  Resonant inelastic X-ray scattering (RIXS) is a powerful probe of elementary
excitations in solids. It is now widely applied to study magnetic excitations.
However, its complex cross-section means that RIXS has been more difficult to
interpret than inelastic neutron scattering (INS). Here we report
high-resolution RIXS measurements of magnetic excitations of La2CuO4, the
antiferromagnetic parent of one system of high-temperature superconductors. At
high energies (~2 eV), the RIXS spectra show angular-dependent dd orbital
excitations which are found to be in good agreement with single-site multiplet
calculations. At lower energies (<0.3 eV), we show that the
wavevector-dependent RIXS intensities are proportional to the product of the
single-ion spin-flip cross section and the dynamical susceptibility of the
spin-wave excitations. When the spin-flip crosssection is dividing out, the
RIXS magnon intensities show a remarkable resemblance to INS data. Our results
show that RIXS is a quantitative probe the dynamical spin susceptibility in
cuprate and therefore should be used for quantitative investigation of other
correlated electron materials.
