Abstract Details
Noble Gas Cluster Explosions Driven by Intense Pulses of XUV Light
Author: Brendan F Murphy
Requested Type: Poster Only
Submitted: 2009-04-21 15:18:47
Co-authors: K. Hoffmann, B. Erk, N. Kandadai, A. Helal, J. Keto, T. Ditmire
Contact Info:
Texas Center for High Intensity Laser Science
1 University Station
Austin, TX 78712
USA
Abstract Text:
We have examined the interaction of argon and xenon clusters of up to 100,000 atoms with intense pulses of XUV light of 38nm wavelength produced by high harmonic generation with a 15TW Ti:Sapphire laser. We measured ion charge states and kinetic energy spectra of electrons and ions produced in the interaction, and observe substantial ion population above the single photon ionization limit (up to Ar[3+] and Xe[5+]), with a small number of Xe[6+]-Xe[8+] ions for the largest xenon clusters. The ion kinetic energy spectra indicate that hydrodynamic expansion dominates in the case of large xenon clusters, while large argon clusters exhibit a combination of hydrodynamic effects and the Coulomb explosion of the outermost ion shells. Electron spectra exhibit strong peaks related to the central harmonic wavelength, with weaker side peaks from residual harmonic sidebands, and a low energy electron background. Very few electrons with energy above the incident photon energy are observed. This is in contrast to intense infrared/cluster interactions, where electrons scattered by the ion core can develop keV kinetic energies and cluster ions have develop far higher ion kinetic energy for similar degrees of ionization.
Infrared pulses from the THOR laser (35fs, 800nm, up to 0.6J, 10Hz) are focused into a pulsed jet of argon using a 2 meter focal length lens, producing odd harmonics of the laser fundamental frequency, up to the 31st order. To separate the harmonic light from the infrared, a beam stop masking the center of the infrared profile is imaged to an iris after the gas jet. A 200nm Al film completely blocks residual infrared light. Individual harmonics are isolated and focused to 8µm FWHM spot size by Sc/Si multilayer mirrors, yielding a focused XUV intensity of 10^11W/cm^2 for the 21st harmonic at 38nm. A pulsed conical jet nozzle and molecular beam skimmer mounted above the XUV focus deliver clusters to the interaction point. A magnetically shielded time of flight spectrometer mounted perpendicular to the cluster source and XUV beam allows ion charge state and kinetic energy measurements. Electrons and ions leaving the interaction region drift through a field-free tube and are collected on a microchannel plate connected to an oscilloscope and a time delay counter.
The kinetic energy spectrum for the largest cluster sizes for xenon is inconsistent with a Coulomb explosion model, but fits well to a model of quasi-neutral, spherical plasma expanding into vacuum with fixed initial electron temperature. Continuum lowering accounts for charge states up to Xe[5+], while a small fraction of higher charge states are produced by electron collisional ionization. Large argon clusters undergo Coulomb explosion of the outermost ion shells with reduced kinetic energy due to plasma screening by the ionized cluster core, accompanied by hydrodynamic expansion of the core.
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