Inelastic x-ray scattering study on the single excitations of helium

The inelastic x-ray scattering technique has been used to study the excitation mechanism of atomic helium in this work. The squares of the form factor for the single excitations of 1s(2) -> 1snl (n <= 6) of helium have been determined by the high resolution (70 meV) inelastic x-ray scattering. The present squares of the form factor measured by inelastic x-ray scattering provide an independent cross-check to the ones measured by high-energy electron energy loss spectroscopy. It is found that the squares of the form factor for the single excitations of 1s(2) -> 1snl (n <= 6) of helium measured by inelastic x-ray scattering are in excellent agreement with the ones measured by high-energy electron energy loss spectroscopy, which confirms the quality of these two experimental methods. The excellent agreement of the present measurements with the theoretical calculations also gives a rigorous test of the theoretical method. This work demonstrates that inelastic x-ray scattering is a powerful tool to study the excitation mechanism in atomic or molecular systems at the third-generation synchrotron radiation light source.