Coulomb explosion and generation of multiply charged ions in nanosecond laser fields

Koch, Ankurjyoti (2021) Coulomb explosion and generation of multiply charged ions in nanosecond laser fields. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Ankurjyoti Koch (16MS073)) 16MS073_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (1MB)

Abstract

The phenomenon of Coulomb Explosion (C.E.) and its characteristic generation of multiply charged ions(MCAI) is reported to occur with higher potency at high ionisation fields with ultrafast lasers with intensities on the order of ~10 14 W/cm2 and higher. Nanosecond laser pulse has peak intensities (~1012 W/cm2) that is several orders of magnitude lower than the threshold intensities required for field ionisation of most atoms. Therefore, the phenomenon of C.E. and MCAI is considered rare under such nanosecond laser field conditions. However, several reports have claimed to have observed C.E. and MCAI at intensities spanning from 109 to 1020 W/cm2, at various pulse duration, i.e. nanosecond, picosecond and femtosecond. Recently, Zhang and coworkers, in their paper (J. Phys.Chem. Lett.2020,11,1100-1105), claimed to have experimentally observed Coulomb Explosion and MCAI when nanosecond lasers at 532nm with intensities of 10 12 W/cm2 was employed in irradiation of Ar clusters doped with molecules containing aromatic chromophores. They have proposed an evaporation model which states that the formation of a highly positively charged cluster through resonant absorption of Arn+ and the subsequent stripping off of electrons by this charged cluster from the evaporating Ar+ eventually causes the formation of Arn+ before C.E. In this context, Vatsa and Mathur (Vatsa & Mathur, 2020) have pointed out some inconsistencies in the evaporation model and an alternate explanation which posits plasma behaviour is used to address the issues and rationalise the observations in the experiments of Zhang and Coworkers. Here, we consider both the evaporation model and the arguments put forward by Vatsa and Mathur, and computationally demonstrate using rate theories that the assumptions in the evaporation model are not universally valid as the evaporation model fails to describe the generation of MCAI for three dopants: 2-bromofluorene, pyrene and fluorine. Hence, the evaporation model is not of a greater utility in describing the dynamics of the generation of multiply charged ions from laser-irradiated gas-phase clusters and so demands a rigorous investigation.

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