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The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail. The first steps in photochemical processes involve changes in electronic and geometric structure on extremely short timescales. Here, the authors report femtosecond dynamics in prototypical acetylacetone, by pump-probe photoexcitation-photoemission experiments and static and dynamics calculations.

The current histoclinical breast cancer classification is simple but imprecise. Several molecular classifications of breast cancers based on expression profiling have been proposed as alternatives. However, their reliability and clinical utility have been repeatedly questioned, notably because most of them were derived from relatively small initial patient populations. We analyzed the transcriptomes of 537 breast tumors using three unsupervised classification methods. A core subset of 355 tumors was assigned to six clusters by all three methods. These six subgroups overlapped with previously defined molecular classes of breast cancer, but also showed important differences, notably the absence of an ERBB2 subgroup and the division of the large luminal ER+ group into four subgroups, two of them being highly proliferative. Of the six subgroups, four were ER+/PR+/AR+, one was ER−/PR−/AR+ and one was triple negative (AR−/ER−/PR−). ERBB2-amplified tumors were split between the ER−/PR−/AR+ subgroup and the highly proliferative ER+ LumC subgroup. Importantly, each of these six molecular subgroups showed specific copy-number alterations. Gene expression changes were correlated to specific signaling pathways. Each of these six subgroups showed very significant differences in tumor grade, metastatic sites, relapse-free survival or response to chemotherapy. All these findings were validated on large external datasets including more than 3000 tumors. Our data thus indicate that these six molecular subgroups represent well-defined clinico-biological entities of breast cancer. Their identification should facilitate the detection of novel prognostic factors or therapeutical targets in breast cancer.

The relaxation of photoexcited nanosystems is a fundamental process of light–matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, we study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He * ) within 1 ps. Subsequently, the bubble collapses and releases metastable He * at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses. There is interest in understanding the relaxation mechanisms of photoexcitation in atoms, molecules and other complex systems. Here the authors unravel the photoexcitation and ultrafast relaxation of superfluid helium nanodroplets using a pump-probe experiment with FEL pulses.

Short wavelength free-electron lasers (FELs), providing pulses of ultrahigh photon intensity, have revolutionized spectroscopy on ionic targets. Their exceptional photon flux enables multiple photon absorptions within a single femtosecond pulse, which in turn allows for deep insights into the photoionization process itself as well as into evolving ionic states of a target. Here we employ ultraintense pulses from the FEL FERMI to spectroscopically investigate the sequential emission of electrons from gaseous, atomic argon in the neutral as well as the ionic ground state. A pronounced forward-backward symmetry breaking of the angularly resolved emission patterns with respect to the light propagation direction is experimentally observed and theoretically explained for the region of the Cooper minimum, where the asymmetry of electron emission is strongly enhanced. These findings aim to originate a better understanding of the fundamentals of photon momentum transfer in ionic matter. Exploring the photoionization process leads to better understanding of the fundamental interactions between light and matter. Here the authors show the non-dipole contribution in the form of asymmetric photoelectron angular distribution from the ionization of argon atoms and ions.

The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 106 He atoms per droplet) and XUV intensity (109-1012 W cm−2) is observed, ranging from narrow atomic-like peaks that are due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of a numerical simulation based on rate equations taking into account all relevant processes-multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI).

Researchers demonstrate correlation of two colours (63.0 and 31.5 nm wavelengths) in a free-electron laser and control photoelectron angular distribution by adjusting phase with 3 attosecond resolution. Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization 1 , pumping an atomic laser 2 and four-wave mixing spectroscopy 3 . However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence 4 , 5 , 6 , 7 . Single-colour pulses from the FERMI seeded FEL are longitudinally coherent 8 , 9 , and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution 10 , 11 by adjusting the phase, with a temporal resolution of 3 as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.

A better molecular characterization of breast cell lines (BCL) may help discover new markers to apply to tumour samples. We performed gene and protein expression profiling of 31 BCL using whole-genome DNA microarrays and immunohistochemistry (IHC) on ‘cell microarrays’ (CMA), respectively. Global hierarchical clustering discriminated two groups of BCL: group I corresponded to luminal cell lines, group II to basal and mesenchymal cell lines. Correlations with centroids calculated from a published ‘intrinsic 500-gene set’ assigned 15 cell lines as luminal, eight as basal and four as mesenchymal. A set of 1.233 genes was differentially expressed between basal and luminal samples. Mesenchymal and basal subtypes were rather similar and discriminated by only 227 genes. The expression of 10 proteins (CAV1, CD44, EGFR, MET, ETS1, GATA3, luminal cytokeratin CK19, basal cytokeratin CK5/6, CD10, and ERM protein moesin) encoded by luminal vs basal discriminator genes confirmed the subtype classification and the validity of the identified markers. Our BCL basal/luminal signature correctly re-classified the published series of tumour samples that originally served to identify the molecular subtypes, suggesting that the identified markers should be useful for tumour classification and might represent promising targets for disease management.

We explore the light induced dynamics in superfluid helium nanodroplets with wide-angle scattering in a pump–probe measurement scheme. The droplets are doped with xenon atoms to facilitate the ignition of a nanoplasma through irradiation with near-infrared laser pulses. After a variable time delay of up to 800 ps, we image the subsequent dynamics using intense extreme ultraviolet pulses from the FERMI free-electron laser. The recorded scattering images exhibit complex intensity fluctuations that are categorized based on their characteristic features. Systematic simulations of wide-angle diffraction patterns are performed, which can qualitatively explain the observed features by employing model shapes with both randomly distributed as well as structured, symmetric distortions. This points to a connection between the dynamics and the positions of the dopants in the droplets. In particular, the structured fluctuations might be governed by an underlying array of quantized vortices in the superfluid droplet as has been observed in previous small-angle diffraction experiments. Our results provide a basis for further investigations of dopant–droplet interactions and associated heating mechanisms.

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions.

Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism. Intense extreme UV and X-ray coherent sources are set to revolutionize numerous research areas, yet characterization of their polarization remains elusive. Here, Mazza et al. measure the polarization state of circularly polarized extreme UV light from a free-electron laser using circular dichroism.