Heating and ionization of metal clusters in the field of an intense femtosecond laser pulse
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Heating and Ionization of Metal Clusters in the Field of an Intense Femtosecond Laser Pulse O. F. Kostenko and N. E. Andreev Institute for High Energy Densities, Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 13/19, Moscow, 125412 Russia Received September 12, 2006; in final form, November 20, 2006
Abstract—Inverse bremsstrahlung heating and thermal electron-impact ionization of a metal cluster are analyzed with account for the spatial structure of the electromagnetic field. It is shown that, for a femtosecond IR radiation pulse with an intensity of ~1018 W/cm2 and for an iron cluster with an optimum radius of ~25 nm, the electron temperature is higher than 1 keV. In this case, the L shell of the ions is highly stripped. The X-ray bremsstrahlung yield from clusters with a radius greater than the skin depth is estimated. PACS numbers: 52.50.Jm, 36.40.Gk DOI: 10.1134/S1063780X07060074
1. INTRODUCTION One of the lines of research on the irradiation of clusters by intense femtosecond laser pulses is the development and creation of efficient ultrashort X-ray sources [1]. The yield of X radiation and its spectral properties have been studied over a broad range of parameters of femtosecond laser pulses and noble gas clusters [2–4]. In experiments on irradiating 50-nmdiameter iron clusters by 80-fs 1.24-µm-wavelength terawatt laser pulses, bremsstrahlung X radiation at wavelengths of 6–13 Å was recorded [5]. X radiation can serve to study the mechanisms whereby laser pulses are absorbed by clusters. The results of experiments with noble gas clusters are analyzed by using cluster nanoplasma models [6, 7]. The optical properties of metal clusters with a radius greater than 10 nm (which can be larger than or on the order of the skin depth) are described by Mie’s electrodynamic theory with the dielectric permittivity for a bulk substance [8]. For electron temperatures Te higher than the Fermi energy, the dielectric permittivity of metals is described by the Drude formula in which the effective electron collision frequency ν depends weakly on Te within a range of several tens of electronvolts [9, 10]. In such a nonideal plasma, the electron mean free path is on the order of the interatomic distance and the collision frequency ν reaches its maximum value. For higher electron temperatures, the frequency is – 3/2 described by the dependence ν ~ T e , which is characteristic of an ideal plasma [9, 10]. In the present paper, we estimate the yield of X-ray bremsstrahlung with a photon energy higher than 1 keV during the irradiation of metal clusters with a radius greater than the skin depth by femtosecond laser pulses with an intensity of ~1018 W/cm2. For an electron tem-
perature on the order of 1 keV, the Debye coupling parameter of a solid-state density plasma is much less than unity, ΓD 1. Inverse bremsstrahlung heating of a uniform solid-state density cluster, as well as electronimpact ionization of the cluster and its ionization by the external field, is analyzed with allowance f
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