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An experimental and theoretical study has been carried out of the ablation of a solid insulator with a wide band gap (LiF) under the action of ultrashort laser pulses of the UV range, obtained in a free-electron laser (pulse width τ_L=0.3ps, photon energy ħω_L=20.2eV), and the soft X-ray region, obtained in a silver-plasma laser (τ_L=7ps, ħω_L=89.3eV). A comparison is made of the results obtained on the two laser systems. It is shown that the ablation threshold is about the same for both lasers. A theory is presented that explains the weak growth of the ablation mass with increasing surface energy density of the laser radiation (the fluence) in the case of X-ray lasers as a result of the transition from spallation close to the ablation threshold to evaporative ablation at high fluence values.

The paper is devoted to experimental and theoretical studies of ablation of condensed matter by optical (OL), extreme ultraviolet (EUV) and X-ray lasers (XRL). Results obtained at two different XRL are compared. The first XRL is collision Ag-plasma laser with pulse duration L= 7 ps and energy of quanta hv=89.3 eV, while the second one is EUV free electron laser (EUV-FEL) and has parameters L= 0.3 ps and energy of quanta 20.2 eV. It is shown that ablation thresholds for these XRL at LiF dielectric are approximately the same. A theory is presented which explains slow growth of ablated mass with fluence in case of XRL as a result of transition from spallative ablation near threshold to evaporative ablation at high fluencies. It is found that the metal irradiated by short pulse of OL remains in elastic state even in high shear stresses. Material strength of aluminum at very high deformation rates V/V ~ 109s-1is defined.

A multiphoton ionization process of He in intense extreme ultraviolet free-electron laser fields is investigated in a photon energy range of hv = 20.1-20.8eV using shot-by-shot photoelectron spectroscopy. A photoelectron peak resulting from sequential double ionization of He due to five-photon absorption is observed around hv = 20.4. Shot-by-shot analysis of photoelectron spectra shows that two-photon resonance to the He+(2s²S) state from the He+ ground state is responsible for the substantial enhancement of five-photon double ionization. Effective cross sections for two-photon absorption between the 1s and 2s states of He+ and for one-photon absorption from the He+ 2s state are obtained. [ABSTRACT FROM AUTHOR]

Electron emission spectra and total electron yields were measured for relatively large (about 5000 atoms) neon clusters that were irradiated by extreme ultraviolet pulses with 20.3 eV, corresponding to the 2p → 3d transition. The present electron emission spectra are remarkably different from those for Ar clusters undergoing sequential ionization [ Bostedt et al. Phys. Rev. Lett. 100 133401 (2008)] and can be interpreted as a superposition of a low-energy exponential tail, dominant at a low free-electron-laser intensity I, and an intermediate-energy Gaussian-like peak which grows with I. Correspondingly, the total electron yield exhibits interesting I dependence. Especially, a plateau appears in the intermediate I range. These results suggest a crossover in the ionization mechanism from thermal electron to two-photon photoelectron emission. An exciton-Mott transition is proposed for the formation of nanoplasma. [ABSTRACT FROM AUTHOR]

A mechanism for three-photon double ionization of atoms by extreme-ultraviolet free-electron laser pulses is revealed, where in a sequential process the second ionization step, proceeding via resonant two-photon ionization of ions, is strongly enhanced by the excitation of ionic autoionizing states. In contrast to the conventional model, the mechanism explains the observed relative intensities of photoelectron peaks and their angular dependence in three-photon double ionization of argon. [ABSTRACT FROM AUTHOR]