|
We have investigated the velocity distribution functions of atoms in laser produced plasma. In our experiments plasma is cheated during the laser sputtering of YBa2Cu3O7-x ceramics target. We used KrF excimer laser for target sputtering. All sputtering conditions that were the same as usual for high Tc films deposition are used. We used time-of-flight method to investigate the velocity distribution function. To measure the instantaneous significance of particles' density we used the laser induced fluorescence technique. We investigated the velocity distribution of Y atoms. These measurements were made at different distances from the target L (5, 10, 20, 30, 45, and 95 mm), at different significance of laser radiation energy fluence (Phi) (0.5 and 3 J/cm2) and at different pressure of buffer gas (10-4, 10-2, and 0.15 Torr). These experiments have showed that velocity distribution function depends upon the energy fluence (Phi) . At low (Phi) equals 0.5 J/cm2 we didn't observe the atoms with velocity more than 10 km/s at any L. The acceleration of atoms was detected. The velocity corresponding to the maximum of distribution function changed from v equals 2 km/s at L equals 5 mm up to 6 km/s at L equals 30 mm. There were not any changes in distribution function at L > 30 mm. At high (Phi) equals 3 J/cm2 one can distinguish two groups of atoms. The first group with low velocity (v equals 2 - 5 km/s) has approximately the same behavior as have atoms at (Phi) equals 0.5 J/cm2. The second group of atoms experiences a strong acceleration during the plasma expansion. Their velocities change from 5 km/s at L equals 5 mm up to 25 km/s at L equals 45 mm. The stabilization of the distribution function shape takes place at L equals 45 mm. This result is in contradiction with the gas dynamic model. We have decided that these atoms arise due to recombination during plasma expansion. The ions experience the acceleration in own plasma electrical fields before recombination. This is the main reason of high energy atoms appearance. The buffer gas influences on the high velocity atoms above all. At the pressure p equals 10-2 Torr the amount of low velocity atoms is approximately the same as at p equals 10-4 Torr, but the amount of high velocity atoms at p equals 10-2 Torr is 100 times less than at p equals 10-4 Torr. This result also can be easily explained if we suppose that high velocity atoms arise due to recombination process.
|
