Few-femtosecond passage of conical intersections in the benzene cation.

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.

XUV-induced reactions in benzene on sub-10 fs timescale: nonadiabatic relaxation and proton migration.

Unraveling ultrafast dynamical processes in highly excited molecular species has an impact on our understanding of chemical processes such as combustion or the chemical composition of molecular clouds in the universe. In this article we use short (<7 fs) XUV pulses to produce excited cationic states of benzene molecules and probe their dynamics using few-cycle VIS/NIR laser pulses. The excited states produced by the XUV pulses lie in an especially complex spectral region where multi-electronic effects play a dominant role. We show that very fast τ ≈ 20 fs nonadiabatic processes dominate the relaxation of these states, in agreement with the timescale expected for most excited cationic states in benzene. In the CH3+ fragmentation channel of the doubly ionized benzene cation we identify pathways that involve structural rearrangement and proton migration to a specific carbon atom. Further, we observe non-trivial transient behavior in this fragment channel, which can be interpreted either in terms of propagation of the nuclear wavepacket in the initially excited electronic state of the cation or as a two-step electronic relaxation via an intermediate state.

Communication: XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation.

Time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy of iodomethane and iodobenzene photodissociation at the iodine pre-N4,5 edge is presented, using femtosecond UV pump pulses and XUV probe pulses from high harmonic generation. For both molecules the molecular core-to-valence absorption lines fade immediately, within the pump-probe time-resolution. Absorption lines converging to the atomic iodine product emerge promptly in CH3I but are time-delayed in C6H5I. We attribute this delay to the initial π → σ(*) excitation in iodobenzene, which is distant from the iodine reporter atom. We measure a continuous shift in energy of the emerging atomic absorption lines in CH3I, attributed to relaxation of the excited valence shell. An independent particle model is used to rationalize the observed experimental findings.

Time-resolved excited state energetics of the solvated electron in sodium-doped water clusters.

The energetics and dynamics of the first electronically excited state of solvated electron in sodium-doped water clusters has been studied, by means of time-resolved electron spectra created in a pump-probe fs-laser experiment. The Na ··· (H2O)n clusters were excited by pulses at a wavelength of 795 nm, while ionization was achieved at a wavelength of 398 nm, and the overall cross-correlation fwhm was about 50 fs. Mass-resolved electron spectra were taken using photoelectron-photoion coincidence (PEPICO) spectroscopy for cluster sizes ranging from n = 1 up to 22. The electron spectra give new insights into the dynamics of the excited state of solvated electrons in Na ··· (H2O)n clusters. These dynamics are compared to known results for water cluster anions. In both cases, the observed dynamics are a combination of solvent rearrangement and internal energy conversion.

Ultrafast photo-excitation dynamics in isolated, neutral water clusters.

Using the efficient nonlinear conversion scheme which was recently developed in our group [M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 134, 1491 (2010); M. Ghotbi, M. Beutler, and F. Noack, ibid 35, 3492 (2010)] to provide intense sub-50 fs vacuum ultraviolet laser pulses we have performed the first real time study of ultrafast, photo-induced dynamics in the electronically excited Ã-state of water clusters (H(2)O)(n) and (D(2)O)(n) , n=2-10. Three relevant time scales, 1.8-2.5, 10-30, and 50-150 fs, can be distinguished which-guided by the available theoretical results-are attributed to H (D)-ejection, OH (OD) dissociation, and a nonadiabatic transition through a conical intersection, respectively. While a direct quantitative comparison is only very preliminary, the present results provide a crucial test for future modeling of excited state dynamics in water clusters, and should help to unravel some of the many still unresolved puzzles about water.

Ultrafast dynamics in Na-doped water clusters and the solvated electron.

