Analysis of Atomic Beams Produced by Laser-Induced Ablation

Abstract

Pulsed atomic beams with flux densities above 1016 cm-2 s-1 have been produced in vacuum with high reproducibility by laser induced ablation from titanium and iron targets. The power of the TEA CO2 laser was kept at a level where plasma production at the target surface was avoided: no ions were detectable. The beams were analysed by laser fluorescence. The high directionality of the beam, a discrepancy between velocity distribution and flux density as well as very low excitation temperatures of the fine structure levels of the ground state point to a non-thermal ejection process of the atoms and/or to impurity surface effects.

Some Properties of Atomic Beams from Laser Irradiated Zirconium

“…Integration over the time also reveals that the total particle emission does not obey a cos2B-distribution: it drops more rapidly for large values of 0 . This directional emission confirms the previous observations with the COzlaser radiation [9].…”

“…The particle density in the ground state was determined as function of time for various distances from the target surface. Generally, the density dependence obtained was similar in appearance to the previous results [9], and from the density maxima again a beam velocity in the direction of the axis of the target rod could be derived. v = 2.8 x lo3 ms-', which corresponds to a kinetic energy of the zirconium atoms of E = 3.7eV.…”

“…Figure 5 shows that T,, increases with the fluence but remains far below any value one would expect from a thermal model. It had been verified previously [9] that for truly thermal evaporation the atoms indeed exhibit an excitation temperature identical to the temperature of the solid.…”

“…The velocity distribution of the emitted particles seemed to agree with melting and heating to the boiling temperature in a small surface layer. However, the flux density of these particles was too low by six orders of magnitude, and, most surprisingly, the excitation temperature derived from the population densities of the fine structure levels of the ground state was about 600 K only [9]. In the present paper, we report measurements on zirconium as target material and compare results obtained when applying pulsed radiation from a COzand Nz-laser.…”

“…Part of the experimental arrangement has been described previously [9]. The target was a cylindrical zirconium rod placed in a vacuum vessel which was evacuated to a pressure lower than Pa.…”