Reaction Dynamics Study Of Dissociative Electron Attachment To BrCN And CH₂F₂ Molecules

Dissociative electron attachment(DEA)is the process in which the low-energy electron is captured by a molecule and the formed resonant transient negative ion dissociates into an anion and neutral fragment(s).DEA is an important process in atmosphere,interstellar space,radiation damage and plasmas.Reaction dynamics study provides an important approach to the understanding of DEA processes.The problems concerning DEA reaction dynamics of polyatomic molecules have been commonly thought challenging and received continuous attention because they involve complex potential energy surfaces,intramolecular energy flows in multiple degrees of freedom and possible many-body dissociations.In this thesis,using the High-Resolution Spectrometer of Dissociative Electron Attachment to Molecule,based on crossed-beams scheme and Velocity Map Imaging(VMI)detection method,we firstly developed an experimental method for studying electron-impact ion-pair dissociations,and then systematically studied the reaction dynamics of gas-phase BrCN in the energy range of 1.07-6.57eV,and also studied the concerted three-body dissociation of CH2F2-.Besides,to extend the capability of the present spectrometer,we designed a new detection system for measuring momentum images of light-mass ions and "pump-probe" double electron-beams method for preparing excited targets.The determination of energetic threshold in electron-impact ion-pair dissociation is thought difficult because it may be interfered by some reactions like dissociative electron attachment.Based on monochromized electron beam,we developed an experimental method for the threshold determination and the good agreement between measured and thermodynamic values in energetic threshold proved the reliability of our method.Besides,it optimized the experimental time-sequences parameter of the apparatus.The dissociation of BrCN-is an important carrier for studying spin-orbit effects and intramolecular energy flows in molecular dissociations.Based on VMI images and high-precision potential energy curves of BrCN-,the reaction dynamics of e+BrCN?BrCN-?Br+CN-in the 1-6.57eV energy range was systematically studied.The main results are as follows:1)In 1-2eV,two dissociation channels are discovered and their corresponding mechanisms are clarified.The first one is a fast process and happens on the X2?+ state of BrCN-.Whereas the second one is a slow process and appears after about 1.87eV;at the beginning BrCN-is a bound state situated in the shallow well on the potential energy curve of A2 ?3/2,but driving by the bending vibration it gradually transforms to repulsive state leading to dissociation.2)In 3.57-6.57eV,three dissociation channels corresponding to high(-1.2eV),medium(?0.4eV)and low(-0.05eV)kinetic energies of CN-are discovered and the states of CN-products are assigned.CN-with high kinetic energies are in vibrationally excited electronically ground state,while other CN-are in electronically excited states.Most importantly,we discovered for the first time in the gas-phase the vibrationally and electronically excited auto-detaching states of CN-,and this provides a new mechanism to explain CN-' s amplitude in nebula.Based on VMI image of CHF-and the kinematic model of synchronous concerted three-body dissociation,we investigated e+CH2F2-CH2F2-?H+F+CHF-in 10.03-12.03eV.Concerted three-body dissociation of CH2F2-is confirmed and contributions of synchronous and asynchronous concerted dissociations are quantitively evaluated.The asynchronous mechanism is found dominant and the synchronous contribution increases with increasing electron energy.The study provides two intuitive pictures of nuclear motions in the three-body dissociation of CH2F2-,inspiring further theoretical studies on the multidimensional nuclear motions of transient polyatomic molecules.In the present spectrometer,the mass-number range of detectable ions are limited to larger than 4 and targets are limited to ground-state molecules.To overcome the limitations,we improve the design of ion detection system and incorporate the design of double electron-beams system.By shortening the length of VMI lens and introducing Delay Line Anode detector,the newly designed ion detection system enables detection of light-mass ions like H-and He-.In the design of double electron-beams system,a new independent electron gun located under the present one,with large beam current and operating in DC mode,is used to excite molecules before they enter the reaction center.