Tracing photoionisation behaviour of methyl iodide in gas phase: From isolated molecule to molecular aggregate

Abstract Methyl iodide (CH 3 I) is one of the most studied molecule in the field of photochemistry. This molecule has been investigated by a variety of light sources including lamps, different lasers as well as synchrotron radiation of varying intensity and wavelength. In this review, photoionisation studies of CH 3 I have been summarised for monomer (CH 3 I) and its aggregates i.e., cluster (CH 3 I) n which were excited with a range of photon energy and intensity using different sources. For the case of monomer, ionisation process follows either ionisation followed by dissociation (ladder climbing mechanism) or dissociation followed by ionisation (ladder switching mechanism) depending upon the wavelength and intensity of ionisation source. In case of CH 3 I ionisation by synchrotron radiation, singly and doubly charged molecular ions were observed depending upon the energy of photon. For ionisation using laser sources, singly and multiply charged ions were observed for laser pulses ranging from femtosecond to nanosecond duration. For the case of cluster (CH 3 I) n ionisation with laser sources, multiply charged ions were observed leading to Coulomb explosion phenomena. Lowest possible laser intensity observed to induce Coulomb explosion in CH 3 I clusters was ∼10 9  W/cm 2 using nanosecond laser pulses. Since observation of multiply charged ion at gigawatt intensity is surprising, a detailed description of experimental results has been summarised. A three stage cluster ionisation model which comprises of multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization has been adapted to explain experimental observations and to understand nanosecond laser induced Coulomb explosion of CH 3 I clusters.