Photon-scattering cross sections ofH2and He measured with synchrotron radiation

Total (elastic + inelastic) differential photon-scattering cross sections have been measured for ${\mathrm{H}}_2$ gas over a range of $2.5\le S=\left(\frac{4\pi }{\lambda }\right)sin\left(\frac{\theta }{2}\right)\le 6.7$ ${\mathrm{\AA }}^{-1\mathrm{}}$ and for He over $3\le S\le 11.2$ ${\mathrm{\AA }}^{-1\mathrm{}}$, using an x-ray beam in the Stanford Synchrotron Radiation Laboratory. Absolute measured cross sections agree with theory within the probable errors. Relative cross sections (normalized to theory at large $S$) agree to better than 1% with theoretical values calculated from wave functions that include the effect of electron-electron Coulomb correlation, but the data deviate significantly from theoretical independent-particle (e.g., Hartree-Fock) results. The ratios of measured absolute He cross sections to those of ${\mathrm{H}}_2$, at any given $S$, also agree to better than 1% with theoretical He-to-${\mathrm{H}}_2$ cross-section ratios computed from correlated wave functions. It appears that photon scattering constitutes a very promising tool for probing electron correlation in light atoms and molecules. The degree of polarization of the synchrotron radiation beam has been measured by rotating the scattering plane about the beam axis; results are compared with theory.