Single-electron pulses for ultrafast diffraction

Visualization of atomic-scale structural motion by ultrafast electron diffraction and microscopy requires electron packets of shortest duration and highest coherence. We report on the generation and application of single-electron pulses for this purpose. Photoelectric emission from metal surfaces is studied with tunable ultraviolet pulses in the femtosecond regime. The bandwidth, efficiency, coherence, and electron pulse duration are investigated in dependence on excitation wavelength, intensity, and laser bandwidth. At photon energies close to the cathode's work function, the electron pulse duration shortens significantly and approaches a threshold that is determined by interplay of the optical pulse width and the acceleration field. An optimized choice of laser wavelength and bandwidth results in sub-100-fs electron pulses. We demonstrate single-electron diffraction from polycrystalline diamond films and reveal the favorable influences of matched photon energies on the coherence volume of single-electron wave packets. We discuss the consequences of our findings for the physics of the photoelectric effect and for applications of single-electron pulses in ultrafast 4D imaging of structural dynamics.     

Breaking resolution limits in ultrafast electron diffraction and microscopy

Ultrafast electron microscopy and diffraction are powerful techniques for the study of the time-resolved structures of molecules, materials, and biological systems. Central to these approaches is the use of ultrafast coherent electron packets. The electron pulses typically have an energy of 30 keV for diffraction and 100-200 keV for microscopy, corresponding to speeds of 33-70% of the speed of light. Although the spatial resolution can reach the atomic scale, the temporal resolution is limited by the pulse width and by the difference in group velocities of electrons and the light used to initiate the dynamical change. In this contribution, we introduce the concept of tilted optical pulses into diffraction and imaging techniques and demonstrate the methodology experimentally. These advances allow us to reach limits of time resolution down to regimes of a few femtoseconds and, possibly, attoseconds. With tilted pulses, every part of the sample is excited at precisely the same time as when the electrons arrive at the specimen. Here, this approach is demonstrated for the most unfavorable case of ultrafast crystallography. We also present a method for measuring the duration of electron packets by autocorrelating electron pulses in free space and without streaking, and we discuss the potential of tilting the electron pulses themselves for applications in domains involving nuclear and electron motions.     

Temporal lenses for attosecond and femtosecond electron pulses

Here, we describe the “temporal lens” concept that can be used for the focus and magnification of ultrashort electron packets in the time domain. The temporal lenses are created by appropriately synthesizing optical pulses that interact with electrons through the ponderomotive force. With such an arrangement, a temporal lens equation with a form identical to that of conventional light optics is derived. The analog of ray diagrams, but for electrons, are constructed to help the visualization of the process of compressing electron packets. It is shown that such temporal lenses not only compensate for electron pulse broadening due to velocity dispersion but also allow compression of the packets to durations much shorter than their initial widths. With these capabilities, ultrafast electron diffraction and microscopy can be extended to new domains,and, just as importantly, electron pulses can be delivered directly on an ultrafast techniques target specimen.     

Thermal Nonlinear Klein–Gordon Equation for Nano-/Micro-Sized Metallic Particle–Attosecond Laser Pulse Interaction

In this study, a rigorous analytical solution to the thermal nonlinear Klein–Gordon equation in the Kozłowski version is provided. The Klein–Gordon heat equation is solved via the Zhukovsky “state-of-the-art” mathematical techniques. Our study can be regarded as an initial approximation of attosecond laser–particle interaction when the prevalent phenomenon is photon–electron interaction. The electrons interact with the laser beam, which means that the nucleus does not play a significant role in temperature distribution. The particle is supposed to be homogenous with respect to thermophysical properties. This theoretical approach could prove useful for the study of metallic nano-/micro-particles interacting with attosecond laser pulses. Specific applications for Au “nano” particles with a 50 nm radius and “micro” particles with 110, 130, 150, and 1000 nm radii under 100 attosecond laser pulse irradiation are considered. First, the cross-section is supposed to be proportional to the area of the particle, which is assumed to be a perfect sphere of radius R or a rotation ellipsoid. Second, the absorption coefficient is calculated using a semiclassical approach, taking into account the number of atoms per unit volume, the classical electron radius, the laser wavelength, and the atomic scattering factor (10 in case of Au), which cover all the basic aspects for the interaction between the attosecond laser and a nanoparticle. The model is applicable within the 100–2000 nm range. The main conclusion of the model is that for a range inferior to 1000 nm, a competition between ballistic and thermal phenomena occurs. For values in excess of 1000 nm, our study suggests that the thermal phenomena are dominant. Contrastingly, during the irradiation with fs pulses, this value is of the order of 100 nm. This theoretical model’s predictions could be soon confirmed with the new EU-ELI facilities in progress, which will generate pulses of 100 as at a 30 nm wavelength.     