The lifetimes of the first electronically excited state of (H(2)O)(n)...Na and (D(2)O)(n)...Na clusters up to n = 40 have been measured by two-color pump-probe spectroscopy (800 and 400 nm) with 35 fs laser pulses. The excited-state lifetime decreases rapidly from 1.2 ps at n = 2 to approximately 100 fs at n > or = 10. For (D(2)O)(n)...Na, the average lifetime is about 3.6 times longer. The fast energy redistribution is explained by conversion of the electronic excitation into vibrations of the ground state. A simple model based on Fermi's Golden Rule predicts the observed trends but fails to reproduce the observed lifetimes quantitatively. The longer lifetimes for deuterated clusters are discussed in the framework of the famous energy gap law and indicate that the stretching modes of water play an important role in the energy-transfer process.

Fragmentation and ionization dynamics of C60 in elliptically polarized femtosecond laser fields.

Ionization and fragmentation of C60 fullerenes is studied in elliptically polarized, intense fs laser fields at 797 nm [I=(0.5-4.3)x10;{14} W cm;{-2}] and contrasted with Xe+, utilizing time-of-flight mass spectrometry. Very pronounced changes of parent and fragment ion yield as a function of ellipticity are observed. At lower intensities reduction of the ion yield for circular polarization establishes a coherent two-photon process connected with the key role of the LUMO+1(t_{1g}) "doorway state" and multielectron dynamics. Comparison with the behavior at 399 nm corroborates this finding. At high intensities enhanced fragmentation is observed which is tentatively attributed to returning loops of electron trajectories by the combined action of the C60+ field and the circular laser field.

Coherent control of bond breaking in amino acid complexes with tailored femtosecond pulses.

Intense femtosecond laser pulses, judiciously tailored in an adaptive, optimal control feedback loop were used to break preferentially the acyl-N ("peptide") bond of Ac-Phe-NHMe that may be regarded as a dipeptide model. We show that coherent excitation of complex wave packets in the strong-field regime allows to cleave strong backbone bonds in the molecular system preferentially, while keeping other more labile bonds intact. These results show the potential of pulse shaping as a powerful complementary analytical tool for protein sequencing of large biopolymers in addition to the well-known mass spectrometry and chemical analysis.

Ultrafast changes of molecular crystal structure induced by dipole solvation.

Femtosecond photoexcitation of organic chromophores in a molecular crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer as is evident from transient vibrational spectra. The structural response of the crystal to the dipole change is mapped directly for the first time by ultrafast x-ray diffraction or diffuse scattering. Changes of diffracted and transmitted x-ray intensity demonstrate an angular rearrangement of molecules around excited dipoles following the 10 ps kinetics of charge transfer and leaving lattice plane spacings unchanged. Transient x-ray scattering is governed by solvation, masking changes of the chromophore molecular structure.

Coupled ultrafast lattice and polarization dynamics in ferroelectric nanolayers.

We report the first analysis of the polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the soft mode. Optical excitation of the SrRuO(3) metal layers generates giant stress (>1 GPa) compressing the PbZr(0.2)Ti(0.8)O(3) layers by up to 2%. The resulting change of tetragonality reaches a maximum after 1.3 ps. As a result, the ferroelectric polarization P is reduced by up to 100% with a slight delay that is due to the anharmonic coupling of the two modes.

Control of giant breathing motion in c60 with temporally shaped laser pulses.

Femtosecond laser pulses tailored with closed-loop, optimal control feedback were used to excite oscillations in C60 with large amplitude by coherent heating of nuclear motion. A characteristic pulse sequence results in significant enhancement of C2 evaporation, a typical energy loss channel of vibrationally hot C60. The separation between subsequent pulses in combination with complementary two-color pump-probe data and time-dependent density functional theory calculations give direct information on the multielectron excitation via the t(1g) resonance followed by efficient coupling to the radial symmetric a(g)(1) breathing mode.

Recent progress in ultrafast X-ray diffraction.

X-ray diffraction with femtosecond time-resolution represents a direct probe of ultrafast structural changes in condensed matter. The generation of ultrashort X-ray pulses in laser-driven plasma and/or accelerator-based sources has made substantial progress, and has allowed for studies of transient structures with an unprecedented accuracy. Herein, recent work on transient crystalline structures is reviewed, with the focus on laser-based experiments.

Coherent atomic motions in a nanostructure studied by femtosecond X-ray diffraction.

Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (DeltaR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.

Generation of single intense short optical pulses by ultrafast molecular phase modulation.

Pulses with durations below 4 fs have been generated using the method of ultrafast molecular phase modulation. A laser pulse shorter than the molecular vibrational or rotational period obtains spectral broadening during propagation along a hollow waveguide filled with previously impulsively excited Raman active gases. The induced time dependent phase, frequency, and frequency chirp are controllable by changing the delay between excitation and probe pulse within the molecular vibrational period.

Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers.

We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.

Chirped-pulse stimulated Raman scattering in barium nitrate with subsequent recompression.

We report a 30% internal conversion efficiency for the first Stokes pulse in stimulated Raman scattering of femtosecond pulses that are dispersively stretched to 250 ps, obtained by use of an all-solid-state laser system. A transfer of the linear chirp is observed, leading to a Raman pulse duration of 190 fs after recompression. Compressed pulse energies of 80 muJ at a repetition rate of 1 kHz are obtained, with a potential for an easy increase to more than 150 muJ. The temporal and spectral characteristics of the Raman and pump pulses are calculated, and the results explain the observed transient features in the presence of chirp.

Powerful and tunable operation of a 1-2-kHz repetition-rate gain-switched Cr:forsterite laser and its frequency doubling.

We present a comprehensive study of the optimum operating regime in gain-switched Cr:forsterite lasers pumped at kilohertz repetition rates, comparing five crystals of similar quality but different dopant levels. The optimization of the cavity design includes selection of the proper pump fluence to account for excited-state absorption, optimum matching of the pump and laser modes, and consideration of thermal effects. As a result >1-W average output power is demonstrated at 2 kHz. The maximum conversion efficiencies achieved at 1 kHz are 24.2% (slope) and 20% (absolute). Narrow-band operation of this laser is possible with a birefringent filter, which is a prerequisite for efficient frequency doubling to cover the 585-660-nm part of the visible spectral range. Tunable second-harmonic generation in a temperature-tuned noncritical scheme that employs LiB(3)O(5) produces 60 mW of average power near 619 nm with 13.5% conversion efficiency.

Femtosecond regenerative amplification in Cr:forsterite.

We describe an oscillator-amplifier laser system for the generation of high-power femtosecond pulses near 1.25 microm based on chromium-doped forsterite. Chirped-pulse amplification at a 1-kHz repetition rate raises the pulse energy to >350 microJ. The nearly transform-limited 200-microJ, 135-fs-long recompressed pulses have a peak power of approximately 1.5 GW.

[Role of membrane protectors and antioxidants in the process of perfusion and preservation of donor lungs]. / Rol' membranoprotektorov i antioksidantov v protsesse perfuzii i konservatsii donorskikh legkikh.

Comparative study of the effect of four perfusion and preservation solutions on lung function, conducted in experiments on 69 inbred dogs, revealed the following regular features. Euro-Collins and electrolyte solutions cause a marked increase of the capillary-hydrostatic pressure, a significant decrease of colloido-osmotic pressure of the plasma, and, as a result, reduction of the ient between them. This is manifested by edema of the lungs. The use of LPD solution is attended by moderate edema of the lungs in moderately increased capillary hydrostatic pressure and mildly reduced colloido-osmotic pressure of the plasma. Perfusion and preservation of the lungs with the LPD solution, containing membrane protectors and antioxidants, for 12 hours is marked by practically unchanged indices of capillary-hydrostatic pressure, plasma coloido-osmotic pressure, and the gradient between them. Edema of the lungs does not develop in this case.

Chromium-doped forsterite laser with 1.1 W of continuous-wave output power at room temperature.

We demonstrate stable continuous-wave operation of a Cr:forsterite laser pumped by a cw Nd:YAG laser at 288 K. We use no choppers to limit the duty cycle of the cw system. The proper adjustment of the pump- and cavity-mode overlap based on the ABCD concept simulation eliminates the output power decrease at the high-level pump power. An output power of 1.1 W and slope efficiency of 26% are derived.