Photodissociation Dynamics of Several Organic Molecules




Degree Year:2018





This dissertation contains two sections.The first section describes the photodissociation dynamics study of o,m,p-bromofluorobenzene in the 234-267nm ultraviolet wavelength range,using a home-made time-sliced ion velocity imaging setup.Based on the experimentally measured speed and angular distributions of the ionized photofragments Br and Br*,photossociation mechanisms of the three aromatic bromides have been discussed.The second section described the photodissociation dynamics study of 1-methylallyl as well as 2-methylallyl in the wavelength range of 226-244nm,using a H-atom high Rydberg tagging time of flight(HRTOF)setup.The experimental recorded HRTOF spectra of the H atoms that are photo fragmented from the two radicals allow us to infer their corresponding dissociation mechanisms in the production of highly unsaturated hydrocarbons.234-267nm photodissociation dynamics of o,m,p-bromofluorobenzene:The photodissociation dynamics of three simple aromatic bromides,o,m,p-bromofluorobenzene,has been investigated in the 234-267nm ultraviolet(UV)wavelength range using a home-made time-sliced velocity ion imaging.Images of ionized photofragments Br(2P3/2)(Br)and Br(2P1/2)(Br*)atoms produced in UV photodissociation of o,m,p-bromofluorobenzene have been accurated measured to determined their speed and spatial angular distributions.It is found from our experimental results that,for all three species,the speed(i.e.,translational energy)distributions for both Br and Br*products can be fitted by two or three Gaussian components with different translational energy.In detail,for o-amd m-bromofluorobenzene,the high translational energy component of Br exhibits a highly anisotropic behavior,and its relative intensity increases with increasing the dissociation photon energy.Specifically for o-bromofluorobenzene,the relative intensity of the high translational energy component is neglectable with a photon energy as low as 4.76 eV.Therefore,we conclude that the high translational energy component is due to a mechanism of an intersection crossing between S,(A’)and S,(A")followed by a direct dissociation along the Si(A")states surface.With our experimental results,we have inferred that Si(A’)-S1(A")intersection crossing occurs at around 4.96 eV.The high translational energy component of Br*also exhibits high anisotropy,which is due to dissociation directly along the Si(A’)state surface which has a small barrier.The medium translational energy components of both Br and Br*has relatively small anisotropy parameters,but with sufficient intensities.Based on these observations,we conclude that,for o-amd m-bromofluorobenzene,the production of the medium translational energy components of Br*is produced along the Si(A’)state surface involving multiple intersystem crossing between T2(A’)and S1(A’),while the medium translational energy components of Br is due to dissociation along the Si(A")state surface involving an intersystem crossing between T2(A’)and S1(A").For m-bromofluorobenzene with a C2v symmetry,similar conclusions on the dissociation mechanisms have also be discussed.226-244nm photodissociation dynamics of 1-methylallyl and 2-methyla;lylThe 226-244nm ultraviolet photodissociation dynamics of 1-methylallyl and 2-methylallyl under 226-244nm has been investigated using a H-atom high Rydberg tagging time of flight(HRTOF)setup.From the experimentally recorded HRTOF spectra of H atoms that are produced from photodissociation of the two radicals in selected UV wavelengths,the corresponding photofragment yild spectra,as well as tranlational energy and angular distribution of H atoms have been obtained for both radicals.Combined with the ab initio calculated potential energy surfaces,we have determined the mechanisms that lead to the production of H atom.In detail,for photodissociation of 1-methylallyl at around 230 nm,two dissociation channels exist,leading to accompanied products of 1,3-butadiene for the fast channel,and 1,2-butadiene or 1-butyne for the slow channel,respectively.While for photodissociation of 2-methylallyl in the similar wavelength region,only one slow dissociation channel exists,leading to the accompanied product of vinyl cyclopropane.