Dissociation Dynamics by Electron Attachment:Acetonitrile,Carbon Dioxide,and Formic Acid Dimer

Author:Li Hao

Supervisor:tian shan xi


Degree Year:2019





The universal existence of low energy electron(<30 eV)has been confirmed in interstellar space and Planet’s atmosphere.As a fundamental collision in atomic and molecular physics,low energy electron scatering mainly contributes to molecular vibration and bond cleavage and plays an important role in many fields,such as radiation chemistry,astrophysics,electron etching and deposition.The abundant secondary electrons by high energy radiation were found as the more efficient way than direct radiation to destroy the DNA structure of bio-tissues.In space,the process of dissociative electron attachment initiates a series chemical reaction and was deemed as the primary way to produce stable anions.Moreover,for the bond-selectively breaking in DEA process,it might be of great applicable potential in chemical synthesis and catalytic chemistry.In semiconductor industries,low energy electron plays an important role in electron induced etching and deposition technique.Many methods in the research of electron scattering process were developed in the past decades to elucidate the elementary chemical reaction in traditional chemistry,such as electron transmission spectroscopy,electron energy loss spectroscopy,anion yield spectroscopy.Owing to the invention of velocity map imaging methods for chemical dynamics research in 1990s,which provides angular and kinetic distribution of the differential cross section for chemical reaction.In contrast to the laser-induced or molecular impact process,the critical information might be buried in electron scattering experiment due to the poor resolution of electron beam.We’ve successfully built a high resolution imaging apparatus for DEA study with vibrational-resolved capability.All the works in this thesis were carried out with this instrument.The essential attribute of high resolution anion imaging apparatus is an beam of electron with high energy-resolution,so we designed a set of trochoidal electron monochromator(TEM).Since the complexity of adjusting the TEM is greatly enhanced by large number of electric plates,a GUI for automation and data analysis was developed for the TEM on the LabVIEW platform and the adjusting process has been greatly simplified by this program.High resolution imaging apparatus have two working modes:Time of Flight(ToF)mass spectroscopy and velocity map imaging.An automotive acquisition and analysis for anion yield spectroscopy was developed for ToF mode.Further,a whole set of data processing program has been developed for imaging mode on Matlab platform and a weighting factor has been added in the code.Firstly,we demonstrated the experiment of dissociative electron attachment to acetonitrile with the high resolution anion imaging apparatus,the anion yield spectroscopy of CN-、CHCN-、CH2CN-fragment has been acquired at low energy range.Even we can not discern the CHCN and CH2CN due to the limitted mass resolution,however the neutral H2 fragment might be vibratedfor the lower threshold of CHCN as suggeted by the efficiency curve.Due to the small kinetic energy of CHCN and CH2CN anion product,so the time-sliced imaging of CN-were recorded at 7.1,7.6,8.1 eV and the results indicate the two-and three-body dissociation process might be involved.Secondly,an experimental observation of dissociative electron attachment to vibrationally excited carbon dioxide has been demostrated.Similar to the pump-probe method in laser-induced chemistry,we managed to produce vibrational molecule and the dissociation process of the vibrationally excited carbon dioxide in a single long(400 ns)electron pulse with the FWHM of 70 meV.An anion yield spectroscopy of O-has been acquired around 2Πu resonance state.The signal is strong below the dissociation threshold and can not be explained with a single vibrational mode.so the time sliced image of O-were recorded at 3.38,3.48,3.58 eV.Moreover,only we assign the O-signal with nl 0 and n00 series vibrational state for the symmetry matching requirement,the kinetic energy and angular distribution can be explained simultaneously.In addition,the reactivity of DEA is greatly enhanced by the vibrational motion of carbon dioxide.Thirdly,it is an ideal model to testify the isomer and rotational state selector for the distinct difference in dipole moment of formic acid monomer and dimer.We carried out experiment of dissociative electron attachment to formic acid monomer and dimer.A distinct difference has been found in HCOO-anion spectroscopy under the monomer and dimer dominated condition and the energy intervals of bumps or peaks on HCOO-efficiency curve is coincident with the-CO2 asymmetric vibrational state.In addition,the present anion efficiency curve of dissociative electron attachment to formic acid monomer agrees with previous results well.So we further acquire the time sliced momentum image of HCOO-,the-CO2 asymmetric vibrational structure also appears in the kinetic spectrum of HCOO-,which further suggests that the nuetral coproduct of HCOO-should lies in the transition structure.Finally,a quantum state selector has been designed to study the rotational and steric effect of dissociative electron attachment.As we know,the dipole moment is smaller in laboratory frame for higher rotational polar molecule.Moreover,the molecular isomerization widely exists in natural world for polyatomic molecule and may results in a possible difference in dipole moment.Owing to the stark interaction,we are able to spatially isolate different comformers which could not be purified in traditional chemistry.For the rotational state selection,the high state density of sample can be realized by supersonic molecular beam method and the rotational temperature of the sampled ensemble can be cooled down to 1 K,and the correponding sample should lies in several low-lying rotational states,so that abundant mono-rotational sample can be realized.Further,the separation of different isomers and different rotational states has been demonstrated in simulation.Moreover,an approach to the nuclear spin statistical weight calculation has been routinely demonstrated and quantum state selector has been constructed.