Cluster formation in the vapor produced by laser pulsing of the Y1Ba2Cu3O7 superconducting solid

Formation of neutral clusters in the vapor plume generated by laser-pulse heating of YBa2Cu3O7-x in vacuum was investigated. A Nd-glass laser of 1060 nm wavelength and 1.75 ms pulse width was used to generate peak power densities in the range of 104--105 W/cm2 over an area of 3--28 mm2. Time-of-flight mass spectrometry was used to identify neutral species contained in the plume and quadrupole mass spectrometry provided the time variation of the signal of several low-mass species. In both cases a transient optical pyrometer was used to monitor the surface temperature of the target during the laser pulse. At power densities generating peak surface temperatures less than 2300 K, the vapor consisted primarily of monomer species O, O2, Cu, Ba, BaO, and the dimer (BaO)2. At higher-power densities the major part of the vapor was made up of clusters of mass up to and beyond 3500 amu. These observations were complemented by microscopic examination of deposited plumes. Thin films of the target material were deposited on silicon and graphite substrates and examined by atomic force microscopy and scanning tunneling microscopy. Clusters 3 nm in diameter were observed in submonolayer films. Films of >∼100 nm were made up of stacks of such clusters. Very few liquid droplets from direct ejection from the target surface were observed. In-flight illumination of the plume by light from the laser did not significantly affect the mass distribution in the plumes; however, cluster formation at high-laser-power density (∼1012 W/cm2) was much lower than at 105 W/cm2