VUV Photodissociation Dynamics Of OCS And Cryogenic Reaction Rates Of Charge Transfer Reactions

This dissertation focuses on experimental studies of gas phase chemical kinetics and molecular dynamics.The subject is beneficial to understanding reaction processes in combustion,atmosphere,and interstellar environment,and helps to reveal the nature of chemical reactions.The author investigated two kinds of reactions commonly found in the interstellar environment:vacuum ultraviolet photodissociation of small molecules and cryogenic ion-molecule reactions.The differential cross section（DCS）and the apparent rate constant are measured for these two reactions respectively.The first part presents the vacuum ultraviolet（VUV）photodissociation dynamics of carbonyl sulfide（OCS）investigated by a tunable photolysis light source and the time-sliced velocity map ion imaging technique.OCS is the most abundant sulfurcontaining compound in the atmosphere and is also present in many interstellar environments.The radicals generated from the photodissociation of OCS can initiate a series of chemical reactions.In addition,the study of photodissociation dynamics is also an important means to explore the molecular potential energy surface.Ion images of S（³P_J=2,1,0）products were measured at eight photolysis wavelengths from 133.26 nm to 140.98nm,corresponding to the E and F Rydberg states of OCS.Two dissociation channels:S（³P_J）+CO（X¹∑⁺）and S（³P_J）+CO（A3Π）were observed with the former dominant.The concentric rings in the ion images correspond to different vibrational states of co-products CO.The product total kinetic energy releases,the anisotropy parameters（β）,and the branching ratios of high-lying CO vibrational states were determined for the S（³P_J）+CO（X¹∑⁺）channel.It was found that the anisotropy parameters suddenly changed from negative to positive when OCS was excited to the higher vibrational levels of the F state.The angular distribution of products from the E state is less anisotropic.Furthermore,the anisotropy parameters for S（³P_J）products of J=2,1,0 were different for both the F and E state.These anomalous phenomena may result from the simultaneous existence of both parallel and perpendicular dissociation mechanisms,suggesting the involvement of other electronic states with different symmetries in the initially-excited energy region.This work provides a further understanding of the nonadiabatic couplings in VUV photodissociation process of OCS.The second part describes a set of apparatus for measuring rate coefficients of cryogenic ion-molecule reactions.In this apparatus,ions are generated by electron ionization.After being filtered by a quadrupole mass analyzer,ion reactants enter a three-dimensional quadrupole ion trap.The ion trap is assembled on a cryogenic liquid helium cold head as an ion-molecule reaction container.Due to the good thermal conductivity between the cold head and the ion trap,and the oxygen-free copper shield blocking the external heat radiation,the temperature of the ion trap wall can be reduced to a minimum of 5.2 K.The reaction time is controlled by adjusting the storage time of the ion trap,and the reacted ions are detected by the electron multiplier detector.To test the working condition,the author investigated the charge transfer reaction Ar⁺+N2→Ar⁺N₂⁺.The Ar⁺signal was measured after 10～190 ms reaction time under different gas number densities of the neutral reactant[N₂].By linear fitting,the apparent rate coefficient of this reaction at about 5 K（temperature of ion trap wall）was finally obtained to be 3.37×10^-10 cm³·s^-1.This apparatus can create the cryogenic environment in the molecular clouds,measuring rate coefficients of ion-molecule reactions at low temperatures.After being improved or modified,the apparatus can work at different temperatures,or be used as an ion beam source in ion-molecule scattering research.