Photodissociation Dynamics of Halogenated Heterocyclic Molecules in Gas Phase

Abstract

The process of photodissociation is the breaking of one or several bonds in a molecule through the absorption of photon. The area of photodissociation dynamics concerns with the detailed mechanisms of photodissociation process investigated on molecular level. One of the principal goal of these kind of studies is to obtain a clear picture of the various dynamic processes in the excited electronic state as the molecule leaves the Franck-Condon region, traverses the ‘transition state’ (i.e., the barrier, if there is any), and finally reaches the asymptotic channel(s), where the fragments are formed. Photodissociation experiments are generally performed state selectively i.e. molecule being prepared in well defined quantum state. Therefore, results from these kinds of experiments are readily compared with the theoretical calculations, resulting in significantly enhanced understanding of the elementary chemical processes [1]. Photodissociation dynamics of various molecules of interest are studied in molecular beam (MB) generated by supersonic expansion through a pulsed valve after seeding it in Helium gas. The isolated molecules in the MB are photodissociated using ultraviolet (UV) laser. One of the resultant photofragments, in the present case mainly atomic species, is probed using Resonance Enhanced Multiphoton Ionization Time-of- Flight Mass Spectrometry (REMPI-TOF-MS). The combination of supersonic molecular beam, REMPI and TOF mass spectrometer has potential of detecting the chemical species state and mass selectively In this thesis, the photodissociation dynamics of halogenated heterocyclic molecules has been studied. It is well known that the photodissociation of halogen containing molecules in the UV range generates halogen atoms which are detrimental for the ozone layer in the stratosphere. Also, it is fundamentally interesting to study the halogenated heterocyclic molecules because of their rich excited state dynamics due to their radiationless transitions, vibronic and spin-orbit coupling. Further, effect of fluorine atom substitution on dynamics has also been investigated. In this context, five and six member heterocyclic molecules have been chosen with one to three hetero atoms in the ring.