Spallative Ablation of Metals and Dielectrics

Abstract

The results of theoretical and experimental studies of ablation of LiF crystal by X-ray beam having photons with 89.3 eV and very short duration of pulse τ = 7 ps are presented. It is found that the crater is formed for fluences above the threshold F abl ≈ 10mJ/cm 2 . Such a small threshold is one order of magnitude less than the one obtained for X-ray ablation by longer (nanoseconds) pulses. The theory explains this dramatic difference as a transition from more energy-consuming evaporative ablation to spallative ablation, when the pulse duration decreases from ns to ps time ranges. Previously, the spallative mechanism of ablation was exclusively attributed to removal of target materials of metal and semiconductor by the short laser pulses with optical photons ∼ 1eV. We demonstrate that tensile stress created in dielectrics by short X-ray pulse can produce spallative ablation of target even for drastically small X-ray fluences.

Two‐Temperature Warm Dense Matter Produced by Ultrashort Extreme Vacuum Ultraviolet‐Free Electron Laser (EUV‐FEL) Pulse

“…Meanwhile, increasing laser fluence up to 80 mJ/cm 2 does not efficiently increase the ablation depth, which reaches the value of only about 30-35 nm. This result is different enough in comparison with irradiation of LiF crystals by 7 ps Ag XRL laser, where we observed the depths of ablation about 40-50 nm near the ablation threshold [1,2]. Fig.…”

“…Diffusion is weak as a result of large mass of holes in the narrow valence band. Therefore we neglect diffusion in equation (1) and neglect electron heat conduction in equation (2). Phonon heat conduction is small.…”

“…It makes it possible to use better known wide-range EOS of Al from [11,12] for computer simulation of LiF, under the action of XRL. Impact ionization frequency ν imp in equation (1) was the same as in the paper [2] from the work [13]. Threebody recombination coefficient κ rec can then be defined from detailed equilibrium [13].…”

“…Phonon heat conduction is small. Laser flux I[W/cm 2 ] defines energy production per unit of volume ∂I/∂x = −I/D att in equation (2). Volume energy absorption and energy of photon hν, absorbed at one-photon transition of electron from valence to conduction band, give volume rate of free electron production in ionization equation (1).…”

“…Two-temperature warm dense matter, created by the action of the collision Ag-plasma X-ray laser (XRL) onto the lithium fluoride (LiF) crystal, has been considered in papers [1,2]. Experimental investigations have been done at Kansai Photon Science Institute (KPSI) of Japan Atomic Energy Agency (JAEA).…”