Models of Laser Cluster Interactions

Models of Laser Cluster Interactions - see Laser Cluster Interactions
  1. - inner shell excitation model (including coherent electron motion) [4,24,25],
  2. - the ionization ignition model [26],
  3. - Coulomb explosion models [27],
  4. - particle codes 28,29
  5. - nanoplasma models [15,30,31]. [TD69.pdf, page 312]

developed by Ditmire et al. at Lawrence Livermore National Laboratory [15]. This numerical model (nanoplasma model) has proved quite successful in reproducing most of the experimental results concerning ion (and electron) emission from ‘‘single’’ clusters [22] and provides an intuitive picture of the interaction in terms of the formation of a high-density, nanometer scale plasma (hence ‘‘nanoplasma’’), ionization and heating in the laser field and its subsequent rapid expansion (‘‘explosion’’) leading to the formation of energetic electrons and ions. The impetus for the development of this model was experimental data demonstrating that large clusters in strong fields behave more like bulk solids than smaller clusters or large molecules, and appreciation of the fact that atomic clusters have near solid density, despite comprising a medium with the average density of a gas. [see Coincident Action] This motivated a plasma description of the ionised cluster, despite its nanometer scale. A key aspect was the inclusion of shielding and enhancement of the laser field inside the cluster, which gives rise to a resonance in the collisional heating rate and provides an explanation for the very energetic nature of the interaction. This resonance leads to certain signature features in the ion emission from cluster explosions, such as an optimum cluster size and optimum laser pulse duration that maximize the ion kinetic energies (explosive (dispersion) forces). [see Principle of Regeneration] These signature features have been observed in experiments [32,33]. The existence of this resonance has also been verified experimentally in a study in which two pulses of different colors have been used to excite the resonance twice [34]. [TD69.pdf, page 312]

A plasma treatment is only valid if the vast majority of the ionized electrons in the cluster are confined to the cluster during the laser pulse, to ensure quasi-neutrality. While for molecules and small clusters (less than a few hundred atoms) the electrons can escape, for large (thousands of atoms), high Z clusters, the majority of the ionized electrons are confined by the space charge of the ions, as is typically the case for a solid. For low Z clusters (e.g. H, D), however, it has been shown that most of the electrons can escape leading to highly charged clusters that rapidly Coulomb explode [35]. The second technical requirement for a plasma treatment is that the cluster size has to be much larger than the Debye length. For typical parameters (solid density Xe plasma, ionized to 5+ with an electron temperature of 1 keV) the Debye length is ≈0.5 nm, much less than the 5–10 nm size of the clusters of interest. Finally, if the laser pulse is to interact with this nanoplasma then it must ‘‘hold together’’ on the time scale of the laser pulse (at least to the peak of the pulse). Assuming an expansion of the cluster at the plasma sound speed, the time taken for it to expand to a background gas density of ≈1017 cm-3 is typically around 1 ps (for the same parameters as before). Clearly, intense laser pulses of femtosecond duration are required. [TD69.pdf, page 312]

The model assumes a spherical plasma with all its constituent particles simultaneously experiencing the same (time-dependent) laser electric field strength. This is valid since the clusters considered (<106 atoms, <10 nm in diameter) are much smaller than both the laser wavelength (≈800 nm) and the plasma skin depth. It is also assumed that the temperature distribution across the plasma is isotropic, since the plasma is small enough and collisional enough to prohibit temperature gradients.

Further, the ion density distribution is assumed to be uniform across the cluster and the expansion is assumed to be self-similar, so the density remains uniform across the cluster throughout its expansion. The final important assumption is that the electron energy distribution is Maxwellian at all times, which will be true in the limit of extremely rapid thermalisation. [TD69.pdf, page 312-313]

See Also

3.14 - Vortex Theory of Atomic Motions
13.04 - Atomic Subdivision
Atomic Cluster Ionization
Atomic Cluster X-Ray Emission
Atomic Clusters
Atomic Force
atomic mass
atomic number
atomic theory
atomic triplet
atomic weight
Debye length
Debye length in a plasma
Debye length in an electrolyte
Etheric Orbital Rotations
Figure 13.06 - Atomic Subdivision
Formation of Atomic Clusters
Inert Gas
Interaction of Intense Laser Pulses with Atomic Clusters - Measurements of Ion Emission Simulations and Applications TD69.pdf
Laser Cluster Interactions
Law of Atomic Dissociation
Law of Atomic Pitch
Law of Oscillating Atomic Substances
Law of Pitch of Atomic Oscillation
Law of Variation of Atomic Oscillation by Electricity
Law of Variation of Atomic Oscillation by Sono-thermism
Law of Variation of Atomic Oscillation by Temperature
Law of Variation of Atomic Pitch by Electricity and Magnetism
Law of Variation of Atomic Pitch by Rad-energy
Law of Variation of Atomic Pitch by Temperature
Law of Variation of Pitch of Atomic Oscillation by Pressure
Models of Laser Cluster Interactions
Plasma holes
Quasi-neutrality and Debye length
Violation of quasi-neutrality

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