Discussion

The size and shape of impact craters mainly depend on the impact velocity, and the distribution of Ag residues is related to the characteristics of target materials such as the melting point and the evaporation energy. The melting point of aluminum is the lowest of all target materials used in these experiments; A1 ( 660« C)< Au ( 1064* C)< Mo ( 2620* C ) Silver has a higher melting point than aluminum, which is lower than gold and molybdenum. This leads to the distribution of Ag residues; the observation that Ag residues remain inside the craters on A1 target is due to the nature that the melting point of aluminum is lower than that of silver, Ag residues distribute inside and around the craters on the other targets because silver has lower melting point than gold and molybdenum. The Ag projectiles buried into the A1 target deeply, while they scattered around the craters on Au and Mo targets.

The crater depth on Mo target is relatively the shallowest of all targets. The shallow crater may be also related to the highest melting point of molybdenum and its largest evaporation energy. This means that much energy is required to dig the Mo target, and that A1 target is dug deeply because a little energy is required to evaporate much volume of aluminum. This is supported by the observations of craters with the SEM. Evaporation energy has a great influence on excavations of the metal targets.

The ionization of impacting projectiles depends on the impact velocity, the melting point and the density difference of the projectile and target materials. The SEM images show that Ag residues remained inside and around the craters on all targets. Such a large quantity of Ag residue is mainly due to the low velocity of Ag projectile. In other words, Ag projectiles could not be vaporized sufficiently at these low impact velocities because the impact energy is divided into the energies to vaporize the projectile and to dig the target. The division may be related to the density difference, which is supported by the TOF spectra; Mo, Au and Ag ions were often detected for the impacts of Ag projectile on Mo and Au targets, while Al and Ag ions were not obtained for Al target.

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