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We present molecular-frame measurements of the recombination dipole matrix element (RDME) in CO2, N2O, and carbonyl sulfide (OCS) molecules using high-harmonic spectroscopy. Both the amplitudes and phases of the RDMEs exhibit clear imprints of a two-center interference minimum, which moves in energy with the molecular alignment angle relative to the laser polarization. We find that whereas the angle dependence of this minimum is consistent with the molecular geometry in CO2 and N2O, it behaves very differently in OCS; in particular, the phase shift which accompanies the two-center minimum changes sign for different alignment angles. Our results suggest that two interfering structural features contribute to the OCS RDME, namely, (i) the geometrical two-center minimum and (ii) a Cooper-like, electronic-structure minimum associated with the sulfur end of the molecule. We compare our results to ab initio calculations using time-dependent density functional theory and present an empirical model that captures both the two-center and the Cooper-like interferences. We also show that the yield from unaligned samples of two-center molecules is, in general, reduced at high photon energies compared to aligned samples, due to the destructive interference between molecules with different alignments. [ABSTRACT FROM AUTHOR]

The first experimental observation of laser induced fluorescence from the furan cation, C4H4O+, is reported. The ion is formed in a supersonic free jet via a two photon ionization involving the 193 nm output of an ArF excimer laser. The B→X electronic transition is probed and vibrational assignments are presented for both states. Comparisons to the infrared, Raman, multiphoton ionization, and photoelectron spectrum of the parent molecule are made. The term value T00 for the B→X electronic transition is located at 24 675 cm-1, which differs drastically with previously reported values from both experimental and theoretical investigations. These differences are discussed and probable explanations proposed. The lifetime of the vibrationless level of the B state is measured to be 330 ns. [ABSTRACT FROM AUTHOR]

Short-wavelength free-electron lasers are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense X-ray pulses is their expected few-femtosecond duration. No measurement has succeeded so far in directly determining the temporal structure or even the duration of these ultrashort pulses in the few-femtosecond range. Here, by deploying the so-called streaking spectroscopy technique at the Linac Coherent Light Source, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. This method is independent of photon energy, decoupled from machine parameters, and provides an upper bound on the X-ray pulse duration. We measured the duration of the shortest X-ray pulses currently available to be on average no longer than 4.4 fs. Analysing the pulse substructure indicates a small percentage of the free-electron laser pulses consisting of individual high-intensity spikes to be on the order of hundreds of attoseconds. [ABSTRACT FROM AUTHOR]

We have developed an analytical theory explaining how the singleatom efficiency of high-harmonic generation scales with laser frequency, and verified this by numerically solving the time-dependent Schrödinger equation in three spatial dimensions. According to our saddle-point analysis of quantum paths, the imaginary part of the action has a significant impact on the scaling law. Furthermore, we found that the scaling law depends on the analytical properties of the ground-continuum transition matrix element. Our analysis elucidates how the relative contributions of different quantum orbits and their relative phases vary with the driving laser frequency and how the resulting quantumpath interferences in high-harmonic spectra can be controlled with an attosecond accuracy. [ABSTRACT FROM AUTHOR]

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum. [ABSTRACT FROM AUTHOR]

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and online means of determining the duration of femtosecond (>40 fs) x-ray pulses. In combination with a theoretical model of the process based on the softphoton approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120± 20 fs full-width at half-maximum (FWHM) and an upper limit of 40±20 fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results. [ABSTRACT FROM AUTHOR]

Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization, electron wave-packet drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques and sculpted laser pulses coupled with theoretical advances have led to a seemingly complete picture of this fundamental laser–atom interaction. Here, we describe an effect that seems to have escaped observation: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy, which becomes prominent using mid-infrared laser wavelengths (λ>1.0 μm). The low-energy structure is observed in all atoms and molecules investigated and thus seems to be universal. The structure is qualitatively reproduced by numerical solutions of the time-dependent Schrödinger equation but its physical origin is not yet identified. [ABSTRACT FROM AUTHOR]

The recent development of intense, ultrashort, table top lasers in the mid-infrared opens new avenues for research in strong field atomic physics. Electrons submitted to such radiation acquire huge quiver energies, even at moderate intensity and interesting properties arise: first, the wavelength offers a convenient experimental knob to tune the ionisation regime by controlling the Keldysh parameter. Second, many processes like above-threshold ionisation or high harmonic generation, whose characteristics depend directly on this energy, can be pushed to unprecedented limits. Third, the wavelength controls the spectral phase of the harmonics and hence the possibility to improve the generation of pulses in the attosecond regime. Recent studies of rare gas and alkali atoms' photoelectron spectra and harmonic generation at 2 and 3.6 μm have begun to confirm the theoretical predictions. However, unexpected features have also been found showing that strong field interaction still keeps some secrets after more than 40 years of investigation. [ABSTRACT FROM AUTHOR]

