| The electronic,photonic,atomic and molecular collisions widely exist in the inter-stellar space,nuclear fusion,plasma,and chemical reaction processes,so the atomic and molecular structures and their excitation dynamics are of great significance for the de-velopment of many fields,such as astrophysics,plasma physics,nuclear fusion,material science,etc.Due to the demands of many high-tech industries involving the astronomi-cal observations,energy projects,military technology and aviation industry,the investi-gations on atomic and molecular structures and excitation dynamic parameters become one of the most active area of physics.However,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,nowa-days there still exist uncertainties and arguments about the mechanism in the scattering process.So it is very important to develop a new technique or improve the existing experimental method to determine the benchmark data of dynamic parameters,which can make further deep understanding of the mechanism in the scattering process.Fast electron impact technique is an important experimental method in studying the excitation dynamic parameters of atoms and molecules,which has been used for decades and provided a large number of data in history.In recent years,with the de-velopment of the third generation synchrotron radiation and the free electron laser,the inelastic x-ray scattering technique has been a powerful tool in studying the excitation dynamics of atoms and molecules.Although fast electron collision method and inelas-tic x-ray scattering technique are completely different,the innate connection between them makes them can be used to probe the structures and excitation dynamics of atoms and molecules.Using the inelastic x-ray scattering technique and fast electron impact method,a series of experimental studies in the excitation dynamics of some atoms and molecules have been carried out:1.Based on the fact that the inelastic x-ray scattering at a negligibly small mo-mentum transfer can simulate the photoabsorption process,the dipole(γ,γ)method is proposed and realized to determine the absolute optical oscillator strengths of the vanlence-shell excitations of atoms and molecules.Compared with the conventionally used photoabsorption method,this new method is free from the line saturation effect,which can seriously limit the accuracies of the measured photoabsorption cross sections for discrete transitions with narrow natural linewidth.Furthermore,the Bethe-Born con-version factor of the dipole(γ,γ)method varies much more slowly with the excitation energy than does that of the dipole(e,e)method.By using the high-resolution dipole(γ,γ)method,absolute optical oscillator strengths for the excitations of 1s2 →1snp(n=3-7)of atomic helium have been determined,and the excellent agreement of the present measurements with both those measured by the dipole(e,e)method and the previous theoretical calculations indicates that the dipole(γ,γ)method is a powerful tool to measure the absolute optical oscillator strengths of the valence-shell excitations of atoms and molecules;2.A joint experimental and theoretical investigation of the excitation dynam-ics of hydrogen has been performed by the high-resolution inelastic x-ray scattering,the fast electron impact method,as well as the multi-reference single-and double-excitation configuration-interaction method.Momentum-transfer-dependent inelastic squared form factors for the vibronic series belonging to the B1∑+u,C1IIⅡu,and EF1∑+g electronic states of molecular hydrogen have been derived for the first time.Through the comparison between the experimental results and theoretical calculations,some in-teresting phenomena are observed:for the lower vibronic excitations ofB1∑+u state,the present and previous calculations agree with the present experimental results very well,while they deviate from the experimental ones gradually with the higher vibronic excited states ofB1∑+u.One possible reason may be the electronic-vibrational coupling effect,considering that hydrogen is the lightest molecule and the velocity of the nu-clei increase with the higher and higher vibronic state.For the C1Ⅱu state and some vibronic excitations of the EF1∑+g state,the present experimental results are in good agreement with the present and previous calculations,while the slight differences be-tween the inelastic x-ray scattering and electron energy-loss spectroscopy results in the larger squared momentum-transfer region may be attributed to the increasing role of the higher-order Born terms in the electron scattering process;3.A joint experimental and theoretical investigations of the excitation dynamics of molecular deuterated hydrogen and deuterium have been performed by the fast elec-tron impact method and the multi-reference single-and double-excitation configuration-interaction method.Momentum-transfer-dependent inelastic squared form factors for the vibronic series belonging to the B1∑+u,C1Ⅱu,and EF1∑+g electronic states of molec-ular deuterated hydrogen and deuterium have been derived for the first time.Similar to the results of molecular hydrogen,the present calculations can not satisfactorily re-produce the inelastic squared form-factor profiles for the higher vibronic transitions of the B1∑+u state of HD and D2.However,there are some new lights on this phenom-ena:the discrepancies between the experimental and calculated results of HD and D2 are showing a weaker dependence on the vibronic numbers than those of molecular hy-drogen,i.e.,the present EELS results of HD and D2 are in better agreements with the calculated ones than those of the H2.Compared with H2,HD and D2 have additional neutrons in their nuclei,so they are heavier than H2.Therefore,it is very likely that the electronic-vibrational coupling effect is less important for these heavier isotopic molecules.Considering that the H2,HD and D2 are the simplest diatomic molecules,we initially expected that their experimental results could be reproduced by the theoret-ical calculations very well.However,obvious discrepancies are observed in the present work.Therefore,further high-resolution experimental studies and highly accurate cal-culations including the electronic-vibrational coupling effect are greatly recommended;4.During the period of the author’s doctoral study,a series of upgrades have been made on the high-resolution electron energy loss spectrometer in our laboratory,includ-ing modifications on electron optics,electronics,constant temperature system,vacuum system and the detection system.Finally,the spectrometer now can be run in a long-term stable working performance at the incident electron energies of 1500-2500 eV with the best energy resolution of 60 meV,which makes some experiments with low cross section or at large scattering angle more easier. |
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