Photoionization and Photofragmentation of B x N y Clusters Produced by Photoablation of Boron Nitride Followed by Supers
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PHOTOIONIZATION AND PHOTOFRAGMENTATION OF BX.^ CLUSTERS PRODUCED BY PHOTOABLATION OF BORON NITRIDE FOLLOWED BY SUPERSONIC EXPANSION COOLING
PAUL A. ROLAND, SAM J. LA PLACA, AND JAMES J. WYNNE IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598-0218 ABSTRACT
Photoablation of isotopically pure boron metal and hexagonal boron nitride with a 532 nm laser, followed by supersonic expansion cooling, produces a variety of B, and BN, clusters, which are detected by photoionization of the neutral clusters with a 194 nm laser, followed by time-of-flight (TOF) mass spectrometry. Under certain experimental conditions, the mass spectrum derived from boron nitride shows resolved peaks corresponding to the clusters B.+, N/. Under different experimental conditions, the mass spectrum shows an unresolved region corresponding to clusters ranging from atomic mass unit 100-1000 as well as resolved peaks, corresponding to B,+ clusters for x= 2 - 100. These resolved peaks grow with increasing photoionization fluence, while the envelope of the unresolved region changes shape and grows with a diminished dependence on fluence. These data are interpreted as evidence that the unresolved clusters are being photofragmented as well as photoionizated by the 194 nm radiation, and the products of such photofragmentation are the aforementioned B,+ clusters. INTRODUCTION
Research in cluster science has been extensively pursued for the past two decades, motivated by the quest to understand the properties of "pieces" of matter that are too large to behave like isolated atoms or molecules and too small to behave like small pieces of crystals. Research activities in cluster science grew significantly in the last decade due to the development of new experimental techniques that could produce clusters from even the most refractory materials, by such means as the laser ablation/supersonic expansion cooling technique of Smalley and co-workers. [1,2] Boron clusters were previously studied by Berkowitz and Chupka [3] who employed mass spectrometry to detect B 2 - 5 clusters produced by laser vaporization of boron metal. Recently, Anderson and co-workers [4] have carried out systematic studies of B, cluster production and collisions with inert and chemically reactive atoms and molecules. Boron-nitrogen cluster ions have been studied by Becker and Dietze, [5] who irradiated a boron nitride target under vacuum conditions, using 1060 nm radiation from a Nd-YAG laser to both vaporize the target and ionize the vapor. The resulting plasma cooled as it expanded into the vacuum, thereby generating cluster ions. The most abundant species they detected, by far, was B+. But many other species were detected, with the series B,+ Nj,+ for y = I - 8 being most prominent. In this paper we describe photoionization and photofragmentation of clusters generated by supersonic expansion cooling of photoablated isotopically pure boron metal and hexagonal boron nitride. As expected, photoablation of pure boron metal Mat. Res. Soc. Symp. Proc. Vol. 285. 01993 Mater
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