Periodic microstructure was found on the ablation crater, and that thought to cause the shifting measurement of ablation impulse and direction with oblique laser-pulse irradiation

In this study, a dual-axis impulse stand, having an elastic pivot with a precision of +/- 0.07 mu Ns, was newly developed, and the impulse induced by a laser pulse irradiation with a fluence of 10 J/cm2 was measured for incidence angles of 0 degrees, 30 degrees, 45 degrees, and 60 degrees. In all cases, the impulse vector was initially directed perpendicularly to the surface of the target. However, as the number of the laser pulse irradiations reached 200, only when the incidence angle was 60 degrees, did the impulse direction shift to the laser-beam direction by approximately 10 degrees. This impulse performance was consistent with the ablation crater morphology consisting in a periodic structure with 100 mu m in width and 20 mu m in depth, which was observed, through a laser microscope, only at this incidence angle.