laser–cluster interac- tion. Gas phase clusters upon interacting with laser pulses exhibit several processes such as fragmentation, intra-cluster reaction, valence and/or core shell ionization etc. Coulomb explosion of molecules/clusters is one of such phenomena which Present address of Purav Badani is Department of Chemistry, University of Mumbai, Mumbai 400098, India  Correspondence to: R.K. Vatsa, Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail: rkvatsa@barc.gov.in Mass Spectrometry Reviews # 2015 Wiley Periodicals, Inc.