Four-dimensional ultrafast electron microscopy

Electron microscopy is arguably the most powerful tool for spatial imaging of structures. As such, 2D and 3D microscopies provide static structures with subnanometer and increasingly with ångstrom-scale spatial resolution. Here we report the development of 4D ultrafast electron microscopy, whose capability imparts another dimension to imaging in general and to dynamics in particular. We demonstrate its versatility by recording images and diffraction patterns of crystalline and amorphous materials and images of biological cells. The electron packets, which were generated with femtosecond laser pulses, have a de Broglie wavelength of 0.0335 Å at 120 keV and have as low as one electron per pulse. With such few particles, doses of few electrons per square ångstrom, and ultrafast temporal duration, the long sought after but hitherto unrealized quest for ultrafast electron microscopy has been realized. Ultrafast electron microscopy should have an impact on all areas of microscopy, including biological imaging.     

Scanning ultrafast electron microscopy

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the space-charge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsed-electron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability.     

Ultrafast Third-Order Nonlinear Optical Response Excited by fs Laser Pulses at 1550 nm in GaN Crystals

The ultrafast third-order optical nonlinearity of c-plane GaN crystal, excited by ultrashort (fs) high-repetition-rate laser pulses at 1550 nm, wavelength important for optical communications, is investigated for the first time by optical third-harmonic generation in non-phase-matching conditions. As the thermo-optic effect that can arise in the sample by cumulative thermal effects induced by high-repetition-rate laser pulses cannot be responsible for the third-harmonic generation, the ultrafast nonlinear optical effect of solely electronic origin is the only one involved in this process. The third-order nonlinear optical susceptibility of GaN crystal responsible for the third-harmonic generation process, an important indicative parameter for the potential use of this material in ultrafast photonic functionalities, is determined.     

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10⁻¹⁸ s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.High-order harmonic generation (HHG) is the most extreme nonlinear optical process in nature, making it possible to coherently upconvert intense femtosecond laser light to much shorter wavelengths (1, 2). High harmonics are radiated as a result of a coherent electron recollision process that occurs each half-cycle of the driving laser field while an atom is undergoing strong-field ionization. The short pulse duration of HHG (which must be shorter than the driving laser pulse) has made it possible to directly access the fastest timescales relevant to electron dynamics in atoms, molecules, and materials. The unique properties of attosecond HHG in the extreme UV (EUV) have uncovered new understanding of fundamental processes in atoms, molecules, plasmas, and materials, including the timescales on which electrons are emitted from atoms (3), the timescale for spin–spin and electron–electron interactions (4, 5), the timescale that determines molecular dissociation and electron localization (6–9), the timescale and mechanisms for spin and energy transport in nanosystems (10–12), as well as new capabilities to implement EUV microscopes with wavelength-limited spatial resolution (13).The temporal structure of HHG is related to the number of times a high-energy electron undergoes a coherent recollision process, as well as the time window over which bright harmonics emerge. Using multicycle 0.8-µm driving lasers, HHG generally emerges as a train of attosecond (as) pulses (14, 15) corresponding to a series of harmonic peaks in frequency space. This emission can narrow to a single isolated as burst when the driving laser field is a few optical cycles (∼5 fs) in duration (16, 17), with an associated broad continuous spectrum. Other techniques can isolate a single burst using a combination of multicolor fields and polarization control (18–26) or spatial lighthouse gating of the driving laser pulses (27, 28). Phase matching can also result in bright isolated as pulse generation for short driving laser pulses (29, 30). To obtain bright, phase-matched, high harmonic beams, the laser and HHG fields must both propagate at the speed of light c so that emission from many atoms interferes constructively. Above a critical ionization level, the phase velocity of the laser exceeds c, which terminates the HHG temporal emission. The chirp present on attosecond bursts can be compensated by using thin materials, gases, or chirped mirrors (31–33). To date, however, most schemes for creating isolated attosecond pulses require either very short-duration few-cycle 0.8-µm driving laser pulses that are difficult to reliably generate, or complex polarization modulation schemes. In addition, the carrier envelope phase (CEP) of the driving laser pulse must be stabilized.A more general understanding of how to efficiently sculpt the temporal, spatial, and spectral characteristics of HHG emission over an extremely broad photon energy range (from the EUV to the keV and higher) has emerged in recent years (34–39). This understanding is critical both for a fundamental understanding of strong-field quantum physics, as well as for applications which have fundamentally different needs in terms of the HHG pulse duration, spectral bandwidth, and flux. By considering both the microscopic single-atom response as well as the macroscopic coherent buildup of HHG, efficient phase-matched HHG can now be implemented from the EUV to >keV photon energies, simply by driving HHG with midinfrared (mid-IR) femtosecond driving lasers. This advance represents, to our knowledge, the first general-purpose, tabletop, coherent soft X-ray light source (39). Furthermore, theory suggested that bright isolated attosecond X-ray bursts would be achievable using multicycle mid-IR driving lasers in a phase-matched geometry (35). However, the low repetition rate of the driving lasers precluded experimental testing of these predictions. Moreover, formidable computation requirements meant that advanced simulations could not be fully extended into the mid-IR region at 2–4 µm.In this paper, we experimentally demonstrate a beautiful convergence of physics for mid-IR (2-µm) driving lasers by showing that the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of bright isolated attosecond soft X-ray bursts. We combine advanced theory with a novel experimental method equivalent to high-resolution Fourier transform spectroscopy to measure bright, attosecond soft X-ray pulses for the first time, to our knowledge. Specifically, we measure a field autocorrelation pulse width of 70 as, corresponding to a transform-limited 35-as pulse, that is supported by a coherent supercontinuum spectrum extending to photon energies around 180 eV. We also validate experimentally, for the first time, to our knowledge, the most intuitive dynamic picture of phase matching of HHG in the time domain by clearly demonstrating that the temporal window during which phase matching occurs shrinks rapidly with increasing driving laser wavelength. Finally, we show through advanced theory that the isolated attosecond pulse is chirped to 300 as. Most surprisingly, we find that bright attosecond pulse generation in the soft X-ray region requires the use of longer-duration, multicycle, mid-IR driving lasers to mitigate group velocity walk-off issues that would otherwise reduce the conversion efficiency. By harnessing the beautiful physics of phase matching, this work represents the simplest and most robust scheme for attosecond soft X-ray pulse generation, and will make attosecond science and technology accessible to a broader community.     

