Introduction

Atomic clusters have provoked great interest since their first observation in the mid-1950s. Physicists' and chemists' fascination with them derives from the unique position clusters hold as an intermediate state between molecules and solids. Many studies have been concerned with the optical properties of clusters. An important finding was the discovery of collective electron dynamics in clusters, which is virtually absent from laser–atom interactions. These are responsible for the ‘giant resonance’ seen in absorption spectra of clusters and can lead to remarkable optical properties.

During the last five or so years, the study of laser–cluster interactions has been extended to laser intensities in excess of 1015 W cm−2 (laser pulse widths in the range 0.1–10 ps) in the so-called ‘strong-field’ interaction regime, for which the electric field of the laser is no longer small relative to the atomic field and the interaction becomes highly non-perturbative. This regime, which was made widely accessible by the development of chirped-pulse-amplification (CPA) lasers, had been studied for atoms, small molecules and bulk solids since the late 1980s. In stark contrast to earlier studies of laser–cluster interactions at lower intensities that had revealed dynamics similar to those seen in molecules – with relatively inefficient coupling of laser energy to electrons and ions – studies on the generation of X-rays from gases of clusters (>1000 atoms) at ≃1016 W cm−2 began to reveal startling evidence of a laser–cluster interaction that was very much more energetic.