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The photoisomerization reaction of a fluorescent protein chromophore occurs on the ultrafast timescale. The structural dynamics that result from femtosecond optical excitation have contributions from vibrational and electronic processes and from reaction dynamics that involve the crossing through a conical intersection. The creation and progression of the ultrafast structural dynamics strongly depends on optical and molecular parameters. When using X-ray crystallography as a probe of ultrafast dynamics, the origin of the observed nuclear motions is not known. Now, high-resolution pump–probe X-ray crystallography reveals complex sub-ångström, ultrafast motions and hydrogen-bonding rearrangements in the active site of a fluorescent protein. However, we demonstrate that the measured motions are not part of the photoisomerization reaction but instead arise from impulsively driven coherent vibrational processes in the electronic ground state. A coherent-control experiment using a two-colour and two-pulse optical excitation strongly amplifies the X-ray crystallographic difference density, while it fully depletes the photoisomerization process. A coherent control mechanism was tested and confirmed the wave packets assignment.Pump–probe measurements conventionally achieve femtosecond time resolution for X-ray crystallography of reactive processes, but the measured structural dynamics are complex. Using coherent control techniques, we show that the ultrafast crystallographic differences of a fluorescent protein are dominated by ground-state vibrational processes that are unconnected to the photoisomerization reaction of the chromophore.

We have investigated the interaction and aggregation of novel fluorescent silicon nanoclusters in liquids by measuring the size distribution of dried clusters on graphite. The clusters were produced by gas aggregation and co-deposition with a beam of water vapour. Drops of the solutions were placed on freshly cleaved highly oriented pyrolitic graphite, subsequently vacuum dried and investigated by atomic force microscopy (AFM) in ultra high vacuum. The AFM images show single clusters and agglomerates. The height distributions are Gaussian-shaped with average heights of 1 nm and widths of 1 nm. The heights never exceed 3 nm. In some regions a second cluster layer is observed. In all samples the separation between first and second layers is larger than the separation between the first layer and the graphite substrate, which we attribute to a stronger interaction between clusters and surface than the cluster self-interaction. We conclude that the separation between first and second layer represents a much better fingerprint of the original size distribution of the clusters in solution than the height of the first layer. The observation of a second cluster layer is important for using silicon clusters as building blocks for cluster-assembled materials.[PUBLICATION ABSTRACT]

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wave packets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. Guided by ab initio calculations for the smallest cluster comprising of a single acetylene molecule and a helium atom, essentially all rotational eigenstates up to the dissociation threshold could be established, providing a comprehensive picture of the rotational energy level structure. The C2H2-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.

The suitability of silicon nanoparticles of 1 nm in diameter for fluorescent sensing was investigated. Silicon nanoparticles were produced in a cluster beam and co-deposited with a beam of vapourised liquids (water, ethanol, isopropanol) onto a cold substrate. Melting of the frozen cluster-ice mixture yielded an aqueous suspension which emitted strong fluorescence in the deep blue spectral range when exposed to UV light. The fluorescence wavelength of the strongest peak was found to correlate with the dipole moment of the solvent molecules which allowed us to derive the transition energy for an isolated nanoparticle. The strong solvent sensitivity showed that the fluorescence originated from a surface state. A second fluorescence peak showed almost no sensitivity to different solvents, hence the peak was attributed to a transition within the bulk–volume of the nanoparticles. Our findings establish that silicon nanoparticles may serve as highly specific bio-sensors in living organism.

We have investigated the interaction and aggregation of novel fluorescent silicon nanoclusters in liquids by measuring the size distribution of dried clusters on graphite. The clusters were produced by gas aggregation and co-deposition with a beam of water vapour. Drops of the solutions were placed on freshly cleaved highly oriented pyrolitic graphite, subsequently vacuum dried and investigated by atomic force microscopy (AFM) in ultra high vacuum. The AFM images show single clusters and agglomerates. The height distributions are Gaussian-shaped with average heights of 1 nm and widths of 1 nm. The heights never exceed 3 nm. In some regions a second cluster layer is observed. In all samples the separation between first and second layers is larger than the separation between the first layer and the graphite substrate, which we attribute to a stronger interaction between clusters and surface than the cluster self-interaction. We conclude that the separation between first and second layer represents a much better fingerprint of the original size distribution of the clusters in solution than the height of the first layer. The observation of a second cluster layer is important for using silicon clusters as building blocks for cluster-assembled materials.

The suitability of silicon nanoparticles of 1 nm in diameter for fluorescent sensing was investigated. Silicon nanoparticles were produced in a cluster beam and co-deposited with a beam of vapourised liquids (water, ethanol, isopropanol) onto a cold substrate. Melting of the frozen cluster-ice mixture yielded an aqueous suspension which emitted strong fluorescence in the deep blue spectral range when exposed to UV light. The fluorescence wavelength of the strongest peak was found to correlate with the dipole moment of the solvent molecules which allowed us to derive the transition energy for an isolated nanoparticle. The strong solvent sensitivity showed that the fluorescence originated from a surface state. A second fluorescence peak showed almost no sensitivity to different solvents, hence the peak was attributed to a transition within the bulk-volume of the nanoparticles. Our findings establish that silicon nanoparticles may serve as highly specific bio-sensors in living organism.[PUBLICATION ABSTRACT]

The observation of non-perturbative harmonic emission in solids from ultrashort laser pulses [1] sparked a wave of studies [2,3] as a probe of charge carrier dynamics in solids under strong fields and a route to extreme ultraviolet (XUV) attosecond photonic devices [4]. High harmonic generation (HHG) in liquids [5,6] is far less explored, despite their relevance to biological media, and the mechanism is hotly debated. Using few-cycle pulses below the breakdown threshold, we demonstrate HHG in alcohol with data showing carrier-envelope-phase-dependent XUV spectra extending to 50 eV from isopropanol. We study the mechanism of the harmonic emission through its dependence on the driving field and find it to be consistent with a strong-field recombination mechanism. This maps emitted photon energy to the electron trajectories. We explore the role of the liquid environment in scattering the trajectories and find evidence that information on electron scattering from neighbouring molecules is encoded in the harmonic spectra. Using simulations we exploit this to estimate the scattering cross section and we confirm that the cross-section in liquid isopropanol is significantly reduced compared to vapour. Our findings suggest an \\textit{in situ} measurement strategy for retrieving accurate values of scattering cross sections in liquids, and also a pathway to liquid-based attosecond XUV devices.

The chemical and physical properties of molecular clusters can heavily depend on their size, which makes them very attractive for the design of new materials with tailored properties. Deriving the structure and dynamics of clusters is therefore of major interest in science. Weakly bound clusters can be studied using conventional spectroscopic techniques, but the number of lines observed is often too small for a comprehensive structural analysis. Impulsive alignment generates rotational wavepackets, which provides simultaneous information on structure and dynamics, as has been demonstrated successfully for isolated molecules. Here, we apply this technique for the firsttime to clusters comprising of a molecule and a single helium atom. By forcing the population of high rotational levels in intense laser fields we demonstrate the generation of rich rotational line spectra for this system, establishing the highly delocalised structure and the coherence of rotational wavepacket propagation. Our findings enable studies of clusters of different sizes and complexity as well as incipient superfluidity effects using wavepacket methods.