Strong-field control and spectroscopy of attosecond electron-hole dynamics in molecules

Molecular structures, dynamics and chemical properties are determined by shared electrons in valence shells. We show how one can selectively remove a valence electron from either Π vs. Σ or bonding vs. nonbonding orbital by applying an intense infrared laser field to an ensemble of aligned molecules. In molecules, such ionization often induces multielectron dynamics on the attosecond time scale. Ionizing laser field also allows one to record and reconstruct these dynamics with attosecond temporal and sub-Ångstrom spatial resolution. Reconstruction relies on monitoring and controlling high-frequency emission produced when the liberated electron recombines with the valence shell hole created by ionization.     

Photochemistry and electron-transfer mechanism of transition metal oxalato complexes excited in the charge transfer band

The photoredox reaction of trisoxalato cobaltate (III) has been studied by means of ultrafast extended x-ray absorption fine structure and optical transient spectroscopy after excitation in the charge-transfer band with 267-nm femtosecond pulses. The Co-O transient bond length changes and the optical spectra and kinetics have been measured and compared with those of ferrioxalate. Data presented here strongly suggest that both of these metal oxalato complexes operate under similar photoredox reaction mechanisms where the primary reaction involves the dissociation of a metal-oxygen bond. These results also indicate that excitation in the charge-transfer band is not a sufficient condition for the intramolecular electron transfer to be the dominant photochemistry reaction mechanism.     

