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We experimentally study the interaction between intense infrared few-cycle laser pulses and an ultrathin (∼2 µm) flat liquid sheet of isopropanol running in vacuum. We observe a rapid decline in transmission above a critical peak intensity of 50 TW/cm2 of the initially transparent liquid sheet, and the emission of a plume of material. We find both events are due to the creation of a surface plasma and are similar to processes observed in dielectric solids. After calculating the electron density for different laser peak intensities, we find an electron scattering rate of 0.3 fs-1 in liquid isopropanol to be consistent with our data. We study the dynamics of the plasma plume to find the expansion velocity of the plume front.

An atomic-level picture of molecular and bulk processes, such as chemical bonding and charge transfer, necessitates an understanding of the dynamical evolution of these systems. On the ultrafast timescales associated with nuclear and electronic motion, the temporal behaviour of a system is often interrogated in a 'pump-probe' scheme. Here, an initial 'pump' pulse triggers dynamics through photoexcitation, and after a carefully controlled delay a 'probe' pulse initiates projection of the instantaneous state of the evolving system onto an informative measurable quantity, such as electron binding energy. In this paper, we apply spectral ghost imaging to a pump-probe time-resolved experiment at an X-ray free-electron laser (XFEL) facility, where the observable is spectral absorption in the X-ray regime. By exploiting the correlation present in the shot-to-shot fluctuations in the incoming X-ray pulses and measured electron kinetic energies, we show that spectral ghost imaging can be applied to time-resolved pump-probe measurements. In the experiment presented, interpretation of the measurement is simplified because spectral ghost imaging separates the overlapping contributions to the photoelectron spectrum from the pump and probe pulse.

Even within the sudden approximation, the removal of one neutral Hartree-Fock orbital does not leave the molecule in a pure one-hole state of the cation; coupling to virtual orbitals leads to a breakdown in the molecular orbital picture. This can be understood as interaction with two-hole one-particle states (singly excited cations) and becomes significant when those states are energetically similar to the one-hole states. This splitting of states is most relevant for inner valence holes which lie above the double ionisation potential. As such, impulsive creation of an inner valence hole (IVH) wavepacket results in a time dependent charge density which is purely electronic in nature (charge migration) [1] , [2] , though it depends on the nuclear geometry at the time of excitation, and subsequent nuclear motion will result in decoherence [3] .

The rate of successful identification of peptide sequences by tandem mass spectrometry (MS/MS) is adversely affected by the common occurrence of co-isolation and co-fragmentation of two or more isobaric or isomeric parent ions. This results in so-called `chimera spectra’, which feature peaks of the fragment ions from more than a single precursor ion. The totality of the fragment ion peaks in chimera spectra cannot be assigned to a single peptide sequence, which contradicts a fundamental assumption of the standard automated MS/MS spectra analysis tools, such as protein database search engines. This calls for a diagnostic method able to identify chimera spectra to single out the cases where this assumption is not valid. Here, we demonstrate that, within the recently developed two-dimensional partial covariance mass spectrometry (2D-PC-MS), it is possible to reliably identify chimera spectra directly from the two-dimensional fragment ion spectrum, irrespective of whether the co-isolated peptide ions are isobaric up to a finite mass accuracy or isomeric. We introduce ‘3-57 chimera tag’ technique for chimera spectrum diagnostics based on 2D-PC-MS and perform numerical simulations to examine its efficiency. We experimentally demonstrate the detection of a mixture of two isomeric parent ions, even under conditions when one isomeric peptide is at one five-hundredth of the molar concentration of the second isomer.

We have generated isolated attosecond pulses and performed attosecond streaking measurements using a two-colour synthesized laser field consisting of a strong near-infrared few-cycle pulse and a weaker multi-cycle pulse centred at 400 nm. An actively stabilized interferometer was used to coherently combine the two pulses. Using attosecond streaking we characterised the electric fields of the two pulses and accurately retrieved the spectrum of the multi-cycle pulse. We demonstrated a two-fold increase in the flux of isolated attosecond pulses produced and show that their duration was minimally affected by the presence of the weaker field due to spectral filtering by a multilayer mirror.

