Ablation of metals with picosecond laser pulses: Evidence of long-lived nonequilibrium conditions at the surface

作     者：E. G. Gamaly N. R. Madsen M. Duering A. V. Rode V. Z. Kolev B. Luther-Davies 

作者机构：Laser Physics Centre Research School of Physical Sciences and Engineering The Australian National University Canberra ACT 0200 Australia Centre for Ultra-high Bandwidth Devices for Optical Systems Australian National University Canberra ACT 0200 Australia Fraunhofer Institute for Laser Technique Steinbachstrasse 15 D-52074 Aachen Germany 

出 版 物：《Physical Review B》 (Phys. Rev. B Condens. Matter Mater. Phys.)

年 卷 期：2005年第71卷第17期

页      面：174405-174405页

核心收录：

学科分类：0808[工学-电气工程] 0809[工学-电子科学与技术（可授工学、理学学位）] 07[理学] 0805[工学-材料科学与工程（可授工学、理学学位）] 0702[理学-物理学] 

摘      要：We report here experimental results on laser ablation of metals in air and in vacuum in similar irradiation conditions. The experiments revealed that the ablation thresholds in air are less than half those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient nonequilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the time needed for energy transfer from the bulk to the surface layer to build the high-energy tail, exceeds other characteristic timescales such as the electron-ion temperature equilibration time and surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this nonequilibrium surface state allowing thermal evaporation to proceed before the surface cools. We find, therefore, that the threshold in vacuum corresponds to nonequilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.