The state-resolved differential cross sections for both the $1s^2$ ${}^1{S}_0\to 1s2s$ ${}^1{S}_0$ monopolar transition and the $1s^2$ ${}^1{S}_0\to 1s2p$ ${}^1{P}_1$ dipolar transition of atomic helium had been measured over a large momentum transfer region by high-resolution inelastic x-ray scattering (IXS). The almost-perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wave functions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

Electron–ion recombination of Be-like ⁴⁰Ar¹⁴⁺ has been measured by employing the electron–ion merged-beams method at the cooler storage ring CSRm. The measured absolute recombination rate coefficients for collision energies from 0 to 60 eV are presented, covering all dielectronic recombination (DR) resonances associated with 2s ² → 2s2p core transitions. In addition, strong trielectronic recombination (TR) resonances associated with 2s ² → 2p ² core transitions were observed. Both DR and TR processes lead to series of peaks in the measured recombination spectrum, which have been identified by the Rydberg formula. Theoretical calculations of recombination rate coefficients were performed using the state-of-the-art multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE to compare with the experimental results. The plasma rate coefficients for DR+TR of Ar¹⁴⁺ were deduced from the measured electron–ion recombination rate coefficients in the temperature range from 10³ to 10⁷ K, and compared with calculated data from the literature. The experimentally derived plasma rate coefficients are 60% larger and 30% lower than the previously recommended atomic data for the temperature ranges of photoionized plasmas and collisionally ionized plasmas, respectively. However, good agreement was found between experimental results and the calculations by Gu and Colgan et al. The plasma rate coefficients deduced from experiment and calculated by the current AUTOSTRUCTURE code show agreement that is better than 30% from 10⁴ to 10⁷ K. The present results constitute a set of benchmark data for use in astrophysical modeling.

The integral cross sections (ICSs) of the electron scattering of nitrogen molecule are of great importance to the understanding of many different aspects of atmospheric physics. In the present work, the generalized oscillator strengths (GOSs) of the valence shell excitations of , , , , and of nitrogen have been determined by fast electron scattering at an incident electron energy of 1500 eV and an energy resolution of 70 meV. By cross‐checking strictly the present results with the previous electron scattering and inelastic X‐ray scattering (IXS) ones, the accuracy and reliability of the GOSs for the corresponding dipole‐allowed transitions of nitrogen are tested stringently. Based on the accurate GOSs of these dipole‐allowed transitions, their ICSs are obtained systematically from the threshold to 5000 eV for the first time with the aid of the BE‐Scaling (B is the binding energy and E is the excitation energy) method, and the corresponding ICSs at the moderate and high energies are the only available data to the best of our knowledge.

The high-resolution inelastic x-ray scattering has been applied to study the excitation mechanism of atomic neon. The different electric multipolar transitions of neon, i.e., the electric monopolar, dipolar, and quadrupolar ones, were clearly resolved, and their squared form factors were determined. It is found that for the monopolar and quadrupolar transitions the present inelastic x-ray scattering results are in agreement with the ones measured by high-energy electron scattering, while for the dipolar transition large discrepancies between the results measured by these two methods were observed. Such phenomena are well elucidated with the aid of the second-order Born approximation: Born amplitude of a transition near zero momentum transfer is the key factor which determines the behavior of the intramolecular multiple scattering in the high-energy electron scattering. It is revealed that the intramolecular multiple scattering in high-energy electron scattering must be considered for the dipole-allowed transition, while it is negligible for the dipole-forbidden ones.

We report a comparative study of low-lying valence electronic states of carbon dioxide by high-resolution inelastic x-ray and electron scattering. Momentum-transfer-dependent inelastic squared form factors for the two states ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ and ${}^1{\mathrm{\Pi }}_{\mathrm{u}}$ and generalized oscillator strength for the 9 eV feature from the ground state $X^1{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$ have been derived from the inelastic x-ray scattering method at an impact photon energy around 10 keV, and the electron energy-loss spectra measured at an incident electron energy of 1500 eV. It is found from the comparison between the present results and the previous outcomes that the recent calculations taking the vibronic effects into consideration satisfactorily reproduce the inelastic squared form-factor profile for the ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ transition and the generalized oscillator strength profile for the 9 eV feature. However, the vibronic effects seem to play no role in the ${}^1{\mathrm{\Pi }}_{\mathrm{u}}$ transition. The difference existing between the inelastic x-ray scattering and electron energy-loss spectroscopy results in the larger momentum-transfer squared region may be attributed to the increasing role of the higher-order Born terms. Furthermore, the controversy concerning the designations of electronic states around 11 eV is solved by assigning the two peaks centered at 10.98 and 11.05 eV to the vibrational progression ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{{+}^{\prime }}$ and ${}^1{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$, and the peak centered at 11.16 eV to the forbidden transition $2^1{\mathrm{\Delta }}_{\mathrm{u}}$ based on the present results.