International audience

The development of attosecond pulses across different photon energies is an essential precursor to performing pump–probe attosecond experiments in complex systems, where the potential of attosecond science1 can be further developed2,3. We report the generation and characterization of synchronized extreme ultraviolet (90 eV) and vacuum ultraviolet (20 eV) pulses, generated simultaneously via high-harmonic generation. The vacuum ultraviolet pulses are well suited for pump–probe experiments that exploit the high photo-ionization cross-sections of many molecules in this spectral region4 as well as the higher photon flux due to the higher conversion efficiency of the high harmonic generation process at these energies5. We temporally characterized all pulses using the attosecond streaking technique6 and the FROG-CRAB retrieval method7. We report 576 ± 16 as pulses at 20 eV and 257 ± 21 as pulses at 90 eV. Our demonstration of synchronized attosecond pulses at different photon energies, which are inherently jitter-free due to the common-path geometry implemented, offers unprecedented possibilities for pump–probe studies.

We demonstrate angle-resolved coherent (ARC) wave mixing using 4 fs light pulses derived from a laser source that spans 550-1000 nm. We believe this to be the shortest pulse duration used to date in coherent multi-dimensional spectroscopy. The marriage of this ultra-broad band, few-cycle coherent source with the ARC technique will permit new investigations of the interplay between energy transfers and quantum superposition states spanning 8200 cmsup-1/sup. We applied this configuration to measurements on the photosynthetic low light (LL) complex from Rhodopseudomonas palustris in solution at ambient temperature. We observe bi-exponential population dynamics for energy transfer across 5500 cmsup-1/sup(0.65 eV), which we attribute to energy transfer from the Qsubx/subtransition of bacteriochlorophylls to the B850 pigment of the complex. We believe for the first time, to the best of our knowledge, we demonstrate that ARC maps can be recorded using a single laser pulse.

In this work, nanosecond laser ablation plasmas generated at 266 and 1064 nm of the two pyrimidine nucleobases uracil and thymine were characterized using time-of-flight mass spectrometry, optical emission spectroscopy and temporally resolved third harmonic generation of a probe laser. This multiple technique approach provides insight into the role played by the irradiation wavelength on the composition and dynamics of plasma species and on the differences between the laser plasmas of the two nucleobases.

International audience

International audience; We theoretically study high-harmonic generation in toluene, ortho-xylene and fluorobenzene driven by a 1.8 μm ultrashort pulse. We find that the chemical substitutions have a strong influence on the amplitude and phase of the emission from the highest occupied molecular orbital, despite having a small influence on the orbital itself. We show that this influence is due to the tunnel ionization step, which depends critically on the sign and amplitude of the asymptotic part of the wave function. We discuss how these effects would manifest in phase-sensitive high-harmonic generation spectroscopy experiments.

We investigate how short and long electron trajectory contributions to high harmonic emission and their interferences give access to intra-molecular dynamics. In the case of unaligned molecules, we show experimental evidences that the long trajectory signature is more dependent upon the molecule than the short one, providing a high sensitivity to cation nuclear dynamics within 100's of as to few fs. Using theoretical approaches based on Strong Field Approximation and Time Dependent Schrodinger Equation, we examine how quantum path interferences encode electronic motion whilst molecules are aligned. We show that the interferences are dependent on channels superposition and upon which ionisation channel is involved. In particular, quantum path interferences encodes electronic migration signature while coupling between channels is allowed by the laser field. Hence, molecular quantum path interferences is a promising method for Attosecond Spectroscopy, allowing the resolution of ultra-fast charge migration in molecules after ionisation in a self-referenced manner., Comment: 10 pages 9 figures 1 table

The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.

