We report a new method to enhance the x-ray emission from nano-cluster plasmas formed upon irradiation by intense femtosecond-duration laser pulses. Our experiments demonstrate that when Ar clusters are doped with H2O the time-integrated yield of Ar K x-ray emission is enhanced by approximately 12-fold in comparison to that obtained from pure Ar clusters under otherwise identical experimental conditions. A significant alteration in the time-dependent electron density is achieved by the presence of an H2O dopant, and this could be the possible reason for the enhancement that is observed.

We report the results of experiments that establish the possibility of bringing about multiple ionization and Coulomb explosion of molecular clusters with nanosecond laser pulses at intensities as small as 10(9) W cm(-2). We demonstrate several new facets of the laser-cluster interaction in the low-intensity, long-pulse domain: (i) The choice of laser wavelength for a given cluster species is very crucial. (ii) Excited electronic states play a very important role in the ionization dynamics. (iii) When field ionization is insignificant and ponderomotive energies are very small, it is energy pooling rather than inverse bremsstrahlung that determines how clusters absorb energy from the optical field.

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle (~10 fs) laser pulses. We find that less than 10 dopant atoms "ignite" the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs.

We present electron energy measurements from nano-cluster plasmas that are formed when molecule-doped rare-gas clusters are irradiated by intense, 100 fs laser pulses of intensity ∼1015 W cm−2. In pure Ar clusters the high temperature component (energy ∼1400 eV) is less than 1% of the low temperature component (energy ∼130 eV), while for water-doped Ar clusters the high temperature component is as high as 7% of the low temperature component. Numerical estimates based on collisional ionization and inverse bremsstrahlung absorption indicate that the easily ionizable dopant molecules enhance the propensity for ionization ignition by significantly altering the temporal profile of the inner-ionized electron density within the cluster.

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump-probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump-probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump-probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron-ion recombination.