Multi-keV proton energies from exploding hydrogen clusters

We report the first direct experimental measurements of the proton energies produced from explosions of isolated hydrogen (H 2 ) clusters in an intense laser field. The measured proton energy distribution from the interaction of a 35 mJ laser pulse of duration 90 fs with a cluster of mean radius 100 Å was measured and has a mean proton energy of 7±1 keV. A maximum proton energy of >20 keV was recorded. We show the cluster expansion in this experiment is not due to a purely Coulomb explosion but a combination of Coulomb and hydrodynamic mechanisms. These findings are relevant to fusion studies in laser-heated clusters.