Photon energy dependence of graphitization threshold for diamond irradiated with an intense XUV FEL pulse

Abstract

We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24-275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions. It is often observed that a femtosecond irradiation of a material induces an atomic disorder therein: amorphization, or defect creation. Graphitization of diamond is a counterexample as it is an order-to-order (solid-to-solid) phase transition. It illustrates the fundamental interplay between the bonding, respectively sp 3 and sp 2 bonds for diamond and graphite, and the structure, respectively cubic and hexagonal. The advent of extreme ultraviolet (XUV) and x-ray free-electron lasers (XFELs), delivering femtosecond intense pulses in the soft to hard x-ray domain allows investigating the structural transition of diamond within this unexplored regime and clarifying whether it leads to an ordered or disordered state. In our study we show the experimental results of the XUV irradiation of diamond followed by a dedicated theoretical analysis.Irradiation by an optical femtosecond laser pulse triggers a specific process known as a nonthermal phase transition, which has been demonstrated for a class of materials.1 However, there are still active debates over the nature of the observed nonthermal transitions, e.g., see Ref. 2. For this type of transition, models predict that the excitation of a few percent of the valence band electrons leads to a drastic modification of the potential energy surface, triggering the displacement of the atoms. This process occurs on a much faster time scale (subpicosecond) than the transfer of the absorbed laser energy to the lattice via electron-phonon coupling. Such a nonthermal phase transition still needs to be observed in the x-ray regime.In the XUV and x-ray domain the excitation of electrons is only due to single photon absorption, and the absorption by free electrons does not occur. As a result, the first stage of the interaction, electron excitation and heating, which are driving nonthermal processes, is quite different compared to the optical regime.1 It is then questionable if a nonthermal phase transition can be triggered by an XFEL pulse. In the present Rapid Communication we identify the phase transition, which diamond undergoes under femtosecond XUV irradiation, as the graphitization. We show that the final state of the...

Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse

“…The VUV-XUV range was studied in our previous work. 43 It was found that in this regime the critical fluence for the material damage was E th = 0.7 eV/atom in terms of the absorbed dose, in good agreement with the experimental findings. 43 For photon energies above the K edge, our simulations indicate that, although the transient kinetics is different as it includes K-shell hole creation and relaxation (for details see Secs.…”

“…As already shown in Ref. 43, the band gap starts to collapse when the fraction of the conduction-band electrons reaches a value of ∼1.5% of the initial number of valence electrons, which corresponds to an absorbed dose of ∼0.7 eV/atom. The induced collapse of the band gap and the increase of the free-electron density trigger a fast relocation of atoms to their new equilibrium positions in the graphite state.…”

“…3. The evolution of the free-electron density proceeds in two steps: 43 first, the fraction of free electrons increases up to ∼1.6% of the initial number of valence electrons due to photoionization and impact ionization processes. The plots show clearly that the higher the photon energy is, the longer the secondary cascading takes.…”

“…The induced collapse of the band gap and the increase of the free-electron density trigger a fast relocation of atoms to their new equilibrium positions in the graphite state. 43 In all calculations, only the graphite with AB-layer stacking was observed, not AA or ABC stacking. 66,67 At the end of the nonthermal phase transition, the material appears to be "swollen."…”

“…The predictions of this model were found to be in good agreement with the experimental measurements of the graphitization threshold for diamond at VUV-XUV photon energies. 43 In the present work, we extend this model to treat higher photon energies, above the K-shell threshold. The damage threshold is calculated in a wide range of photon energies up to tens of keV.…”