M. Krishnamurthy

Asymmetric emission of high-energy electrons in the two-dimensional hydrodynamic expansion of large xenon clusters irradiated by intense laser fields

Energy spectra and angular distributions have been measured of electrons that are emitted upon disassembly of Xe{sub n} clusters (n=20 000-150 000) following irradiation by intense (10{sup 15}-10{sup 16} W cm{sup -2}) laser pulses whose durations are varied over the 100-2200 fs range. The cluster explosion dynamics occur in the hydrodynamic regime. For the smaller clusters in the range that we have studied, a single-electron temperature adequately describes the measured electron energy distribution; in the case of larger clusters, a two-temperature fit becomes necessary. The total electron emission is found to be unexpectedly asymmetric and exhibits a resonance when the laser-pulse duration is {approx}1 ps. These results are rationalized by extending the hydrodynamic model to also take into account the force that the light field exerts on the polarization charge that is induced on the surface of the cluster. We show that the magnitude of this electrostrictive force is comparable to those of the Coulombic and hydrodynamic forces, and it exhibits resonance behavior. Contrary to the findings of the only other earlier report, we find that the low-energy component in the electron energy distribution is connected to the resonance in energy absorption by the cluster. The high-energy component seems tomorexa0» be produced by a mechanism that is not so strongly influenced by the resonance.«xa0less

Anisotropic 'charge-flipping' acceleration of highly charged ions from clusters in strong optical fields

Measurement of energies of ions that result from disassembly of atomic and molecular clusters [Ar{sub n}, n=2000-40 000; (N{sub 2}){sub n}, n=50-3000] in strong optical fields provides evidence for charge-flipping acceleration that gives rise to ions with energies in excess of the Coulombic limit. Measurements of ion spectra as a function of cluster size, and as a function of laser polarization, demonstrate different facets of charge-flipping acceleration.

Energy pooling in multiple ionization and Coulomb explosion of clusters by nanosecond-long, megawatt laser pulses

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.

Enhancement of x-ray yields from heteronuclear cluster plasmas irradiated by intense laser light

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 H2 Ot he time-integrated yield of Ar K x-ray emission is enhanced by approximately 12fold in comparison to that obtained from pure Ar clusters under otherwise identical experimental conditions. A significant alteration in the timedependent electron density is achieved by the presence of an H2O dopant, and this could be the possible reason for the enhancement that is observed. (Some figures in this article are in colour only in the electronic version)

Hotter electron generation in doped clusters

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.

Explosions of water clusters in intense laser fields

Energetic, highly charged oxygen ions O{sup q+} (q{<=}6), are copiously produced upon laser field-induced disassembly of highly charged water clusters, (H{sub 2}O){sub n} and (D{sub 2}O){sub n}, n{approx}60, that are formed by seeding high-pressure helium or argon with water vapor. Ar{sub n} clusters (n{approx}40 000) formed under similar experimental conditions are found to undergo disassembly in the Coulomb explosion regime, with the energies of Ar{sup q+} ions showing a q{sup 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{sup 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.

Engineering clusters for table-top acceleration of ions

Upon irradiation by ultrashort, intense laser light, inert gas clusters show an uncanny ability to absorb nearly all the incident optical energy, and to subsequently disburse this energy by producing very energetic ions and electrons. Practical realization of a table-top accelerator requires such a laser-cluster “ion source” to demonstrate sufficiency in terms of brightness, ion yield, and charge state. We show that by “engineering” the constituents of the cluster, using low ionization energy dopants, it is possible to significantly enhance the high-energy ion yield and ion charge states.

Dopant-Induced Ignition of Helium Nanodroplets in Intense Few-Cycle Laser Pulses

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.

A Thomson parabola ion imaging spectrometer designed to probe relativistic intensity ionization dynamics of nanoclusters

Conventional techniques of probing ionization dynamics at relativistic intensities for extended target systems such as clusters are difficult both due to problems of achieving good charge resolution and signal integration over the focal volume. Simultaneous measurement of arrival time, necessary for these systems, has normally involved complicated methods. We designed and developed a Thomson parabola imaging spectrometer that overcomes these problems. Intensity sampling method evolved in this report is proved to be mandatory for probing ionization dynamics of clusters at relativistic intensities. We use this method to measure charge resolved kinetic energy spectra of argon nanoclusters at intensities of 4 × 10(18) W cm(-2).

Evolution of dopant-induced helium nanoplasmas

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.