Attosecond vacuum UV coherent control of molecular dynamics

High harmonic light sources make it possible to access attosecond timescales, thus opening up the prospect of manipulating electronic wave packets for steering molecular dynamics. However, two decades after the birth of attosecond physics, the concept of attosecond chemistry has not yet been realized; this is because excitation and manipulation of molecular orbitals requires precisely controlled attosecond waveforms in the deep UV, which have not yet been synthesized. Here, we present a unique approach using attosecond vacuum UV pulse-trains to coherently excite and control the outcome of a simple chemical reaction in a deuterium molecule in a non-Born–Oppenheimer regime. By controlling the interfering pathways of electron wave packets in the excited neutral and singly ionized molecule, we unambiguously show that we can switch the excited electronic state on attosecond timescales, coherently guide the nuclear wave packets to dictate the way a neutral molecule vibrates, and steer and manipulate the ionization and dissociation channels. Furthermore, through advanced theory, we succeed in rigorously modeling multiscale electron and nuclear quantum control in a molecule. The observed richness and complexity of the dynamics, even in this very simplest of molecules, is both remarkable and daunting, and presents intriguing new possibilities for bridging the gap between attosecond physics and attochemistry.The coherent manipulation of quantum systems on their natural timescales, as a means to control the evolution of a system, is an important goal for a broad range of science and technology, including chemical dynamics and quantum information science. In molecules, these timescales span from attosecond timescales characteristic of electronic dynamics, to femtosecond timescales characteristic of vibrations and dissociation, to picosecond timescales characteristic of rotations in molecules. With the advent of femtosecond lasers, observing the transition state in a chemical reaction (1), and controlling the reaction itself, became feasible. Precisely timed femtosecond pulse sequences can be used to selectively excite vibrations in a molecule, allow it to evolve, and finally excite or deexcite it into an electronic state not directly accessible from the ground state (2). Alternatively, interferences between different quantum pathways that end up in the same final state can be used to control the outcome of a chemical reaction (3–9).In recent years, coherent high harmonic sources with bandwidths sufficient to generate either attosecond pulse trains or a single isolated attosecond pulses have been developed that are also perfectly synchronized to the driving femtosecond laser (10–12). This new capability provides intriguing possibilities for coherently and simultaneously controlling both the electronic and nuclear dynamics in a molecule in regimes where the Born–Oppenheimer approximation is no longer valid, to select specific reaction pathways or products. Here, we realize this possibility in a coordinated experimental–theoretical study of dynamics in the simplest neutral molecule: deuterated hydrogen (D₂).The hydrogen molecule, as the simplest possible neutral molecule that can be fully described theoretically, has been the prototype molecule for understanding fundamental processes that lie at the heart of quantum mechanics (13–16). However, in such a small molecule, the coupled electron–nuclear dynamics are in the attosecond-to-few-femtosecond regime, whereas the electronically excited states lie in the vacuum UV (VUV) region of the spectrum. Because of the challenge of generating attosecond VUV waveforms using traditional laser frequency doubling or tripling in nonlinear crystals, it has not been possible to date to explore the dynamics of an electronically excited hydrogen molecule. Exploiting attosecond physics, however, only a handful of time-resolved experiments have been performed on H₂ (D₂), focusing mainly on controlling dissociation through electron localization in electronically excited ions (17–23). Recently it was realized that IR femtosecond laser pulses, in combination with a phased locked comb of attosecond VUV harmonics, can be used to control the excitation and ionization yields in He on attosecond timescales, by interfering electron wave packets (24, 25). These experiments extended the Brumer–Shapiro (3–9) two-pathway interference coherent control concept to the attosecond temporal and VUV frequency domain. More recently, it was shown that by manipulating the individual amplitude of VUV harmonics, it is possible to induce full electromagnetic transparency in He, by destructively interfering two electronic wave packets of the same amplitude and opposite phases (26). Other recent work used shaped intense femtosecond laser pulses to manipulate populations by controlling the oscillating charge distribution in a potassium dimer (27).In this paper, we demonstrate that we can coherently and simultaneously manipulate multistate electronic and multipotential-well nuclear wave packet dynamics to control the excitation, nuclear wave packet tunneling, and dissociation and ionization channels of the only electronically excited neutral molecule where full modeling of the coupled quantum dynamics is possible—H₂ (for practical reasons, we used deuterated hydrogen in this experiment). By combining attosecond pulse trains of VUV with two near-IR fields, together with strong-field control that exploits a combination of the two-pathway interference Brumer–Shapiro (3–9) and pump-dump Tannor–Rice (2) approaches, we demonstrate that we can selectively steer the ionization, vibration, and dissociation of D₂ through different channels. Interferences between electronic wave packets (evolving on attosecond timescales) are used to control the population of different electronic states of the excited neutral molecule, which can be switched on attosecond timescales. Then, by optimally selecting the excitation wavelengths and time delays, we can control the vibrational motion, total excitation, ionization yield, and desired ionization and dissociation pathways. State-of-the-art quantum calculations, which have only recently become feasible, allowed us to interpret this very rich set of quantum dynamics, including both the nuclear motion and the coherently excited electronic state interferences. Thus, we succeed in both observing and rigorously modeling multiscale coherent quantum control in the time domain. The observed richness and complexity of the dynamics, even in this very simplest of molecules, is both remarkable and daunting.We note that our approach for control, using a combination of phase-locked VUV and IR fields, where the VUV field consists of an attosecond pulse train with a 10-fs pulse envelope, is ideal for coherently exciting and manipulating electronic and vibrational states on attosecond time scales, while simultaneously retaining excellent spectral resolution necessary for state-selective attochemistry. Exciting electron dynamics in a molecule using a single, broad-bandwidth VUV attosecond pulse would simply excite many ionization/dissociation channels with little state selectivity, potentially masking the coherent electronic and nuclear quantum dynamics. For example, the bandwidth required to support an isolated 200-attosecond pulse around 15 eV is ∼5 eV. In contrast, the 10-fs VUV pulse train used here corresponds to a comb of VUV harmonics, each with a FWHM bandwidth of 183 meV, which can be tuned in the frequency domain (26) to coherently and selectively excite multiple electronic states.     

