When a shock wave of several tens of GPa breaks out at a free surface, a material is ejected ahead of this surface. The amount and velocity of such ejecta depend on the breakout pressure, state of the released material (solid, liquid, or mixed), whether the shockwave is supported or unsupported, and the initial geometrical perturbation (or roughness) of the free surface. If surface defects consist of small grooves, pits, or scratches, material ejection occurs in the form of jets breaking up into tiny particles (so-called microjetting), with jet tip velocities up to several times higher than the free surface velocity. The laser-based experiments presented in this paper focus on microjetting in shock-melted tin with periodic surface perturbations. Several complementary diagnostics are combined to measure the velocity and mass of ejecta during the early stages of the jetting process. One relevant advancement is the use of ps-laser x-ray radiography to probe the density of the ejecta in distinct jets a few tens of μ m-wide. The effects of the depth and wavelength of the initial perturbation are investigated in both linear and near-linear growth regimes. The results are compared with predictions derived from the Richtmyer-Meshkov Instability theory. © 2020 Author(s).