Wepresent the first measurement of the vectorial response of strongly dressed helium atoms probed by an attosecond pulse train (APT) polarised either parallel or perpendicular to the dressing field polarisation. The transient absorption is probed as a function of delay between the APT and the Linearly polarised 800 nmfield of peak intensity 1.3∗1014 W cm-2. The APT spans the photon energy range 1642 eV, covering the first ionisation energy of helium (24.59 eV). With parallel polarised dressing and probing fields, we observe modulations with periods of one half and one quarter of the dressing field period. When the polarisation of the dressing field is altered from parallel to perpendicular with respect to the APT polarisation we observe a large suppression in the Modulation depth of the above ionisation threshold absorption. In addition to this we present the intensity dependence of the harmonic modulation depth as a function of delay between the dressing and probe fields, with dressing field peak intensities ranging from 2∗1012 to 2∗1014 W cm-2.We compare our experimental results with a full-dimensional solution of the single-atom time-dependent (TD) Schrdinger equation obtained using the recently developed abinitio TDB-spline ADCmethod and find good qualitative agreement for the above threshold harmonics.

The challenge of measuring ultrafast electron dynamics will be explained. Recent progress on soft X-ray sources based upon XFELs and HHG and their appication to time resolved absorption spectroscopy and related techniques will be described.

We report on the characterization of space-time couplings in high energy sub-2-cycle 770nm laser pulses using a self-referencing single-shot method. Using spatially-encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction (SEA-F-SPIDER) we characterize few-cycle pulses with a wave-front rotation of 2.8x?10^11 rev/sec (1.38 mrad per half-cycle) and pulses with pulse front tilts ranging from to -0.33 fs/um to -3.03 fs/um., Comment: 6 pages, 6 figures

1 pags. -- Correction to: Scientific Reports https://doi.org/10.1038/s41598-018-22055-9, published online 28 February 2018, A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Rapid proton migration is a key process in hydrocarbon photochemistry. Charge migration and subsequent proton motion can mitigate radiation damage when heavier atoms absorb X-rays. If rapid enough, this can improve the fidelity of diffract-before-destroy measurements of biomolecular structure at X-ray-free electron lasers. Here we study X-ray-initiated isomerization of acetylene, a model for proton dynamics in hydrocarbons. Our time-resolved measurements capture the transient motion of protons following X-ray ionization of carbon K-shell electrons. We Coulomb-explode the molecule with a second precisely delayed X-ray pulse and then record all the fragment momenta. These snapshots at different delays are combined into a ‘molecular movie’ of the evolving molecule, which shows substantial proton redistribution within the first 12 fs. We conclude that significant proton motion occurs on a timescale comparable to the Auger relaxation that refills the K-shell vacancy., United States. Dept. of Energy. Office of Basic Energy Sciences

Using electron spectroscopy, we have investigated nanoplasma formation from noble gas clusters exposed to high-intensity hard-x-ray pulses at ~5 keV. Our experiment was carried out at the SPring-8 Angstrom Compact free electron LAser (SACLA) facility in Japan. Dedicated theoretical simulations were performed with the molecular dynamics tool XMDYN. We found that in this unprecedented wavelength regime nanoplasma formation is a highly indirect process. In the argon clusters investigated, nanoplasma is mainly formed through secondary electron cascading initiated by slow Auger electrons. Energy is distributed within the sample entirely through Auger processes and secondary electron cascading following photoabsorption, as in the hard x-ray regime there is no direct energy transfer from the field to the plasma. This plasma formation mechanism is specific to the hard-x-ray regime and may, thus, also be important for XFEL-based molecular imaging studies. In xenon clusters, photo- and Auger electrons contribute more significantly to the nanoplasma formation. Good agreement between experiment and simulations validates our modelling approach. This has wide-ranging implications for our ability to quantitatively predict the behavior of complex molecular systems irradiated by high-intensity hard x-rays., 超強力X線による極微小プラズマ生成を発見 -X線自由電子レーザーを利用したイメージングに重要なメッセージ-. 京都大学プレスリリース. 2015-07-07.

