Photophysics of Polyatomic Species Ejection Following Uv Laser Ablation of Single Crystal Bi 2 Sr 2 Ca 1 Cu 2 O 8
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PHOTOPHYSICS OF POLYATOMIC SPECIES EJECTION FOLLOWING UV LASER ABLATION OF SINGLE CRYSTAL Bi2Sr 2CalCu 2 O8 t LAWRENCE WIEDEMAN, HYUN-SOOK KIM', AND HENRY HELVAJIAN*= Mechanics and Materials Technology Center, The Aerospace Corporation, P.O. Box 92957, Los Angeles, California 90009. ABSTRACT A series of experiments have been conducted which probe the origin of the polyatomic ions (e.g., Sr 20 +, BiCu +) measured following the low fluence, pulsed UV laser irradiation of crystalline Bi 2Sr 2 CaCu 2O 8 (BiSrCaCuO). The photoejection of polyatomic species presents a problem for the pulsed laser deposition (PLD) technique. The deposition of polyatomic products could limit epitaxial growth during thin film material processing. Our results indicate that the polyatomic species are not the consequence of a gas phase recombination, but are in fact directly produced. All experiments are done at laser fluences below that required for above surface plasma formation. As such, our results do not address the formation of large clusters within the dense plume in high fluence laser ablation. INTRODUCTION The PLD technique has been successfully used for growing thin film multilayer structures. Further analysis reveals that, with refinement, the PLD technique could potentially compete with MBE as a mean for epitaxial layering. However, recent experimental results suggest that polyatomic species could be directly removed from the target surface [1]. This view differs from earlier studies, done at higher laser fluences, where it is the above surface chemistry that initiates polymerization/ condensation [2]. In this experiment, we examine whether the observed polyatomic ions are also due to above surface chemistry. We have chosen the compound Bi 2Sr 2CaCu2O 8 for study and a 288 nm pulsed UV laser is used to induce the photoexcitation. Our previous experimental results show that for UV laser wavelengths and low laser fluences, the Bi 2Sr 2CalCU20 8 compound decomposes by electronic excitation [1]. The measured photoejected mass spectrum also reveals the unexpected removal of polyatomics (e.g., SrO+, Sr 20 +, BiCu+). In this experiment, we measure the polyatomic ion population and KE distributions and address whether these products are the result of an above surface reaction. This is accomplished by a) carefully increasing the sputtering laser fluence, or b) abruptly increasing the above surface reagent density with the use of a second pulsed laser. The second laser precedes the first by 100 nsec. The former approach merely increases the product density at a potential cost of product photolysis (i.e., Sr 2 0 + + hv--.SrO + + Sr). The latter approach is feasible due to the measured large measured difference in the photoejected kinetic energy between ions and neutrals. Photoejected ions can overtake neutral products in < 100 ns. We use resonance enhanced multiphoton ionization (REMPI) spectroscopy to measure the neutral product distribution and time-of-flight mass spectrometry (TOF/MS) to monitor all the ions. EXPERIMENTAL Figure 1 shows a
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