Watching energy transfer in metalloporphyrin heterodimers using stimulated X-ray Raman spectroscopy

Understanding the excitation energy transfer mechanism in multiporphyrin arrays is key for designing artificial light-harvesting devices and other molecular electronics applications. Simulations of the stimulated X-ray Raman spectroscopy signals of a Zn/Ni porphyrin heterodimer induced by attosecond X-ray pulses show that these signals can directly reveal electron–hole pair motions. These dynamics are visualized by a natural orbital decomposition of the valence electron wavepackets.Porphyrin rings are pyrole-based cyclic conjugated systems that serve as the main building blocks in many devices that depend on their high excitation energy transfer (EET) efficiency (1–4). Because of their stability and interesting structural, electronic, and optical properties, porphyrin compounds have a wide range of uses as chemical sensors (5), photosensitizers in photodynamic therapy for cancer (6), nonlinear optical materials (7–9), and molecular electronic (10–12) and spintronic devices (13, 14).Porphyrin-based molecules hold a pivotal position in the chemistry of engineered photoactive organic compounds, and extensive electronic structure calculations of monomeric (15) and oligomeric (16, 17) porphyrin molecules, porphyrin structures in biomacromolecules (18, 19), and quasi-1D and -2D porphyrin systems with infinite sizes have been carried out (13, 14, 20–23). Most applications involve multiporphyrin arrays, either in linear or in cyclic shape, or dendrimers (24). Porphyrin dimers, which are still small enough to be treated with relatively high-level modern quantum chemistry methods, can offer basic clues to track down the more complicated EET dynamics in multiporphyrin arrays.The kinetics of EET in multiporphyrin systems have long been studied by time-resolved fluorescence anisotropy decay (25) and pump–probe techniques, using visible light (26).Here we present a simulation study that shows how recently developed attosecond sources of X-ray pulses may be used to probe the energy transfer dynamics in a porphyrin dimer. Intense attosecond X-ray pulses, recently made available by new X-ray free electron laser (XFEL) (27, 28) and higher harmonic generation (29, 30) sources, have bandwidths covering multiple electron volts and can prepare coherent superpositions of valence electronically excited states through an impulsive Raman process (31). The short durations of these pulses make them ideal for tracing valence electronic dynamics that evolve with extremely short periods. X-ray pulses can also exploit the spectrally isolated core-excitation frequencies to create valence excitations in the neighborhood of a selected atom, a type of localized excitation not generally accessible using visible or UV pulses. An experimental realization of a two-color pump–probe X-ray source from XFEL radiation was recently reported (32). These new sources have also been used in time-dependent X-ray diffraction studies (33), to monitor ultrafast changes in the conductivity of semiconductors (34) and measure metal-to-ligand charge transfer in inorganic complexes (35) and the ultrafast dissociation of molecules adsorbed on a metal surface (36). Many of these techniques use the X-ray light source only for the probing pulse (37, 38) or depend on a detection method relying on ion or electron capture after the molecule interacts with the X-rays (39). All-X-ray photon-in, photon-out measurements are experimentally very difficult, and X-ray pump–probe measurements with attosecond pulses have yet to be performed. A series of theoretical studies on the time-domain, impulsive X-ray Raman signals of small organic molecules at the K-edges of second- and third-row elements have explored some of the unique capabilities of this technique (31).The porphyrin ring can chelate different metal atoms, changing the properties of the chromophore. This variability makes porphyrin systems ideal candidates for stimulated X-ray excitation; the spectrally isolated core transitions of the central metal atom allow them to act as X-ray dyes, creating local excitations through an X-ray Raman process.In this paper we simulate the stimulated X-ray Raman spectroscopy (SXRS) signals of a Zn-Ni porphyrin dimer linked by an ethynyl group. A natural orbital analysis (40, 41) of the electronic wavepacket that evolves during the delay between pulses is used to characterize the electron and hole dynamics after excitation.     

Excitation trapping and primary charge stabilization in Rhodopseudomonas viridis cells, measured electrically with picosecond resolution

The transmembrane primary charge separation in the photosynthetic bacterium Rhodopseudomonas viridis was monitored by electric measurements of the light-gradient type [Trissl, H. W. & Kunze, U. (1985) Biochim. Biophys. Acta 806, 136-144]. Excitation of whole cells with 30-ps laser pulses at either 532 nm or 1064 nm gave rise to a biphasic increase of the photovoltage. The fast phase, contributing about 50% of the total, rose with an exponential time constant ≤40 ps and was independent of the redox state of the quinone electron acceptor. It is assigned to the migration of the excitation energy in the antenna and its subsequent trapping by the reaction center, monitored by the ultrafast charge separation between the primary electron donor and the bacteriopheophytin intermediary acceptor. The slower phase (125 ± 50 ps) only occurred when the quinone was oxidized and disappeared when it was reduced (either chemically or photochemically). It is assigned to the forward electron transfer from the bacteriopheophytin to the quinone. The relative amplitudes of these two electrogenic steps demonstrate that the bacteriopheophytin intermediary acceptor is located halfway between the primary donor and the quinone.     

