High Intensity Laser Absorption by Gases of Atomic Clusters

Abstract

We have measured the energy absorption efficiency of high intensity, picosecond laser pulses in low density gases composed of large atomic clusters. We find that, though the average density of the resulting plasmas is low, the energy absorption can be very high ͑.95%͒, indicating that substantial laser energy is deposited per particle in the plasma. Ion energy measurements confirm that this efficient energy deposition results in plasmas with very high (multi-keV) ion temperatures.[S0031-9007(97)03016-0] PACS numbers: 52.50. Jm, 33.80.Eh, The production of high temperature plasmas with small scale, short pulse, high intensity lasers has been actively pursued during the last ten years. Of particular interest in these studies is the measurement of the energy absorption efficiency of high density plasmas created by intense irradiation of a solid target, and many groups have published work measuring solid target plasma absorption efficiency over a wide range of incident intensities and laser wavelengths on planar [1-5] and microstructured targets [6]. These studies have shown that the plasma typically absorbs a large fraction of the laser energy, between 10% and 80% of the incident energy, depending upon intensity and laser wavelength. Such experiments have shown that a large amount of energy can be deposited per unit area and that high temperatures ͑.100 eV͒ are achievable [1,7]. However, rapid heat conduction into the cold, solid substrate beneath the plasma will typically clamp the plasma temperature to a value of ,1000 eV [7-9]. Furthermore, most of the deposited energy is contained within the plasma electrons, which cool too rapidly by conduction and hydrodynamic expansion to transfer much of this laser energy to the cold ions.A gas of large atomic clusters ͑.1000 atoms͞cluster͒ presents a radically different environment for laser-plasma interaction dynamics [10,11]. In general, low density gases are expected to exhibit very low absorption efficiency ͑,1%͒ and the plasmas produced by intense irradiation will generally be quite cold (10-100 eV). The presence of clusters in a gas changes this situation dramatically [11]. Though the average density of a gas containing clusters is low, the local density within the cluster is near solid, and, consequently, will be subject to the rapid heating experienced by a solid target due to collisional inverse bremsstrahlung. Very bright x rays have been observed from gas target plasmas produced by intense femtosecond illumination of clusters [10], indicating that electron temperatures in these plasmas were quite high, far in excess of those expected from a gas composed only of single atoms [11]. These observations suggested that the clusters were very efficient in absorbing laser energy. In this Letter, we report the first laser energy absorption measurements of intense laser pulses in gaseous media containing atomic clusters. We find that cluster gases are at least as efficient as solid targets in absorbing short pulse laser energy, and that, furthermore, much of this energy is...

Characterization of doping levels in heteronuclear, gas-phase, van der Waals clusters and their energy absorption from an intense optical field

“…The variation of the absorption with pressure that we observe is in consonance with the only earlier report [4] available in the literature.…”

“…One of the motivations for such studies has been the unusually efficient manner in which large, gas-phase clusters absorb energy from the optical field that they are irradiated by. Energy absorption in excess of 90% has been reported for rare-gas clusters exposed to laser intensity in the range of 10 16 W cm −2 [4]. One consequence of this energy absorption is the ejection of a large number of electrons from the cluster.…”

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