PHOTOIONIZATION AND PHOTOFRAGMENTATION OF BXNY 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 B(x)N(y) 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 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.