No abstract

A direct comparative measurement of the dependence on the wavelength of irradiation of the kilovolt x-ray yields (Xe(M) ∼ 9-15 Å and Xe(L) ∼ 2.5-3.2 Å) multiphotoninduced from Xe clusters by excitation with intense (I > 10 18 W cm −2 ) femtosecond pulses at 248 and 800 nm has been made. The spectroscopic findings demonstrate that both the Xe(M) and Xe(L) emissions are strongly reduced with excitation at the longer wavelength (800 nm). The peak strengths of the Xe(M) and Xe(L) emissions are diminished by factors of ∼ 10 2 and ∼ 10 3 , respectively. Significant spectral differences are also observed. This sharp reduction in the amplitude of the excitation is in conflict with a thermal model for the production of kilovolt x-rays (Xe(M) and Xe(L)) from multiphoton 248 nm excited Xe clusters. These results are consistent with a dynamical mechanism of enhanced coupling which involves ordered manyelectron motions in which a dephasing of the electrons can appreciably influence both the amplitude of excitation and the threshold intensity for inner-shell vacancy production. Within the framework of this picture, these experimental findings indicate an effective dephasing time for Xe clusters of ∼ 1-2 fs, a range that is consistent with the measured k-space scattering dynamics of carriers in GaAs.

A theoretical analysis of laser-driven collisional ejection of inner-shell electrons is presented to explain the previously observed anomalous kilovolt L-shell x-ray emission spectra fkom atomic Xe cluster targets excited by intense sub-picosecond 248nrn ultraviolet radiation [A. McPherson et al., Nature 370, pp. 631-634 (1994)].For incident ponderomotively-driven electrons photoionized by strong above threshold ionizatio~the collisional ejection mechanism is shown to be highly l-state and signifka.ntly n-state (i.e. radially) selective for time periods shorter than the collisional dephasing time of the photoionized electronic wavefbnction. The resulting preference for the collisionai ejection of 2p electrons by an ionized 4p state produces the measured anomalous Xc(L) emission which contains direct evidence for (i) the generation of " X&T+(2@3dO) and X@+(2pS3&) ions exhibiting inner-shell population inversion and (ii) a coherent correlated ekctron state collision responsible for the production of double 2p vacancies.For longer time periods, the selectivity of this coherent impact ionization mechanism is rapidly reduced by the combined effects of intrinsic quantum mechanical spreading and dephasing -in agreement with the experimentally observed and extremely strong -A4 pump-laser wavelength dependence of the efficiency of inner-shell (2p) vacancy production in Xe clu~ers excited in underdense plasmas~.

We explore the possibility of using an intense laser beam to focus a molecular beam onto a surface to create nanowires. We show that with a grazing angle of incidence between the laser and molecular beams, it is possible to use available technology to create wires <50 nm wide and >100 μm long with a 100 W continuous wave laser. Narrower and longer features could be created with higher power lasers. This technique is very general, and may be used to deposit any atom or molecule onto an arbitrary substrate, so long as the particles may be entrained in a molecular beam and have an adequate sticking probability. The effects of spherical and chromatic aberration and laser mode structure on the focusing properties of the molecular lens are examined in detail, and design criteria for building a practical device are discussed.

An explanation is presented for the recently reported striking differences in the kilovolt Xe L-shell (3d → 2p) x-ray emission from Xe cluster targets excited by comparable terawatt ultraviolet (248 nm) and infrared (800 nm) femtosecond laser pulses under nearly identical experimental conditions (Kondo K et al 1997 J. Phys. B.: At. Mol. Opt. Phys. 30 2707. A classical analysis of these results, within the framework of the first Born approximation for electron-atom collisions producing inner-shell ionization, strongly suggests that both the ∼ 3000 times stronger Xe(L) emission under ultraviolet laser excitation and the observed differences in the x-ray spectra are caused primarily by the different ultraviolet and infrared pump laser wavelengths. The kinematics of photoionized electrons in the intense laser fields (10 18 -10 19 W cm −2 ) and the Coulomb-driven expansion of the electron distribution photoionized from the atomic cluster both indicate that the strong pump-laser wavelength scaling in the production of kilovolt x-rays from Xe clusters results from the more localized and controlled electron-cluster interactions afforded by a shorter optical period.