Threshold Fluence UV Laser Ablation of Single Crystal Bi 2 Sr 2 Ca 1 Cu 2 O 8 : Product Population and Kinetic Energy Di
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THRESHOLD FLUENCE UV LASER ABLATION OF SINGLE CRYSTAL Bi 2Sr 2CatCu 20 8: PRODUCT POPULATION AND KINETIC ENERGY DISTRIBUTIONS OF EJECTED IONIC SPECIES LAWRENCE WIEDEMAN, HYUN SOOK KIM*, AND HENRY HELVAJIAN** Laser Chemistry & Spectroscopy Department, Aero hysics Laboratory, The Aerospace Corporation, RO. Box 92957, Los Angeles, California ABSTRACT We have conducted an experiment which measures the product population and kinetic energy (KE) distributions from the UV laser induced decomposition of crystalline Bi 2Sr 2 Ca1Cu 20 8. We have measured these distributions at two laser wavelengths 248, 351. At a third wavelength (355 nm) we have measured the photoejected mass spectra from both a single crystal Bi 2Sr 2Ca 1Cu 2O8 sample and a polycrystalline Bi 2 Sr 2Ca 2Cu 3O10 sample. For all the experiments, the laser fluence is maintained near the threshold for ion formation. The laser fluences are well below the level for instigating a laser induced above surface plasma. Our results show that the ejected prod ucts are not the consequence of a laser surface evaporation process. We mcasu re a wavelength dependence in the ejected species population distribution and the ejected kinetic eneray distribution ( < KE > = 5 t 1 eV 2eV FWHM) is indicative of an electronic excitation process. The measured ion mass spectra show atomic, diatomic (e.g. Sr2 +), and oxide (e.g. SrO +. CaO + ) species with lesser quantities of the complex oxides (e.g. Sr20 + ). Distinctly absent from the mass spectra are the oxide compounds BiO+, CuO+, and the atomic species 0+. Furthermore. the mass spectrum shows that at 248 nm laser excitation, the Bi+ species is the abundant photoproduct. However, for both the 351 nm and 355 nm excitations, the Sr+ and SrO+ ions are measured as more abundant. Also, comparing the 355 nm laser excitation of the single and polycrystalline samples, there is very little difference in the photoejected species mass spectrum. INTRODUCTION As physical properties become better understood and fabrication techniques become more refined, the development of multilayer materials with complex superlattice microstructures is expected to find wider application. Novel growth techniques will be required which can prepare a contaminant free interface on the order of a monolayer with precise control in the stoichiometry and thickness. The laser ablation technique is one that has been successfully used to grow thin films of the YBa2Cu 3O5 + 6 superconductor. The achieving of high growth rates has made a requisite of using high fluence laser ablation. One disadvantage in the high fluence laser/surface interaction is the generating of a plasma which contains very high KE species (> 100 eV) including cluster compounds. Furthermore, the KE distribution of the plasma is found to be very broad (> 100 eV). This enhances implantation and increases the distance over which an interface region is formed. It is expected that the growth of microstructures by tile laser ablation technique will be sensitive to various parameters including the recondensat
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