The response of solids to electromagnetic fields is of crucial importance in many areas of science and technology. Many fundamental questions remain to be answered about the dynamics of the photoexcited electrons that underpin this response, which can evolve on timescales of tens to hundreds of attoseconds. How, for example, is the photoexcited electron affected by the periodic potential as it travels in the solid, and how do the other electrons respond in these strongly correlated systems? Furthermore, control of electronic motion in solids with attosecond precision would pave the way for the development of ultrafast optoelectronics. Attosecond electron dynamics can be traced using streaking, a technique in which a strong near-infrared laser field accelerates an attosecond electron wavepacket photoemitted by an extreme ultraviolet light pulse, imprinting timing information onto it. We present attosecond streaking measurements on the wide-bandgap semiconductor tungsten trioxide, and on gold, a metal used in many nanoplasmonic devices. Information about electronic motion in the solid is encoded on the temporal properties of the photoemitted electron wavepackets, which are consistent with a spread of electron transport times to the surface following photoexcitation. (C) 2015 Optical Society of America

We investigate high-order harmonic spectra from aligned diatomic molecules in intense driving fields whose components have orthogonal polarizations. We focus on how the driving-field ellipticity influences structural interference patterns in a macroscopic medium. In a previous publication [Phys. Rev. A 88, 023404 (2013)] we have shown that the non-vanishing ellipticity introduces an effective dynamic shift in the angle for which the two-center interference maxima and minima occur, with regard to the existing condition for linearly polarized fields. In this work we show through simulation that it is still possible to observe this shift in harmonic spectra that have undergone macroscopic propagation, and discuss the parameter range for doing so. These features are investigated for $H_2$ in a bichromatic field composed of two orthogonally polarized waves. The shift is visible both in the near- and in the far-field regime, so that, in principle, it can be observed in experiments., Comment: 9 pages, 6 figures

This review paper provides a general introduction to the interaction of intense (>1015 W cm−2), femtosecond laser pulses with atomic clusters in the size range 500–105 atoms. A nanoplasma model of the laser–cluster interaction is used to elucidate the underlying physics. Measurements of ion emission from the laser–cluster interaction are presented together with numerical simulations. Emerging applications are described.

We present detailed analysis of the recently proposed technique of high-order-harmonic generation spectroscopy of correlation-driven electron hole dynamics in atoms and molecules. This novel technique resolves Auger-type processes with attosecond-scale resolution by clocking the decay process with high-harmonic generation. The harmonic generation is driven by an attosecond, XUV pump pulse and a long-duration, infrared pulse. We present the strong-field-approximation-based theory of such an XUV-initiated high-order-harmonic generation process. We detail different ways of recovering the hole survival probability by altering experimental parameters to change the time-energy mapping of the harmonics. The various reconstruction methods are then simulated for ${M}_{4,5}\mathit{NN}$ Auger decay in krypton and molecular-orbital breakdown dynamics in trans-butadiene and propanal.

Quasifree field-driven electron trajectories are a key element of strong-field dynamics. Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecond-duration extreme ultaviolet emission in the process of high-harmonic generation. The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer and the efficient return of the launched electron trajectories. It has been predicted that these limits can be significantly exceeded by an appropriately ramped-up cycle shape [L. E. Chipperfield et al., Phys. Rev. Lett. 102, 063003 (2009)]. Here, we present an experimental realization of similar cycle-shaped waveforms and demonstrate control of the high-harmonic generation process on the single-atom quantum level via attosecond steering of the electron trajectories. With our improved optical cycles, we boost the field ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration. We demonstrate, in realistic experimental conditions, 2 orders of magnitude enhancement of the generated extreme ultraviolet flux together with an increased spectral extension. This application, which is only one example of what can be achieved with cycle-shaped high-field light waves, has significant implications for attosecond spectroscopy and molecular self-probing.

Ultrafast hole dynamics created in molecular systems as a result of sudden ionisation is the focus of much attention in the field of attosecond science. Using the molecule glycine we show through ab initio simulations that the dynamics of a hole, arising from ionisation in the inner valence region, evolves with a timescale appropriate to be measured using X-ray pulses from the current generation of SASE free electron lasers. The examined pump–probe scheme uses X-rays with photon energy below the K edge of carbon (275–280 eV) that will ionise from the inner valence region. A second probe X-ray at the same energy can excite an electron from the core to fill the vacancy in the inner-valence region. The dynamics of the inner valence hole can be tracked by measuring the Auger electrons produced by the subsequent refilling of the core hole as a function of pump–probe delay. We consider the feasibility of the experiment and include numerical simulation to support this analysis. We discuss the potential for all X-ray pump-X-ray probe Auger spectroscopy measurements for tracking hole migration.

