The density of helium in bubbles in implanted materials: Theoretical interpretation of VUV absorption and EELS spectroscopy

The shape of the He resonance absorption line of He bubbles in metals is discussed in terms of He-metal and He-He interactions. For bubble radii larger than 10 Å the metal matrix effects are found to be negligible. For small vacancy clusters there is as yet no quantitative calculation of the role of the metal conduction electrons in determining the position and width of the He resonance excitation.

The He-He interactions are treated within static line broadening theory which assumes pairwise additivity of the interactions. These are taken from ab initio quantum mechanical calculations of 2p excimer potential energy curves due to Guberman and Goddard and to Gupta and Matsen. The radial pair distribution function required by our statistical line shape calculation is computed from two theoretical models of the dense fluid: either the exact solution of Percus-Yevick's equation for hard spheres or a Molecular Dynamics computer simulation based on an accurate pair potential. The two approaches give very similar results. The blue shift of the resonance line is found to depend nearly linearly on density but our value for the slope differs substantially from the prediction of a model band structure calculation performed by the Jülich group in the framework of density functional theory.