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Coffee, R. N.

Publications and source records attributed to Coffee, R. N..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Few-femtosecond resolved imaging of laser-driven nanoplasma expansion

Abstract The free expansion of a planar plasma surface is a fundamental non-equilibrium process relevant for various fields but as-yet experimentally still difficult to capture. The significance of the associated spatiotemporal plasma motion ranges from astrophysics and controlled fusion to laser machining, surface high-harmonic generation, plasma mirrors, and laser-driven particle acceleration. Here, we show that x-ray coherent diffractive imaging can surpass existing approaches and enables the quantitative real-time analysis of the sudden free expansion of laser-heated nanoplasmas. For laser-ionized SiO 2 nanospheres, we resolve the formation of the emerging nearly self-similar plasma profile evolution and expose the so far inaccessible shell-wise expansion dynamics including the associated startup delay and rarefaction front velocity. Our results establish time-resolved diffractive imaging as an accurate quantitative diagnostic platform for tracing and characterizing plasma expansion and indicate the possibility to resolve various laser-driven processes including shock formation and wave-breaking phenomena with unprecedented resolution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Megahertz-compatible angular streaking with few-femtosecond resolution at x-ray free-electron lasers

Highly brilliant, coherent, femtosecond x-ray pulses delivered by free-electron lasers (FELs) constitute one of the pillars of modern ultrafast science. Next generation FEL facilities provide up to megahertz repetition rates and pulse durations down to the attosecond regime utilizing self-amplification of spontaneous emission. However, the stochastic nature of this generation mechanism demands single-shot pulse characterization to perform meaningful experiments. Here we demonstrate a fast yet robust online analysis technique capable of megahertz-rate mapping of the temporal intensity structure and arrival time of x-ray FEL pulses with few-femtosecond resolution. We performed angular streaking measurements of both neon photo- and Auger electrons and show their applicability for a direct time-domain feedback system during ongoing experiments. Here, the fidelity of the real-time pulse characterization algorithm is corroborated by resolving isolated x-ray pulses and double pulse trains with few-femtosecond substructure, thus paving the way for x-ray-pump–x-ray-probe FEL science at repetition rates compatible with the demands of LCLS-II and European XFEL.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon

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

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Attosecond time–energy structure of X-ray free-electron laser pulses

Here, the time–energy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modulations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Nonlinear Ultrafast Spin Scattering in the Skyrmion Phase of Cu 2 OSeO 3

Ultrafast x-ray scattering studies of the topological Skyrmion phase in Cu 2 OSeO 3 show the dynamics to be strongly dependent on the excitation energy and fluence. At high photon energies, where the electron-spin scattering cross section is relatively high, the excitation of the topological Skyrmion phase shows a nonlinear dependence on the excitation fluence, in contrast to the excitation of the conical phase which is linearly dependent on the excitation fluence. Finally, the excitation of the Skyrmion order parameter is nonlinear in the magnetic excitation resulting from scattering during electron-hole recombination, indicating different dominant scattering processes in the conical and Skyrmion phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning

Free-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. Lastly, this opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Emitter-site-selective photoelectron circular dichroism of trifluoromethyloxirane

The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane, C 3 H 3 F 3 O, is studied experimentally and theoretically. Thereby, we investigate the photoelectron circular dichroism (PECD) for nearly symmetric O 1s and F 1s electronic orbitals, which are localized on different molecular sites. The respective dichroic β 1 and angular distribution β 2 parameters are measured at the photoelectron kinetic energies from 1 to 16 eV by using variably polarized synchrotron radiation and velocity map imaging spectroscopy. The present experimental results are in good agreement with the outcome of ab initio electronic structure calculations. We report a sizable chiral asymmetry β 1 of up to about 9% for the K -shell photoionization of oxygen atom. For the individual fluorine atoms, the present calculations predict asymmetries of similar size. However, being averaged over all fluorine atoms, it drops down to about 2%, as also observed in the present experiment. Our study demonstrates a strong emitter and site sensitivity of PECD in the one-photon inner-shell ionization of this chiral molecule.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS

Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination with a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Dual crystal x-ray spectrometer at 1.8 keV for high repetition-rate single-photon counting spectroscopy experiments

The recent development of high-repetition rate x-ray free electron lasers (FEL), makes it possible to perform x-ray scattering and emission spectroscopy measurements from thin foils or gasses heated to high-energy density conditions by integrating over many experimental shots. Since the expected signal may be weaker than the typical CCD readout noise over the region-of-interest, it is critical to the success of this approach to use a detector with high-energy resolution so that single x-ray photons may be isolated. We describe a dual channel x-ray spectrometer developed for the Atomic and Molecular Optics endstation at the Linac Coherent Light Source (LCLS) for x-ray spectroscopy near the K-edge of aluminum. The spectrometer is based on a pair of curved PET (002) crystals coupled to a single pnCCD detector which simultaneously measures x-ray scattering and emission in the forward and backward directions. Furthermore, the signals from single x-ray photons are accumulated permitting continuous single-shot acquisition at 120 Hz.

42 ENGINEERING↗
DOE OSTI2015

Disentangling formation of multiple-core holes in aminophenol molecules exposed to bright X-FEL radiation

Competing multi-photon ionization processes, some leading to the formation of double core hole states, have been examined in 4-aminophenol. The experiments used the linac coherent light source (LCLS) x-ray free electron laser, in combination with a time-of-flight magnetic bottle electron spectrometer and the correlation analysis method of covariance mapping. Furthermore, the results imply that 4-aminophenol molecules exposed to the focused x-ray pulses of the LCLS sequentially absorb more than two x-ray photons, resulting in the formation of multiple core holes as well as in the sequential removal of photoelectrons and Auger electrons (so-called PAPA sequences).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