Ultrafast dynamics in atomic clusters: analysis and control

We present a study of dynamics and ultrafast observables in the frame of pump-probe negative-to-neutral-to-positive ion (NeNePo) spectroscopy illustrated by the examples of bimetallic trimers Ag2Au-/Ag2Au/Ag2Au+ and silver oxides Ag3O2-/Ag3O2/Ag3O2+ in the context of cluster reactivity. First principle multistate adiabatic dynamics allows us to determine time scales of different ultrafast processes and conditions under which these processes can be experimentally observed. Furthermore, we present a strategy for optimal pump-dump control in complex systems based on the ab initio Wigner distribution approach and apply it to tailor laser fields for selective control of the isomerization process in Na3F2. The shapes of pulses can be assigned to underlying processes, and therefore control can be used as a tool for analysis.     

Infrared PINEM developed by diffraction in 4D UEM

The development of four-dimensional ultrafast electron microscopy (4D UEM) has enabled not only observations of the ultrafast dynamics of photon–matter interactions at the atomic scale with ultrafast resolution in image, diffraction, and energy space, but photon–electron interactions in the field of nanoplasmonics and nanophotonics also have been captured by the related technique of photon-induced near-field electron microscopy (PINEM) in image and energy space. Here we report a further extension in the ongoing development of PINEM using a focused, nanometer-scale, electron beam in diffraction space for measurements of infrared-light-induced PINEM. The energy resolution in diffraction mode is unprecedented, reaching 0.63 eV under the 200-keV electron beam illumination, and separated peaks of the PINEM electron-energy spectrum induced by infrared light of wavelength 1,038 nm (photon energy 1.2 eV) have been well resolved for the first time, to our knowledge. In a comparison with excitation by green (519-nm) pulses, similar first-order PINEM peak amplitudes were obtained for optical fluence differing by a factor of more than 60 at the interface of copper metal and vacuum. Under high fluence, the nonlinear regime of IR PINEM was observed, and its spatial dependence was studied. In combination with PINEM temporal gating and low-fluence infrared excitation, the PINEM diffraction method paves the way for studies of structural dynamics in reciprocal space and energy space with high temporal resolution.Since its invention in the 1930s by Knoll and Ruska (1), the electron microscope has become a powerful tool in the fields of physics, chemistry, materials, and biology. A great variety of techniques related to the electron microscope has been developed in image, diffraction, and energy space (2, 3), with the spatial and energy resolutions of the transmission electron microscope now reaching 0.5 Å with Cs corrector (4) and sub-100 meV with electron monochromators (5, 6), respectively.To these capabilities of spatial and energy resolution has been added the high resolution in the fourth dimension (time) by the development of four-dimensional ultrafast electron microscopy (4D UEM) (7–9), currently enabling nanoscale dynamic studies with temporal resolution that is 10 orders of magnitude better than the millisecond range of video-camera-rate recording in conventional microscopes. In 4D UEM, ultrafast time resolution is reached by using two separate but synchronized ultrashort laser pulses, one to generate a probing electron pulse by photoemission at the microscope cathode and the other to excite the specimen into a nonequilibrium state. The state of the specimen within the window of time of the probe pulse can be observed by recording the probe electron packet scattered from the specimen in any of the different working modes of the microscope, such as image and diffraction (10), energy spectrum (11), convergent beam (12), or scanning transmission electron microscopy (TEM) (13). Scanning the time delay between arrival of the pump and probe pulses at the specimen, which is controlled by a precise optical delay line, allows the evolution of the specimen to be traced.One of the important techniques developed in, and unique to, UEM is photon-induced near-field electron microscopy (PINEM) (14). PINEM has extended the capability of UEM to observation of light–electron interactions near nanostructures or at an interface, which offers exciting prospects for the study of dynamics of photonics and plasmonics at the nanometer scale (15). The three-body interaction of photon, electron, and nanostructure relaxes