Energetic, highly-charged oxygen ions, O q+ (q ≤ 6), are copiously produced upon laser field-induced disassembly of highly-charged water clusters, (H 2 O) n and (D 2 O) n , n ∼ 60, that are formed by seeding high-pressure helium or argon with water vapor. Ar n clusters (n∼40000) formed under similar experimental conditions are found undergo disassembly in the Coulomb explosion regime, with the energies of Ar q+ ions showing a q 2 dependence. Water clusters, which are argued to be considerably smaller in size, should also disassemble in the same regime, but the energies of fragment O q+ ions are found to depend linearly on q which, according to prevailing wisdom, ought to be a signature of hydrodynamic expansion that is expected of much larger clusters. The implication of these observations on our understanding of the two cluster explosion regimes, Coulomb explosion and hydrodynamic expansion, is discussed. Our results indicate that charge state dependences of ion energy do not constitute an unambiguous experimental signature of cluster explosion regime.

We present measurements of the laser induced spatial alignment of two diatomic molecules, iodine (I 2) and bromine (Br 2). Dynamic alignment is inferred from the angular distribution of the ionic fragments from multi-electron dissociative ionization (MEDI). The angular distributions were determined for different pulse durations and energies of the short infrared laser pulses that induce the MEDI, and were measured using a velocity map ion imaging detector. The width of the angular distribution of the fragments with respect to the laser polarization axis depends only weakly on the laser pulse energy, and decreases rapidly for longer pulse lengths (a few picoseconds) at constant pulse energy. The interpretation of this result in terms of dynamic alignment is supported by an extended field ionization Coulomb explosion model that includes the rotation of the molecule induced by the laser field.

The fragmentation of the CO molecule by O 7+ ion impact is investigated in two different energy regimes by fragment ion momentum spectroscopy. The improved resolution of the present kinetic energy release measurement together with application of a time-dependent wavepacket dynamics method used in conjunction with new high-level computations of a large number of dication potential energy curves enables one to unambiguously assign each line to an excited state of the transient molecular dication produced during the collision. This is the first direct experimental evidence of the limitations of the Coulomb explosion model to reproduce the molecular fragmentation dynamics induced by ion impact. Electron removal due to a capture process is shown to transfer less excitation to the target than direct ionization. At low collision velocity, the three-body interaction between the projectile and the two fragments is also clearly highlighted.

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.

The disassembly of molecular clusters (N 2 ) n (n=50-3000) in strong optical fields is investigated using two-dimensional time-of-flight spectrometry. Very highly charged ions are formed with a two-component energy distribution. A low-energy, isotropic component correlates with Coulomb explosion. A highenergy, anisotropic component, that results from a "charge flipping" acceleration mechanism, gives rise to ions with energies in excess of the Coulombic limit.