Published online by Cambridge University Press: 28 February 2011
Irradiation of metals in gaseous and liquid nitrogen by KrF excimer laser pulses shows the way in which vapor or plasma pressure fields determine the nature of the rapidly solidified surface topography. Wave patterns formed on aluminum by irradiation in N2 gas arise from recoil pressure differences produced by highly localised differential surface evaporation. The solidification time, calculated from the observed ripple wavelength, is in good agreement with that deduced from a heat flow calculation. For irradiation in liquid N2, the homogeneous pressure within the center of the dense plasma zone results in a smooth surface. Beyond this region the progressively diminishing pressure induces surface ripples, fragmentation and droplet formation. Intriguingly, immersion of Ni, Ti, Zn, and Al in liquid N2 (to a depth of 2 to 4 mm) considerably increases the area and depth of irradiation-modified regions.