We have measured multi-keV protons and ∼100 keV iodine ions from the interaction of intense, femtosecond laser pulses with atomic Xe clusters and mixed-species HI clusters. The explosion dynamics of the HI clusters differs from the pure H and I cluster cases, with the iodine energies being substantially reduced. Mixed-species clusters provide a possible route for boosting the explosion energies of low-Z ions, and extend the advantages of the laser-cluster interaction to species that do not readily form pure clusters.

The tunnelling of a bound electron out of an atom in a laser field is a well-known quantum-mechanical process. But it happens very quickly, and it takes some fast work with X-rays and lasers to see it in action.

The development of high–intensity short–pulse lasers by chirped pulse amplification is described and the use of short intense pulses for the study of atoms and molecules is discussed. As a case study, the dynamics of the ionization and dissociation of clusters of noble gas atoms is reviewed with special emphasis on the generation of energetic electrons and highly charged ions.

Enhanced single harmonic generation is analyzed in indium laser ablation plasmas at excitation conditions of multicycle (30 fs) and few-cycle (3.5 fs) pulses. We demonstrate the strong influence of pulse duration, on the emission spectra from the indium plasma. For few-cycle pulses, the enhanced emissions do not coincide with the expected harmonic wavelengths, which is the case for multicycle pulses. We test the coherent properties of an enhanced emission around 20 eV using polarization and double-slit interference techniques. We also characterize the dynamics of the emissions from the indium plasma by tuning the laser pulse duration. A theoretical analysis is presented to describe the indium plasma emission upon excitation by few-cycle pulses. © 2013 American Physical Society.

Attosecond light pulses within the vacuum ultraviolet (VUV) energy range are predicted by solving the time-dependent Schr\"odinger equation (TDSE) for a model neon atom in short laser pulses of different field polarization states. We compare high-order harmonic generation in linearly polarized laser pulses to the method of polarization gating and find attosecond pulses that approach the Fourier limit of 700 as given by an indium filter, spectrally centered at 15 eV. At such low energies, harmonic generation has low sensitivity to ellipticity, which enables the generation of elliptically polarized attosecond pulses. We also show that emission at the atomic transition energies is strongly damped by including intensity averaging.

The angular dependence of nonsequential double ionisation and dissociation induced by laser driven inelastic electron rescattering was investigated experimentally in aligned CO2 . A clear dependence on the recollision angle was found demonstrating quantum control of ionisation and dissociation processes in the strong field regime.

We report on high-order-harmonic generation in nanoparticle-containing laser ablation plasmas using a Ti:sapphire driving laser at a repetition rate of 1 kHz and on the relation of the harmonic signals with the morphological and spectroscopic characteristics of the ablation deposits. The study of the deposits has allowed the definition of ablation conditions, leading to the generation of a plasma containing nanoparticles of similar sizes to the ones used as target material. The corresponding plasma plumes lead to efficient conversion of the IR photons towards the short-wavelength range through high-order harmonic generation and show superior performance to those corresponding to bulk targets of the same metallic element. © 2013 American Physical Society.

We use a high harmonic generated supercontinuum in the soft X-ray region to measure X-ray absorption near edge structure (XANES) spectra in polythiophene (poly(3-hexylthiophene)) films at multiple absorption edges. A few-cycle carrier-envelope phase-stable laser pulse centered at 1800 nm was used to generate a stable soft X-ray supercontinuum, with amplitude gating limiting the generated pulse duration to a single optical half-cycle. We report a quantitative transmission measurement of the sulfur L2,3 edge over the range 160-200 eV and the carbon K edge from 280 to 330 eV. These spectra show all the features previously reported in the XANES spectra of polythiophene, but for the first time they are measured with a source that has an approximately 1 fs pulse duration. This study opens the door to measurements that can fully time-resolve the photoexcited electronic dynamics in these systems.