In 1964 Keldysh helped lay the foundations of strong-field physics by introducing a theoretical framework that characterized atomic ionization as a process that evolves with the intensity and wavelength of the fundamental field. Within this context, experiments have examined the intensity-dependent ionization but, except for a few cases, technological limitations have confined the majority to wavelengths below 1 μm. The development of intense, ultrafast laser sources in the mid-infrared (1 μm<λ<5 μm) region enables exploration of the wavelength scaling of the Keldysh picture while enabling new opportunities in strong-field physics, control of electronic motion and attosecond science. Here we report a systematic experimental investigation of the wavelength scaling in this region by concurrently analysing the production of energetic electrons and photons emitted by argon atoms interacting with few-cycle, mid-infrared fields. The results support the implicit predictions contained in Keldysh’s work, and pave the way to the realization of brighter and shorter attosecond pulsed light sources using longer-wavelength driving fields. [ABSTRACT FROM AUTHOR]

The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom-laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space. [ABSTRACT FROM AUTHOR]

We review the requirements for achieving full phase and amplitude characterization of attosecond X-ray pulses and propose several methods for carrying out such measurements. We show via numerical simulation that these techniques are capable of retrieving full field information of attosecond pulses and compare their performance and ease of implementation. [ABSTRACT FROM AUTHOR]

A systematic study of the temporal and spectral properties of high-harmonic generation was explored using a low-binding energy atom (caesium) excited by an intense mid-infrared (3-4 µm) laser pulse. The long-wavelength fundamental field produces odd-order high harmonic radiation, which is experimentally accessible with standard temporal and spectral metrology. We present measurements of the pulse width and coherence times of the 5th through 9th harmonic orders with their corresponding bandwidths. Our study demonstrates that the strategy of using a scaled laser-atom interaction can offer not only a practical solution for direct probing of the properties of high harmonic radiation but also a means for exploring the physics in novel conditions. We present results showing the enhanced harmonic emission from an 'incoherent' target of ground and excited state atoms. [ABSTRACT FROM AUTHOR]

Focuses on the significance of advances in ultra-short pulse laser systems to atomic and molecular dynamics. Atoms in strong optical fields; Strong-field physics of photodissociation; Coherent control of dissociation.

A new contactless technique for determination of excess carrier lifetime in semiconductors is demonstrated. The technique involves measuring the change in the transmitted intensity of a continuous probe beam (hνEg). The technique has been applied to silicon samples having different doping densities. The measured lifetime values in the range of 0.5–200 μs on both n-type and p-type silicon samples by this new technique agree well with the values obtained by the traditional photoconductive decay method. [ABSTRACT FROM AUTHOR]

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light-matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms. [ABSTRACT FROM AUTHOR]

High harmonic generation (HHG) is used to measure the spectral phase of the recombination dipole matrix element (RDM) in argon over a broad frequency range that includes the 3p Cooper minimum (CM). The measured RDM phase agrees well with predictions based on the scattering phases and amplitudes of the interfering s- and d-channel contributions to the complementary photoionization process. The reconstructed attosecond bursts that underlie the HHG process show that the derivative of the RDM spectral phase, the group delay, does not have a straightforward interpretation as an emission time, in contrast to the usual attochirp group delay. Instead, the rapid RDM phase variation caused by the CM reshapes the attosecond bursts. [ABSTRACT FROM AUTHOR]

Explosions of large Xe clusters (〈N〉) ∼ 11 000) irradiated by femtosecond pulses of 850 eV x-ray photons focused to an intensity of up to 1017 W/cm2 from the Linac Coherent Light Source were investigated experimentally. Measurements of ion charge-state distributions and energy spectra exhibit strong evidence for the formation of a Xe nanoplasma in the intense x-ray pulse. This x-ray produced Xe nanoplasma is accompanied by a three-body recombination and hydrodynamic expansion. These experimental results appear to be consistent with a model in which a spherically exploding nanoplasma is formed inside the Xe cluster and where the plasma temperature is determined by photoionization heating. [ABSTRACT FROM AUTHOR]

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum. [ABSTRACT FROM AUTHOR]

The article describes the influence of intense midinfrared laser field on the inelastic scattering of broadband electron wave packets. It explains how laser-driven ineslastic scattering could lead to nonsequential ionization (NSI) or higher processes or excitation. The authors conclude that their analysis of multiple ionization can be relevant to the establishment of a laser-driven scattering application in strong-field science.

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray--matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s ⇔ 2p resonance in singly ionized neon. This photondriven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies. [ABSTRACT FROM AUTHOR]

We investigate the harmonic generation induced by the interaction of a laser field with a solid target. The harmonic spectra is composed of the contribution of two processes interpreted as interband and intraband transitions. The interband process corresponds to the recombination from an upper band, populated during the laser interaction, to a lower band. The intraband process originates from nonlinear processes of the current in individual bands. In this Letter, we develop a theory based on Wannier states and reveal in depth the underlying physics of intraband dynamics. In particular, this approach highlights the determinant role of transitions between different lattice wells. Furthermore, our approach provides quantitative predictions concerning high-order harmonic energy cutoffs, harmonic yields, and emission times. [ABSTRACT FROM AUTHOR]

We report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of N2 in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength 1.3 μm driving laser to isolate the HOMO in the near threshold region, 19-67 eV. The precision of our group delay measurements reveals previously unseen angle-resolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations. [ABSTRACT FROM AUTHOR]