Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work, we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n = 2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants.

The low density matter end-station at the new seeded free electron laser FERMI@Elettra is a versatile instrument for the study of atoms, molecules and clusters by means of electron and ion spectroscopies. Beams of atoms, molecules and helium droplets as well as clusters of atoms, molecules and metals can be produced by three different pulsed valves. The atomic and molecular beams may be seeded, and the clusters and droplets may be pure, or doped with other atoms and molecules. The electrons and ions produced by the ionization and fragmentation of the samples by the intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging and time-of-flight techniques respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency. We describe the current status of the apparatus and illustrate the potential for future experiments.

The competition between rearrangement of the excited allyl radical via a 1,3 sigmatropic shift versus sequential 1,2 shifts has been observed and characterized using isotopic substitution, laser excitation, and molecular beam techniques. Both rearrangements produce a 1-propenyl radical that subsequently dissociates to methyl plus acetylene. The 1,3 shift and 1,2 shift mechanisms are equally probable for CH(2)CHCH(2), whereas the 1,3 shift is favored by a factor of 1.6 in CH(2)CDCH(2). The translational energy distributions for the methyl and acetylene products of these two mechanisms are substantially different. Both of these allyl dissociation channels are minor pathways compared to hydrogen atom loss.

The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to 10 13 W=cm 2 power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike 2p states with a photon energy of 21.4 eV, below the ionization potential (I p ), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above I p is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike 2p states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems. With the advent of short-wavelength free-electron lasers (FELs) the interaction between intense, high-energy light pulses and matter has become a very active field of research [1][2][3] and one of the most exciting topics in atomic and molecular science. Key questions are related to ionization dynamics on an atomic level, answers to which will help to develop an understanding of processes in more complex systems. In pioneering experiments and theoretical studies, various new phenomena such as absorption enhancement [1,4], bleaching [3,5,6], as well as modification [7] and suppression [8] of electron emission were discovered.At high power densities a nanoscale sample, such as a large molecule or cluster, can absorb a large number of photons and the system undergoes a transition to a highly excited, nonequilibrium state. Ionization in this case is strongly interlinked with correlated electron dynamics, either due to multielectron collisions with energy exchange [7] or by novel types of autoionization processes related to interatomic Coulombic decay (ICD), as predicted recently [9]. According to this work, clusters resonantly irradiated by intense light pulses with photon energies insufficient to ionize the atoms by single photon absorption are efficiently autoionized due to the energy exchange between two excited electrons [ Fig. 1(a)]. As a result, an unusual form of a collectively excited, plasmalike state may be formed which is expected to autoionize on a fs-ps time scale [9]. Initial evidence for such an ionization process in Ne clusters has been reported recently [10,11].In this Letter we report a study of electron emission from He clusters irradiated by intense pulses from the new seeded-FEL FERMI [12] at power densities where such collective autoionization (CAI) processes are expected to occur [13]. He droplets were either resonantly excited to the 2p atomiclike state [14], which is well below the ionization potential (I p ), or excited into the continuum. The elect...

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.

Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg ͑FLASH at DESY͒ with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths. DOI: 10.1103/PhysRevA.74.011401 PACS number͑s͒: 32.80.Rm, 32.80.Fb., 42.50.Hz The achievement of short-wavelength free-electron-laser ͑FEL͒ action at DESY in the year 2000 ͓1͔, based on the process of self-amplified spontaneous emission ͑SASE͒, represented a synergistic tour de force as optical and accelerator technologies were combined to produce ultrashort laser pulses at high fundamental photon energies with high peak and average power. The first experiments on rare-gas clusters revealed new insights into intense laser-matter interactions ͓2͔. In contrast to visible and infrared laser-matter interactions, where valence electrons are the primary participants, the fundamental FEL photon energy lies far above the ionization potential of all stable matter. Under these conditions, inner-shell electrons can be excited into resonant and nonresonant continuum states ͑e.g., ͓3,4͔͒ and will be the predominant mediators of the underlying photoprocesses ͑linear and nonlinear͒.Starting in mid-2005 the free-electron laser in Hamburg ͑FLASH͒ covers a much larger wavelength range compared to the first lasing in 2000. In view of its unprecedented characteristics ͓5͔, an associated time-synchronized optical laser facility opens up new and particularly exciting research opportunities allowing the investigation of fundamental photoionization processes in intense bichromatic laser fields where one field can directly ionize valence and/or inner-shell electrons in a single step. The dynamics of various processes can be investigated, including ultrafast electronic relaxation of autoionization states ͓6͔, coupling between two autoionization states ͓7,8͔, wave-packet formation of high-lying Rydberg states ͓9͔, fast dissociation of molecules upon inner-and outer-shell photoexcitation, etc. Such a pump-probe setup has recently been implemented at the FEL facility at DESY, which provides either femtosecond or picosecond infraredvisible pulses, synchronized with a rms jitter of less than 1 ps to the femtosecond xuv pulses from the FEL ͓10͔. Here, we present experimental results obtained with this system, combined with a corresponding theoretical analysis, on the photoionization of the most prototypical of atoms for such studies-helium-in the presence of a strong o...

Hot-carriers, that is, charge carriers with an effective temperature higher than that of the lattice, may contribute to the high power conversion efficiency (PCE) shown by perovskite-based solar cells (PSCs), which are now competitive with silicon solar cells. Hot-carriers lose their excess energy in very short times, typically in a few picoseconds after excitation. For this reason, the carrier dynamics occurring on this time scale are extremely important in determining the participation of hot-carriers in the photovoltaic process. However, the stability of PSCs over time still remains an issue that calls for a solution. In this work, we demonstrate that the insertion of graphene flakes into the mesoscopic TiO 2 scaffold leads to stable values of carrier temperature. In PSCs aged over 1 week, we indeed observe that in the graphene-free perovskite cells the carrier temperature decreases by about 500 K from 1800 to 1300 K, while the graphene-containing cell shows a reduction of less than 200 K after the same aging time delay. The stability of the carrier temperature reflects the stability of the perovskite nanocrystals embedded in the mesoporous graphene-TiO 2 layer. Our results, based on femtosecond transient absorption measurements, show that the insertion of graphene can be beneficial for the design of stable PSCs with the aim of exploiting the hot-carrier contribution to the PCE of the PSCs.

The inner shell excitation of pyrimidine and some halogenated pyrimidines near the C and N K-edges has been investigated experimentally by near edge x-ray absorption fine structure spectroscopy and theoretically by density functional theory calculations. The selected targets, 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine, allow the effects of the functionalization of the pyrimidine ring to be studied either as a function of different halogen atoms bound to the same molecular site or as a function of the same halogen atom bound to different molecular sites. The results show that the individual characteristics of the different spectra of the substituted pyrimidines can be rationalized in terms of variations in electronic and geometrical structures of the molecule depending on the localization and the electronegativity of the substituent.