We have measured the total photoattenuation cross section of helium for photons in the energy range of 3 -14 keV. At these energies, the photoionization cross section is rapidly decreasing, so that Compton scattering is significant at 4 keV and dominates at the highest energies. Our measurements verify the dominance of Compton scattering in this energy range, and its importance in recent measurements of the ratio of double to single photoionization of helium. The measured cross sections are close to the combined calculated cross sections for Compton scattering and photoionization; we are able to distinguish the contributions of the two effects.PACS number(s): 32.80.Cy, 32.80.Fb
Following the previous developments to simulate the fully infrared spectra of weak hydrogen bond systems within the linear response theory, an extension of the adiabatic model is presented here. A general formulation including the electrical anharmonicities in the calculation of the damped autocorrelation function of weak H-bonds is adopted to facilitate the support of the additional properties, and thus the IR spectra of the Cl-H[combining right harpoon above] stretching band in the gaseous (CH)OHCl complex. We have explored the origins of the broadening of the Cl-H[combining right harpoon above] stretching band. We found that the main features of the lineshape are attributed to electrical anharmonicity as a consequence of the large mixed second derivatives of the dipole moment with respect to the Cl-H[combining right harpoon above] bond and of the intermonomer elongations . In addition to providing more accurate theoretical band shapes, inclusion of the electrical anharmonicity in the present model paves the way for a more complete interpretation by generating three new Franck-Condon superposed distributions.
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