Optical Absorption and Electron-Energy-Loss Spectra of Helium Microbubbles in Aluminum

Aluminum thin films containing helium microbubbles produced by ${\mathrm{He}}^{+}$-ion implantation have been investigated by ultraviolet absorption and electron-energy-loss spectroscopies in the range 1-30 eV. The shape and position of the $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-n{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ helium line series and several Al plasmon bands are discussed in terms of the physical state of helium in the bubbles. Results imply helium densities 3 to 6 times that of liquid He at 2°K and 1 bar pressure.