Photoionization and photofragmentation of BxNy clusters produced by laser vaporization of boron nitride

Abstract

Vaporization of hexagonal boron nitride with a 532 nm laser, followed by supersonic expansion cooling, produces a variety of BxNy clusters which are detected by photoionization of the neutral clusters with a 194 nm laser, followed by time-of-flight (TOF) mass spectrometry. At low 194 nm photoionization fluence (<50 mJ/cm2), the mass spectrum shows resolved peaks corresponding to the clusters By+1N+y for y=1–8, as well as an unresolved region corresponding to clusters ranging from atomic mass unit 100–1000. At higher photoionization fluence, resolved peaks, corresponding to B+x clusters for x=2–100, appear and grow with increasing fluence. Concommitantly, the envelope of the unresolved region changes shape and grows with a diminished dependence on fluence, indicating that the unresolved clusters are being photofragmented as well as photoionized by the 194 nm radiation. The products of such photofragmentation are the aforementioned B+x clusters. This process is modeled using an Arrhenius relation to describe the probability of fragmentation.