Time Evolution of Coulomb Explosion of Argon Clusters in Moderately Intense Nanosecond Laser Fields

We report experimental observations of extremely high levels of above-threshold ionization (ATI) and extensive delayed ionization of photoelectrons from argon clusters in moderately intense nanosecond laser fields at 532 nm. We have successfully projected the cluster explosion and expansion process onto the time-of-flight axis of the photoelectrons. The photoelectron spectra can be separated into three groups: a fast group that reaches as high as 3500 times the ponderomotive energy (<i>U</i>_p) of the laser field, an ATI group that shows the addition of up to 8 photons (over 200<i>U</i>_p), and a delayed group that are ionized ∼100 ns after laser excitation ∼1 mm downstream from the excitation spot. The delayed electrons are tentatively attributed to field ionization of near-threshold electrons contained in the expanding nanoplasma after an initial Coulomb explosion; hence, the delay times of these electrons demonstrate a dependence on the strength of the extraction field. These surprising discoveries demonstrate that the intermediate intensity regime is not a simple extrapolation of strong fields, and new phenomena warrant detailed investigation.