Photodissociation Dynamics Of Small Molecular Cations In Ultraviolet

Photodissociation dynamics of small molecular ions play an important role in earth's atmosphere and in the interstellar chemistry,and on its own reveal the correlation between molecular properties and photodissociation mechanisms,which is a frontier of nowadays physical chemistry.In this dissertation,the ultraviolet(UV)photodissociation dynamics of two small molecular cations(N2O+,CO2+)have been studied using a state-of-the-art instrument,the so-called cryogenic ion trap velocity map imaging(CIT-VMI)spectrometer.A new slow electron velocity map imaging(SEVI)was also designed and constructed with the purpose of studying the photodetachment and spectroscopy of negative ions.(1)UV photodissociation dynamics of N2O+ in UV energy regionPhotodissociation dynamics of the N2O+ cation in its B2? state has been experimentally studied in an energy region around the NO+(1?+)+N(2P)dissociation limit using a cryogenic cylindrical ion trap velocity map imaging spectrometer.The total translational energy release spectrum and the photofragment recoiling angular distribution were obtained by recording the velocity images of fragmented NO+.The results show that,NO+(1?+)+N(2D)production channel dominates the dissociation dynamics,and a bending angle of N2O+(B2?)prior to dissociation is inferred in the 30-50°range.The NO+(1?+)+ N(2P)production channel,which directly correlates to the B2? state but less competitive,opens immediately when the photon energy reaches the dissociation limits,indicating a flat dissociation pathway on the B2? state surface.(2)Dissociation dynamics of CO2+(C2Eg+)via[1+1]two-photon excitationThe dissociation dynamics of CO2+in the C2?g+ state has been studied in the 8.14-8.68 eV energy region by[1+1]two-photon excitation via vibronically selected intermediate A2?u and B2?u+ states using a cryogenic ion trap velocity map imaging spectrometer.The cryogenic ion trap produces an internally cold mass selected ion sample of CO2+.Total translational energy release(TER)and two-dimensional recoiling velocity distributions of fragmented CO+ ions are measured by time-sliced velocity map imaging.High resolution TER spectra allow us to identify and assign three dissociation channels of CO2+(C2?g+)in the studied energy region:(1)production of CO+(X2?+)+O(3P)by predissociation via spin-orbit coupling with the repulsivv 14?u state;(2)production of CO+(X2E+)+O(1D)by predissociation via bending and/or antisymmetric stretching mediated conical intersection crossing with A2?u or B2?u+,where the C2?g+/A2?u crossing is considered to be more likely;(3)direct dissociation to CO+(A2?)+O(3P)on the C2?g+ state surface,which exhibits a competitive intensity above its dissociation limit(8.20 eV).For the first dissociation channel,the fragmented CO+(X2?+)ions are found to have widely spread populations of both rotational and vibrational levels,indicating that bending of the parent CO2+ over a broad range is involved upon dissociation,while for the latter two channels,the produced CO+(X2?+)and CO+(A2?)ions have relatively narrow rotational populations.The anisotropy parameters ? are also measured for all three channels and are found to be nearly independent of the vibronically selected intermediate states,likely due to complicated intramolecular interactions in the studied energy region.(3)UV photodissociation dynamics of pentafluorobromobenzeneIn addition,the photodissociation dynamics of pentafluorobromobenzene in 234-267 nm ultraviolet region was studied by the technique of time-slice velocity map imaging.It is found that,the dissociation followed a general ??*???*/n?*predissociation mechanism in their first UV absorption band region.At 267 nm,the strong spin-orbit coupling effect in this molecule resulted in a significant increase in the quantum yield of the spin-orbit excited bromine atom Br(2P1/2).With increasing the excitation energy,pentafrobromobenzene may be excited to the second 1??*state with negligible spin-orbit coupling effect,leading to a very small quantum yield of the the Br(2P1/2)product channel.(4)Design and construction a slow electron velocity map imaging spectrometerA new slow electron velocity map imaging(SEVI)was also designed and constructed with the purpose of studying the photodetachment and spectroscopy of negative ions.This instrument combined the electron velocity map imaging technique with a newly designed anion source,which is called plasma-entrained supersonic jet.We have tested this anion source,and found that the design of a supersonic free jet entained by another gas-discharge plsma jet is advanced in producing some special anionic species,which are difficult to be directly produced by regular gas discharge.