Electron Momentum Spectroscopy Study On Distorted-Wave Effect And Nuclear Motion Effect Of Molecules

Electron momentum spectroscopy（EMS）provides an unique experimental method for studying the electronic structures of atoms and molecules,which can simultaneously obtain the electron binding energy and the electron momentum distributions（EMDs）of atomic and molecular orbitals.This method has been successfully applied to the investigation of many atomic and molecular systems.Recently,the theoretical calculations based on plane wave impulse approximation（PWIA）and molecular equilibrium geometry have significant discrepancies with the experimental results in EMS studies on some molecular systems.These discrepancies may be induced by distorted wave or molecular nuclear motion.The influence of nuclear motion on EMDs can be analyzed by harmonic analytical quantum mechanical calculations or by calculations at the distorted geometry induced by nuclear relaxation.However,the distorted wave calculation for molecules remains unresolved.Most recently,the newly developed multi-center three distorted wave（MCTDW）method has solved the problem.In this thesis,the experimental and theoretical studies on nitrous oxide（N₂O）,oxygen（O₂）,sulfur hexafluoride（SF₆）and ethylene（C₂H₄）have been performed.With MCTDW method,the influence of the distorted wave on experimental electron momentum distribution is analyzed in detail,and the accuracy of this method has been demonstrated.Besides,the influence of molecular nuclear motion is also further investigated.This theis includes six chapters,each of which is as follows:Chapter 1 introduces the basic principles and the research progresses of EMS.The main research contents of this thesis is also presented.Chapter 2 introduces the electron momentum spectrometers,experimental methods and theoretical framework used in this thesis.Chapter 3 presents the result of high-resolution EMS study of N₂O.Four main ionization peaks of the outer-valence orbitals and six satellites are observed experimentaly.The assignments and the pole strengths for these ionization states are determined by comparing the shape and the intensity of the experimental electron momentum profiles with those of the theoretical ones.For the EMD of 1π orbital,the calculated result at equilibrium geometry underestimates the measurement in the low momentum region.The high intensity of the experimental EMD is interpreted by using the geometry distortion induced by ultrafast nuclear relaxation motion at the instant of molecular ionization（Renner-Teller effect）.Chapter 4 shows the EMS study of O₂.The EMDs of valence orbitals of O₂ is measured at three different electron impact energies.The PWIA calculations largely underestimate the experimental intensity both in the low and large momentum regions.The newly developed multi-center three distorted wave calculations precisely reproduce the experimental result,revealing the influence of the distorted wave on the EMDs of molecular orbital.This study directly demonstrates the accuracy and reliability of MCTDW method.Chapter 5 introduces the EMS study of SF₆.The EMDs of valence orbitals are measured.The theoretical results are calculated by the MCTDW method at equilibrium geometry and by the PWIA method both at equilibrium geometry and considering vibrational motions.For all the measured orbitals,large discrepancies were observed between the measurments and the PWIA calculations at equilibrium geometry.The vibrational effect can partly explain the high intensity of experimental result in the lowmomentum region for HOMO 1t1g.However,the MCTDW calculations improved the agreement with experiments for all the observed orbitals,indicating that the distorted wave also plays an important role in explaining the experimental results of SF₆.Chapter 6 shows the EMS study of C₂H₄.The experiment is performed with the incident electron energies equal to 600 and 1200 eV.The measured EMD of 1b_3g orbital shows a significant "turn-up" in the low momentum region.The PWIA calculations at equilibrium geometry and considering vibrational motions,as well as the MCTDW calculations at equilibrium geometry can not explain the experimental result well.However,our newly developed MCTDW calculation including molecular vibrational motions reproduces the experimental result of 1b_3g orbital,indicating that the observed"turn-up" is caused jointly by the contributons of distorted wave and molecular vibration.Finally,the summary of this thesis and the prospects for future work on experiment and theory are presented.