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) cluste rs 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 s hows resolved peaks corresponding to the clusters By+1Ny+ for y = 1-8, as well as an unresolved region corresponding to clusters ranging fro m atomic mass unit 100-1000. At higher photoionization fluence, resolv ed peaks, corresponding to B(x)+ clusters for x = 2-100, appear and gr ow with increasing fluence. Concommitantly, the envelope of the unreso lved region changes shape and grows with a diminished dependence on fl uence, indicating that the unresolved clusters are being photofragment ed as well as photoionized by the 194 nm radiation. The products of su ch photofragmentation are the aforementioned B(x)+ clusters. This proc ess is modeled using an Arrhenius relation to describe the probability of fragmentation.