High-sensitivity measurements were made of the Ga0 emission which arose from the (110) surface when laser pulses with various photon energies were directed at it. The energies were above the band-gap energy, EG (range I), below EG but above the energy gap (EVS) between the valence band and the unoccupied surface band (range II), or below EVS (range III). It was found that, below the ablation threshold, laser pulses in energy ranges II and III caused particle emission with a yield that decreased when laser shots were repeated at the same site. The dependence of the yield upon the number of shots had both rapidly (A) and slowly (S) decreasing components. The A-component was enhanced by Ar+ bombardment, and was reduced by subsequent annealing. Measurements of the pulse width showed that the yield was scaled by the fluence, for the A-component. The yields for both components were super-linear functions of the fluence, and exhibited apparent threshold values. The threshold fluence for the A component was smaller than that for the S component. The yield versus fluence relationships could also be fitted by power functions. The exponent was between 2 and 3 for the A-component, and between 4 and 6 for the S-component. Neither component was produced by photon energies in range I. At energies which were above the ablation threshold, the yield from a given site increased with further laser pulses. Almost the same ablation threshold fluence was observed for photon energies in regions I and II, whereas the ablation threshold was scattered for photons in range III. The results were explained in terms of the breaking of the bonds of loosely bound atoms near to defects on the surface. Three types of defect were identified. These were an adatom type, a kink-type on steps, and a vacancy-type. It was suggested that the A and S components were due to adatom and kink-type defects. respectively, while ablation was initiated by vacancies.

K.Hattori, A.Okano, Y.Nakai, N.Itoh: Physical Review B, 1992, 45[15], 8424-36