High-energy ions produced in explosions of superheated atomic clusters

Efficient conversion of electromagnetic energy to particle energy is of fundamental importance in many areas of physics. A promising avenue for producing matter with unprecedented energy densities is by heating atomic clusters, an intermediate form of matter between molecules and solids1, with high-intensity, ultra-short light pulses2–4. Studies of noble-gas clusters heated with high-intensity (>1016Wcm–2) laser pulses indicate that a highly ionized, very high temperature micro-plasma is produced. The explosion of these superheated clusters ejects ions with substantial kinetic energy3–5. Here we report the direct measurement of the ion energy distributions resulting from these explosions. We find, in the case of laser-heated xenon clusters, that such explosions produce xenon ions with kinetic energies up to 1 MeV. This energy is four orders of magnitude higher than that achieved in the Coulomb explosion of small molecules6, indicating a fundamental difference in the nature of intense laser–matter interactions between molecules and clusters. Moreover, it demonstrates that access to an extremely high temperature state of matter is now possible with small-scale lasers.