INVESTIGATION OF THE SURFACE DEFECT STRUCTURES ON SINGLE CRYSTALS OF MOLYBDENUM, NICKEL, AND BISMUTH INDUCED BY PULSED LASER IRRADIATION: CHARACTERIZATION BY LEED AND SINGLY POSITIVE HELIUM ION CHANNELING (THERMAL STRESS, SLIP, DISLOCATIONS)

The technique of LEED-Spot Profile Analysis (LEED-SPA) and He+ ion channeling have been used to investigate the response of molybdenum, nickel and bismuth single crystals to pulsed laser irradiation. Samples of molybdenum and nickel with (100) orientations were irradiated with a Q-switched, frequency doubled Nd:YAG laser under ultra-high vacuum conditions. Good epitaxial regrowth of the melted surfaces was indicated by the appearance of a well defined LEED pattern. Analysis of the spot profiles as a function of incident electron energy and annealing time at 1000 C indicates the initial formation of random island structures on the surface which coalesce toward a flat surface with prolonged heating. Samples which had been disordered in the near surface region by Ar+ ion bombardment exhibited a sharp LEED pattern after pulsed laser melting.; The (0001), (1010), and (2110) surfaces of bismuth were pulsed laser melted using a Q-switched Ruby laser. Channeling, Nomarski Interference Contrast Microscopy, and selective chemical etching indicate that the (0001) surface regrows epitaxially with no increase in disorder after pulsed laser melting at low laser fluences. Both the (1010) and (2110) surfaces show a marked increase in disorder after irradiation. The responses of these metals and the data are explained using a thermomechanical stress model in which the materials plastically deform in response to the high thermal gradients induced during pulsed laser irradiation.