Ion Production by Laser Impact on a Silver Surface

Even at moderate fluence (0.6 -2.4 J/cm2) laser impact on metals in the UV regime results in a significant number of ions emitted from the surface. Even at this low fluence the particles ejected from a surface interact with each other in a so-called laser ablation plume. The ablated particles are largely neutrals at low fluence, but the fraction of ions increases strongly with fluence. We have irradiated silver in a vacuum chamber (~ 10-7 mbar) with a Nd:YAG laser at a wavelength of 355 nm. The ion flow in different directions has been measured with a hemispherical array of Langmuir probes, by which the time-of-flight spectra in all directions can be recorded [1,2]. Typically, the spectra of silver ions peak from 70 eV up to 145 eV in a direction normal to the target surface with increasing fluence. At the highest fluence the ionized fraction of the ablated particles exceeds 0.5. The fluence range considered is also a typical range for pulsed laser deposition (PLD), by which the material is collected on a suitable substrate for thin film growth. PLD has the advantage compared with other film deposition methods, that even a complicated stoichiometry, e.g. metal oxides or alloys, can be transferred from target to substrate. However, at high fluence the energetic ions produced by the ablation process can induce preferential sputtering of the growing film, such that the desired stoichiometry of the film no longer can be maintained [3]. [1] B. Thestrup, B. Toftmann, J. Schou et al., Appl. Surf. Sci. 197-198 (2002) 175 [2] B. Toftmann, J. Schou, T. N. Hansen and J. G. Lunney, Phys. Rev. Lett. 84 (2000) 3998. [3] J. Schou, Appl. Surf. Sci. 255 (2009) 5191.