Excitation Dynamics Of Atoms And Molecules Studied By The High-resolution X-ray Scattering

As an important subfield of physics, atomic and molecular physics forms the foun-dation of plasma physics, condensed matter physics, astrophysics, chemistry, material science, biology, etc. The properties of atoms and molecules are mainly characterized by their energy levels and the electronic structures in position or momentum space. The later, which are usually represented as the oscillator strengths or the squared form fac-tors, relate directly to the wave-functions of the initial and final states, and are mainly explored by the scattering experiments. Nowadays the determination of the energy lev-els has achieved a very high precision (10-16), but the uncertainties of the measured dynamic parameters are commonly worse than a few percent, and the high-precision measurement in a common sense has only an accuracy of about 5%. In addition, there still exist indeterminacy and arguments about the mechanism in the scattering process. So it is very important to develop a new experimental scattering method to acquire the benchmark data of dynamic parameters, to reveal the pros and cons of different exper-imental methods, and to explore the mechanism in the scattering process.At present, the most commonly used experimental techniques to measure the os-cillator strengths or squared form factors are the photoapsorption method, the high en-ergy electron scattering method and the X-ray scattering method. Among them, the photoapsorption method can only be used to determine the optical oscillator strengths of the dipole-allowed transitions at near zero momentum transfer, while the other two methods can be used to measure the dynamic parameters versus momentum transfer of both the dipole-allowed and dipole-forbidden transitions. The photoabsorption and high energy electron scattering methods have been applied to such studies for a long time and have provided plenty of experimental data. In contrary, the high-resolution X-ray scattering method was introduced to the atomic and molecular physics until re-cently (2009) by our group. As a new method in the atomic and molecular physics, the high-resolution X-ray scattering method gains wide applications due to that it can reveal new scattering mechanism and provide cross-check on the obtained experimental data. So this dissertation mainly presents our contributions to the X-ray scattering technique and the systematic researches on the optical oscillator strengths, the elastic and inelastic squared form factors of atoms and molecules. The detailed content includes:1. The experimental benchmark data of the squared form factors of argon, hy-drogen and nitrogen are determined by the inelastic X-ray scattering method. Among them, the vibronic squared form factors of hydrogen and nitrogen are obtained exper-imentally for the first time. All of these obtained results agree well with some latest calculations. Through comparison between the squared form factors determined by the X-ray scattering and high energy electron scattering methods, it is found the intramolec-ular scattering is serious in the scattering process of fast electrons with multi-electron atom or molecules, and thus the first Born approximation is not valid. At the meanwhile, deviations of the results of the X-ray scattering experiment from the coincident results of high energy electron scattering experiments at different incident electron energies are observed. This indicates that the commonly accepted view that the superposition of electron impact results with different probe energies is a reliable condition to confir-m the validity of the first Born approximation dose not strictly hold. The contributions from the higher Born terms also show an independence behavior on the incident electron energies.2. We firstly proposed and realized the dipole (γ,γ) method to determine the op-tical oscillator strength. In the photoabsoption process, the absorbed photon transfers all of its energy and momentum to the atom or molecule. The energy transfer is rela-tively large (several eV), while the momentum transfer is particularly small (10-3a.u.). In view of this, we propose that when the momentum transfer is small enough at near-zero angle (2°) in the X-ray scattering, it can be used to simulate the photoabsorption process and thus measures the optical oscillator strength. We call this near-zero angle X-ray scattering as the dipole (γ,γ) method. This dissertation presents the dipole (γ,γ) measurement of the optical oscillator strengths of the valence shell excitations of car-bon monoxide. The obtained optical oscillator strengths reach high accuracies and can serve as the experimental benchmark data.3. The recoiling Young’s double-slit interference experiment at quantum scale is realized by means of the elastic X-ray scattering with diatomic molecules. On one side, the diatomic molecule can be seen as the natural Young’s double-slit interferometer-s so that the interference fringes appear as the intensities of scattered photon versus the scattering angles. On the other side, the mass of an diatomic molecule is small e-nough to make the recoiling energy of the double-slit measurable, and thus the recoiling double-slit experiment is fulfilled. Interference fringes are observed for both homonu-clear molecule N2 and heteronuclear molecule CO, and the measured recoiling energy of N2 indicates clearly that a single photon is scattered by the whole double-slit. So the description for interference phenomenon, which is derived from the quantum theory and goes far beyond intuition, that a photon passes through both slits when interference exists, is fully verified.