Lifetime of an excited atom in a metal cavity

Abstract

Several broadening mechanisms for an optical absorption line of a gas atom or molecule trapped in a spherical cavity in a metallic host are examined. Among these, the linewidth due to the coupling of the atom to damped surface polaritons of the metal cavity is analyzed in detail, and its dependence on cavity radius and on atom position is obtained. One finds that the surface-polariton coupling effect is important only for bubbles of atomic sizes (a few vacancy clusters), or for resonant cases where the atomic frequency coincides with one of the surface-plasmon frequencies. Numerical application is made to the broadening of the resonance line of helium trapped in an aluminum matrix. For this system the polariton coupling appears to be negligible as compared to the so-called resonance broadening in the gas under the pressures which are presumed to prevail in bubbles produced by He implantation. The optical absorption of a random distribution of small He-gas bubbles in Al is also computed by means of an effective-medium theory. One finds that the absorption linewidth and position should give a good measure of the average He density in the bubbles.