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

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+1Ny+ 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 Bx+ 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 Bx+ clusters. This process is modeled using an Arrhenius relation to describe the probability of fragmentation. © 1993 American Institute of Physics.