Explosions of water clusters in intense laser fields

Abstract

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.

Simulation of the dynamics of laser-cluster interaction

“…One manifestation is the up-conversion of the incident infra-red laser into keV X-ray photons in rare-gas clusters, which takes place with a relatively high efficiency providing large X-ray yields just like in solids, yet is relatively debris-free, a property shared with gas targets [1]. Similar behavior has been observed for the emission of energetic electrons [2] or highly charged ions [3], thus making the interaction of intense short and ultra-short laser pulses with clusters a topic of considerable interest [4; 5].…”

“…One manifestation is the up-conversion of the incident infra-red laser into keV X-ray photons in rare-gas clusters, which takes place with a relatively high efficiency providing large X-ray yields just like in solids, yet is relatively debris-free, a property shared with gas targets [1]. Similar behavior has been observed for the emission of energetic electrons [2] or highly charged ions [3], thus making the interaction of intense short and ultra-short laser pulses with clusters a topic of considerable interest [4; 5].In a simple picture, the dynamics during the interaction of a strong laser pulse with a cluster can be summarized as follows [6]: the atoms of the cluster are first ionized by the incident laser pulse (inner ionization) and a cold "nano-plasma" of solid density is formed. The quasi-free electrons take part in a collective oscillation driven by the laser field and, moreover, interact with the field of the surrounding particles.…”

“…From an experimental point of view, the spectroscopy of the emitted ions [3] and electrons [2] gives information on the system a few microseconds after the femtosecond laser-pulse and the cluster disintegration. X-ray spectroscopy [16], on the other hand, presents the advantage of measurements on a much shorter time-scale, down to a few femtoseconds.…”

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

“…Strong enhancements in the energy, yield, or charge states of emitted species [6,7,8,15] can be achieved with laser fields that induce resonant collective heating of nanoplasma electrons (Mie plasmon resonance). When present, the plasmon resonance is typically stronger than additional higher-order nonlinear resonance effects [16].…”

“…Under intense near-infrared laser pulses (λ ∼ 800 nm), the transition of gas-phase clusters into well-isolated finite nanoplasmas results in high energy absorption, rapid cluster explosion, and the emission of highly charged ions [3,4,5], fast electrons [6,7,8,9], and energetic photons from the vacuum ultraviolet up to the x-ray range [10,11,12,13,14]. Because of the opportunity to study the underlying ionization, heating, and decay mechanisms in a nearly background-free environment, intense laser-cluster interactions are of high interest also for various other fields ranging from plasma physics to applied laser-matter research.…”