High-resolution inelastic X-ray scattering studies of the state-resolved differential cross-section of Compton excitations in helium atoms

The state-resolved differential cross sections for both the 1s 2 1 S0 → 1s2s 1 S0 monopolar transition and the 1s 2 1 S0 → 1s2p 1 P1 dipolar transition of atomic helium have been measured over a large momentum transfer region by the high-resolution inelastic X-ray scattering (IXS) for the first time. The almost perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wavefunctions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.