ION DISTRIBUTIONS IN LASERPRODUCED PLASMAS WITH SMALL DIAMETER FOCAL SPOTS

Abstract
Ion emission from a laserproduced plasma with very small focal spots (20 µm full 1/e width) has been investigated, charge, angle and velocity being resolved. The experiments have been performed for different polarizations and wavelengths (1.06 µm and 0.53 µm) of the laser (Nd-YAG mode locking system). In the case of p-polarization an enhancement of the total ion expansion energy by a factor of 3.5 is obtained compared with s-polarization. This is due to resonance absorption. While most of the additionally absorbed energy leads to an increase in the number of emitted ions, the velocity spectrum is reduced and the average charge is only weakly influenced. The result is quite different when the wavelength of the laser is changed instead of the polarization. For the first harmonic (0.53 µm) an increase in the ion expansion energy is likewise observed, which, however, in this case is preferentially due to an increase in the mean kinetic energy, the ablated mass remaining essentially unchanged.