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M. Rafique, S. Bashir, W. Husinsky, A. Hobro, B. Lendl:
"Surface analysis correlated with the Raman measurements of a femtosecond laser irradiated Ca F₂";
Applied Surface Science, 258 (2012), 3178 - 3183.

Ultra fast electronic and thermal processes for energy deposition mechanism during femtosecond laser
ablation of CaF2 have been identified by means of Atomic Force Microscopy (AFM) and Raman scattering
technique. For this purpose, a single crystal CaF2 (1 1 1) was exposed with 800 nm, 25 fs Ti: Sapphire laser
pulses at different laser fluences both in air and under UHV condition. Various nano and microstructures
on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Around
the ablation threshold fluence, most significant features identified at the CaF2 surface are nanohillock like
structures. These nanohillocks are typical features related to the fast electronic processes and are explainable
on the basis of Coulomb explosion. At moderate irradiation fluence, bump formation is considered
to be due to ultrafast melting. Further increase in the laser fluence resulted into strong enhancement of
the thermal process with the appearance of larger humps and craters. These fast electronic and thermal
processes are well correlated with the structural and crystallographic alterations inferred from Raman
spectroscopy analysis. The nanohillocks appearing at a lower fluence are due to calcium colloid formation
(aggregates of metal clusters). At higher fluences and dozes, the compressive as well as tensile stresses
along with the presence of calcium carbonate are associated to diffusion, transformation and aggregation
of defects which are typical features of thermal processes leading to the growth of larger humps.

Ti: sapphire laser AFM Nanohillocks Bumps Raman spectroscopy