Study of Ultrafast Fragmentation Dynamics of Molecules

Venkatachalam, Anbu Selvam and Sharma, Vandana (2017) Study of Ultrafast Fragmentation Dynamics of Molecules. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

In this work, photo-induced fragmentation study of molecules (O 2 and CH 3 I) using a velocity-map- imaging (VMI) spectrometer is performed. An indigenously built VMI spectrometer, housed in a non-magnetic stainless steel chamber under 10 − 8 mbar of vacuum was utilized. The fragmentation of molecules was induced by a 25 fs, 1 kHz Ti:Sapphire laser with a focal volume of 100 microns, leading to formation of ion fragments and electrons. The ions produced are directed by electrostatic plate potentials towards a special detector that is capable of acquiring the time of impact and the position in a plane perpendicular to direction of propagation. From the combined information of the time-of- flight (TOF) and the position, the total kinetic energy of the ions shall be calculated. Preliminary information about the orbitals involved in the fragmentation of the excited state molecular oxygen ion is inferred from the angular distribution of the fragments acquired in the detector. A comparative study of fragmentation of methyl iodide under different laser pulse widths and average power is shown in this work. The capture of multiply charged ion fragments only at certain conditions and the variation in the radial distribution of the ions on the detector shed some light in the mechanisms involved in the birth of these ion fragments, either be autoionization or photo dissociation or Coulomb explosion.

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