momentum conservation and leads to efficient coupling between photons and electrons (16). In PINEM, an ultrashort optical pulse is used to excite evanescent electromagnetic fields near a nanostructure or at an interface. When the probe electron packet is in spatiotemporal overlap with these evanescent or scatter fields, some of its electrons can absorb/emit one or more scattered photons and then be detected by their contributions to displaced energy peaks in the electron energy spectrum. These displaced peaks appear as discrete sidebands to the zero-loss peak at separations given by the photon energy (hν) of the pump optical pulse. When using energy filtering to select for imaging only those electrons gaining energy, the resulting PINEM image reflects the strength and topology of the excited near field around the nanostructure or interface.The PINEM technique has been used to detect the evanescent near field surrounding a variety of structures with different materials properties and different geometries, such as carbon nanotubes (14), silver nanowires (14, 17), nanoparticles (16, 18), cells and protein vesicles (19), and several-atoms-thick graphene-layered steps (20). In addition, focused-beam PINEM has been used in scanning TEM mode to obtain induced near-field distributions for a copper grid bar (21), a nanometer gold tip (22), and a silver nanoparticle at the subparticle level (21). In a recent publication, three pulses, two optical and one electron, were introduced into the arsenal of techniques to gate the electron pulse and make its width only limited by the optical-pulse durations (23). Numerous general theoretical treatments (24–28) have successfully described the phenomenon, with detailed treatments quantitatively reproducing many unique features of these multifaceted experimental observations (17, 20–22).Despite the growing body of PINEM studies, almost all previously published PINEM results were obtained in the image mode of the electron microscope using optical pulses with wavelengths of 500–800 nm. An exception is a single unresolved PINEM spectrum for 1,038-nm excitation published from this laboratory (25). Because the PINEM response of a material is governed by its optical properties and dimensions relative to the wavelength of light, excitation wavelength is an important parameter largely remaining to be explored experimentally.Here we report the development of IR PINEM using excitation at the wavelength of 1,038 nm (photon energy 1.2 eV). The spatial- and fluence-dependent behavior of well-resolved IR PINEM induced at the edge of a copper grid bar is examined by combining nanometer-scale convergent-beam electron diffraction and diffraction-mode detection for electron-energy spectroscopy with an unprecedented energy resolution down to 0.63 eV at 200 keV. Different e-beam size effects were compared for PINEM generated by green and IR pump pulses. The spatial dependence of IR PINEM at the interface was studied at low-pulse fluence (linear regime) and high-pulse fluence (nonlinear regime). Diffraction of a gold crystal film was observed using the energy-resolved PINEM electrons produced by interaction with the scatter field of the adjacent copper grid edge. Notably, substantial PINEM peak amplitudes were achievable at dramatically lower fluence for IR pulses than for green pulses, opening up a possible path for studies of photosensitive materials. This general accessibility of strong PINEM signals is of particular importance for our primary interest of ultrafast dynamics, for which PINEM photon gating has the potential to vastly improve temporal resolution.All PINEM experiments reported here were performed on the California Institute of Technology UEM-2 apparatus. The operation voltage on UEM-2 is 200 keV. The laser system used emits a train of ∼220-fs pulses with wavelength of 1,038 nm, set to operate at a repetition rate of 1 MHz. The laser output was frequency-doubled two successive times to provide the 259-nm pulses used to generate the electron packet (probe beam) at the 200-keV microscope photocathode source. The residual 1,038-nm and 519-nm optical pulses were each available for use as the PINEM pump beam to excite the near-field plasmons at the interface. All of the experiments were carried out with polarization set to be perpendicular to the interface and in the single-electron regime (8) to eliminate space-charge effects. In diffraction mode, a camera length of 920 mm and a spectrometer entrance aperture of 1 mm were used to obtain a small collection angle for better energy resolution.     

