Mechanism of single-shot damage of Ru thin films irradiated by femtosecond extreme UV free-electron laser.

Ruthenium is a perspective material to be used for XUV mirrors at free-electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser-matter interaction. In this work, we present single-shot damage studies of thin Ru films irradiated by femtosecond XUV free-electron laser pulses at FLASH. Ex-situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte-Carlo code XCASCADE(3D) and the two-temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologies.

Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold.

The durability of grazing- and normal-incidence optical coatings has been experimentally assessed under free-electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10% of the single-shot damage threshold. The experiment was performed at FLASH, the Free-electron LASer in Hamburg, using 13.5 nm extreme UV (EUV) radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20° and 10° grazing incidence, respectively. Mo/Si periodical multilayer structures were tested in the Bragg reflection condition at 16° off-normal angle of incidence. The exposed areas were analysed post-mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X-ray photoelectron spectroscopy. The analysis revealed that Ru and Mo/Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV-induced oxidation of the surface.

Ultrafast reorientation of dangling OH groups at the air-water interface using femtosecond vibrational spectroscopy.

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.

Ultrafast inter- and intramolecular vibrational energy transfer between molecules at interfaces studied by time- and polarization-resolved SFG spectroscopy.

We present experimental results on femtosecond time-resolved surface vibrational spectroscopy aimed at elucidating the sub-picosecond reorientational dynamics of surface molecules. The approach, which relies on polarization- and time-resolved surface sum frequency generation (SFG), provides a general means to monitor interfacial reorientational dynamics through vibrations inherent in surface molecules in their electronic ground state. The technique requires an anisotropic vibrational excitation of surface molecules using orthogonally polarized infrared excitation light. The decay of the resulting anisotropy is followed in real-time. We employ the technique to reveal the reorientational dynamics of vibrational transition dipoles of long-chain primary alcohols on the water surface, and of water molecules at the water-air interface. The results demonstrate that, in addition to reorientational motion of specific molecules or molecular groups at the interface, inter- and intramolecular energy transfer processes can serve to scramble the initial anisotropy very efficiently. In the two exemplary cases demonstrated here, energy transfer occurs much faster than reorientational motion of interfacial molecules. This has important implications for the interpretation of static SFG spectra. Finally, we suggest experimental schemes and strategies to decouple effects resulting from energy transfer from those associated with surface molecular motion.

Measuring molecular reorientation at liquid surfaces with time-resolved sum-frequency spectroscopy: a theoretical framework.

A theoretical framework is presented for the design and analysis of ultrafast time- and polarization-resolved surface vibrational spectroscopy, aimed at elucidating surface molecular reorientational motion in real time. Vibrational excitation with linearly polarized light lifts the azimuthal symmetry of the surface transition-dipole distribution, causing marked, time-dependent changes in the surface sum-frequency generation (SFG) intensity. The subsequent recovery of the SFG signal generally reflects both vibrational relaxation and reorientational motion of surface molecules. We present experimental schemes that allow direct quantification of the time scale of surface molecular reorientational diffusive motion.

Interface-specific ultrafast two-dimensional vibrational spectroscopy.

Surfaces and interfaces are omnipresent in nature. They are not just the place where two bulk media meet. Surfaces and interfaces play key roles in a diversity of fields ranging from heterogeneous catalysis and membrane biology to nanotechnology. They are the site of important dynamical processes, such as transport phenomena, energy transfer, molecular interactions, as well as chemical reactions. Tools to study molecular structure and dynamics that can be applied to the delicate molecular layers at surfaces and interfaces are thus highly desirable. The advent of multidimensional optical spectroscopies, which are the focus of a special issue of Accounts of Chemical Research, and in particular of two-dimensional infrared (2D-IR) spectroscopy has been a breakthrough in the investigation of ultrafast molecular dynamics in bulk media. This Account reviews our recent work extending 2D-IR spectroscopy to make it surface-specific, allowing us to reveal the structure and dynamics of specifically interfacial molecules. 2D-IR spectroscopy provides direct information on the coupling of specific vibrational modes. Coupling between different modes can be resolved and quantified by exciting a particular mode at a specific frequency and probing the effect of the excitation on a different mode at a different frequency. The response is thus measured as a function of two frequencies: the excitation and the probe frequency, which provides a two-dimensional vibrational spectrum. When two vibrational modes are coupled, this will give rise to the intensity in the off-diagonal part of the 2D-IR spectrum. The intensity of the cross-peak is determined by the strength of the coupling between the two modes, which, in turn, is determined by molecular conformation. One can therefore relate the 2D-IR spectrum to the molecular structure. By delaying pump and probe pulses relative to one another, one can obtain additional information about conformational fluctuations. The surface-specific 2D-IR approach presented here combines the virtues of 2D-IR with the surface specificity and sub-monolayer sensitivity of vibrational sum frequency generation (SFG). We demonstrate its application on a self-assembled monolayer of a primary alcohol on water. It allows for the elucidation of different contributions to the coupling between the different interfacial methyl and methylene stretching modes. Although the surface 2D-IR technique presented here is conceptually closely related to its bulk counterpart, it is shown to have distinct characteristics, owing to the preferential alignment of molecules at the interface and the strict selection rules of the SFG probing scheme. We present an analytic theoretical framework that incorporates these effects and present simulations on instructive examples as well as on the alcohol monolayer. Overall, these results illustrate the potential of extending 2D-IR spectroscopy to the investigation of surface molecular dynamics.

Noncollinear optical parametric amplification in potassium titanyl phosphate pumped at 800 nm.

The generation of broadband tunable optical pulses is demonstrated in the range of 1050-1300 nm with noncollinear optical parametric amplification of white-light seed pulses in potassium titanyl phosphate (KTiOPO(4)). Pulse bandwidths of 50 nm (12 THz) are demonstrated with pulse energies up to 20 microJ when pumped with 500 microJ, 150 fs pulses centered at 802 nm. The required signal-pump angles range from 1.9 to 5.0 degrees. The pulse duration was 60 fs after compression.