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 μm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 μJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

We report studies of high-order harmonic generation in laser-produced manganese plasmas using sub-4-fs drive laser pulses. The measured spectra exhibit resonant enhancement of a small spectral region of about 2.5 eV width around the 31st harmonic (~50eV). The intensity contrast relative to the directly adjacent harmonics exceeds one order of magnitude. This finding is in sharp contrast to the results reported previously for multi-cycle laser pulses [Physical Review A 76, 023831 (2007)]. Theoretical modelling suggests that the enhanced harmonic emission forms an isolated sub-femtosecond pulse.

We report on the spatially resolved full amplitude and phase characterization of mid-infrared high intensity laser pulses generated in a three stage OPA. We use a spatially-encoded arrangement (SEA-)SPIDER with spectral filters for ancilla generation for spatially resolved characterization. Using five interchangeable filter sets we are able to characterize pulses from 1 to 2{\mu}m with one single device with minimal adjustments., Comment: 6 pages, 5 figures

High-order harmonic generation in graphite-ablated plasmas was systematically studied using ultrashort (3.5 and 30 fs) laser pulses. We observed the efficient frequency conversion of 3.5 fs Ti:sapphire laser pulses in the range of 15-26eV. Stabilization of the harmonic yield at a 1kHz pulse repetition rate was accomplished using a rotating graphite target. We also show the results of harmonic generation in carbon plasma using 1300nm, 40ps pulses, which allowed the extension of the harmonic cutoff while maintaining a comparable conversion efficiency to the case of 780nm driving radiation. The time-of-flight mass spectrometric analysis of the plasma components and the scanning electron microscopy of plasma debris under optimal conditions for harmonic generation suggest the presence of small carbon clusters (C 10-C 30) in the plasma plume at the moment of femtosecond pulse propagation, which further aggregate on nearby substrates. We present the results of plasma spectroscopy obtained under unoptimized plasma conditions that elucidate the reduction in harmonic signal. We also present calculations of plasma concentration under different excitation conditions of the ablated graphite target.

Ultrafast science has begun to tackle the measurement of electronic and chemical processes taking place on the few-femtosecond-to-attosecond timescale. This field requires high-power, extremely short-duration laser pulses. Here we review progress towards the generation of such pulses by Raman scattering in a medium whose component molecules oscillate in phase, which modulates the optical polarizability of the medium and generates high-order Raman sidebands on a field propagating through it. This process may occur with high efficiency and thus lead to sufficient bandwidth for supporting few-femtosecond to attosecond pulses. Significant progress has recently been made in the use of this technique to deliver useable ultrashort pulses in the visible to ultraviolet regions of the spectrum.

We demonstrate control of short and long quantum trajectories in high harmonic emission through the use of an orthogonally polarized two-color field. By controlling the relative phase ϕ between the two fields we show via classical and quantum calculations that we can steer the two-dimensional trajectories to return, or not, to the core and so control the relative strength of the short or long quantum trajectory contribution. In experiments, we demonstrate that this leads to robust control over the trajectory contributions using a drive field from a femtosecond laser composed of the fundamental ω at 800 nm (intensity ∼1.2×10(14) W cm(-2)) and its weaker orthogonally polarized second harmonic 2ω (intensity ∼0.3×10(14) W cm(-2)) with the relative phase between the ω and 2ω fields varied simply by tilting a fused silica plate. This is the first demonstration of short and long quantum trajectory control at the single-atom level.

High-order harmonic generation in laser-produced plasma was demonstrated on metal surfaces using a high pulse repetition rate laser (1 kHz). It was found that if sufficiently thick bulk targets were used thermal conduction was sufficient to prevent any deterioration in the metal plasma plume production at this repetition rate. Under appropriate conditions harmonic yields were found to remain stable for >105 laser shots without the need to move the target. The high repetition rate allowed both short and long electron trajectories to be clearly identified during harmonic generation from the metal plasmas. © 2011 Taylor & Francis., This research was supported by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme, EPSRC programme (Grant EP/ E028063/1), and FASTQUAST ITN. M.E. Lo´pez-Arias acknowledges CSIC for a JAE-pre contract.