4D multiple-cathode ultrafast electron microscopy

Four-dimensional multiple-cathode ultrafast electron microscopy is developed to enable the capture of multiple images at ultrashort time intervals for a single microscopic dynamic process. The dynamic process is initiated in the specimen by one femtosecond light pulse and probed by multiple packets of electrons generated by one UV laser pulse impinging on multiple, spatially distinct, cathode surfaces. Each packet is distinctly recorded, with timing and detector location controlled by the cathode configuration. In the first demonstration, two packets of electrons on each image frame (of the CCD) probe different times, separated by 19 picoseconds, in the evolution of the diffraction of a gold film following femtosecond heating. Future elaborations of this concept to extend its capabilities and expand the range of applications of 4D ultrafast electron microscopy are discussed. The proof-of-principle demonstration reported here provides a path toward the imaging of irreversible ultrafast phenomena of materials, and opens the door to studies involving the single-frame capture of ultrafast dynamics using single-pump/multiple-probe, embedded stroboscopic imaging.In 4D ultrafast electron microscopy (UEM), ultrafast light pulses generate electron packets by photoemission at the cathode of an electron microscope, and these are used to probe a dynamic process initiated by heating or exciting the microscopic specimen with a second, synchronized ultrafast light pulse (1, 2). In conventional implementations, each pump pulse on the specimen is accompanied by one suitably delayed laser pulse on the cathode to generate one packet of electrons probing a single time point in the evolution of the specimen. A record of the full course of temporal evolution of the specimen is then constructed by repeating the experiment multiple times with variation of the delay time between the two light pulses, reading out a separate CCD image for each delay time. Thus, information about different time points in the dynamic response of the specimen is obtained from different excitation events. This implementation is ideally suited for a specimen that undergoes irreversible but sufficiently well-defined dynamics to allow a new specimen area to be used for each time point (Fig. 1A), or for a specimen that recovers fully to allow repeated identical excitations of the same area (Fig. 1B); see also Methodology. The applications of these two approaches are numerous, as highlighted in a recent review account of the work (3).Open in a separate windowFig. 1.Variant implementations of UEM. (A) Single-pulse UEM, which enables single-shot imaging of homogeneous specimens. (B) Stroboscopic UEM. (D) Multiple-cathode UEM. For comparison, we include, in C, the single-cathode, deflection method. See text for details.For the study of completely nonrepetitive dynamics, for example, a stochastic process in a heterogeneous sample that does not return to its initial configuration, a series of snapshots following a single excitation event can provide the only direct and detailed view of the evolution. Observing the effects of a single excitation pulse with video-mode imaging can currently reach millisecond-scale time resolution, far short of the time scale for many phenomena of interest in nanoscale materials science, chemistry, and physics. Nanosecond resolution has been reached (4) by combining one excitation pulse with a train of light pulses on a single cathode, with deflection of the imaging electrons after passing the specimen plane to direct each successive pulse to a new region of the detector (Fig. 1C). This nanosecond method has been successfully used with a high-speed electrostatic deflector array to obtain time sequences of irreversible and stochastic processes (5, 6).Here we demonstrate a technique that removes any limit on time resolution imposed by image deflection and in a single frame enables the capture of ultrafast phenomena. With this approach, it is possible to probe and distinctly record multiple time points in a dynamic process following a single initiation pulse. The probing electron packets are all generated by a single light pulse that impinges on multiple, spatially distinct, cathode surfaces (Fig. 1D). Time separations between packets in the electron-pulse train are adjusted by the cathode spatial and electrostatic configuration. In the present application, two electron packets, generated from two source locations at the same potential and separated in time by 19 picoseconds (ps), are recorded on each CCD frame after undergoing diffraction in a gold film following femtosecond heating. The packets originate from different cathode locations, pass through the same area of the specimen, and are recorded at distinct locations on the detector, thereby encoding two different time points in the evolution of the specimen. The results obtained provide the basis for exploration of expanded application of the multiple-cathode concept.     

Femtosecond spectroscopy of excitation energy transfer and initial charge separation in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis

Reaction centers from the photosynthetic bacterium Rhodopseudomonas viridis have been excited within the near-infrared absorption bands of the dimeric primary donor (P), of the “accessory” bacteriochlorophylls (B), and of the bacteriopheophytins (H) by using laser pulses of 150-fsec duration. The transfer of excitation energy between H, B, and P occurs in slightly less than 100 fsec and leads to the ultrafast formation of an excited state of P. This state is characterized by a broad absorption spectrum and exhibits stimulated emission. It decays in 2.8 ± 0.2 psec with the simultaneous oxidation of the primary donor and reduction of the bacteriopheophytin acceptor, which have been monitored at 545, 675, 815, 830, and 1310 nm. Although a transient bleaching relaxing in 400 ± 100 fsec is specifically observed upon excitation and observation in the 830-nm absorption band, we have found no indication that an accessory bacteriochlorophyll is involved as a resolvable intermediary acceptor in the primary electron transfer process.     

Nonexponential fluorescence decay of aqueous tryptophan and two related peptides by picosecond spectroscopy

Time-resolved fluorescence spectroscopy of tryptophan and two related dipeptides, tryptophylalanine and alanyltryptophan, has been carried out on the subnanosecond time scale by using picosecond exciting pulses at a wavelength of 264 nm. Detection was with an ultrafast streak camera coupled to an optical multichannel analyzer. The zwitterions of these molecules show a definite nonexponential fluorescence decay which can be analyzed in terms of two exponentials. The two decay rates increase strongly with increasing temperature, as does the weight of the faster component. In tryptophan at pH 11, where the amino group is deprotonated, there remains only a single temperature-dependent exponential. The results are interpreted in terms of two kinds of trapped conformers in the excited state that interconvert no quicker than the time scale of the fluorescence. A model is suggested in which the nonradiative processes in one conformer approximate those in the bare indole moiety. The nonradiative decay rate of the other conformer is substantially faster. It is believed that the process responsible for this fast decay is intramolecular electron transfer from the indole to the amino acid side chain. The predilection for this electron transfer depends on steric relationships as well as on the electron-attracting power of the carbonyl group. This picture is consistent with earlier fluorescence quantum yield results. In fact, a self-consistent picture emerges from the temporal and yield data that quantitatively explains most important facets of tryptophan photochemistry in aqueous solution.