Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 39
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
2.
Nat Commun ; 8: 15461, 2017 06 05.
Artigo em Inglês | MEDLINE | ID: mdl-28580940

RESUMO

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. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.

3.
Phys Rev Lett ; 117(15): 153003, 2016 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-27768351

RESUMO

Time-resolved femtosecond x-ray diffraction patterns from laser-excited molecular iodine are used to create a movie of intramolecular motion with a temporal and spatial resolution of 30 fs and 0.3 Å. This high fidelity is due to interference between the nonstationary excitation and the stationary initial charge distribution. The initial state is used as the local oscillator for heterodyne amplification of the excited charge distribution to retrieve real-space movies of atomic motion on ångstrom and femtosecond scales. This x-ray interference has not been employed to image internal motion in molecules before. Coherent vibrational motion and dispersion, dissociation, and rotational dephasing are all clearly visible in the data, thereby demonstrating the stunning sensitivity of heterodyne methods.

4.
Nat Commun ; 5: 4235, 2014 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-24953740

RESUMO

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

5.
Opt Lett ; 39(18): 5325-8, 2014 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-26466262

RESUMO

Using balanced detection in both the radio frequency (RF) and the optical domain, we remotely synchronize the repetition rate of a Ti:sapphire oscillator to an Er-doped fiber oscillator through a 360 m length-stabilized dispersion compensated fiber link. The drift between these two optical oscillators is 3.3 fs root mean square (rms) over 24 hours. The 68 MHz Er-doped fiber oscillator is locked to a 476 MHz local RF reference clock, and serves as a master clock to distribute 10 fs-level timing signals through stabilized fiber links. This steady remote two-color optical-to-optical synchronization is an important step toward an integrated femtosecond fiber timing distribution system for free-electron lasers (FELs); it does not require x-ray pulses, and it makes sub-10-fs optical/x-ray pump-probe experiments feasible.

6.
Faraday Discuss ; 163: 475-84 discussion 513-43, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24020217

RESUMO

Recent measurements of the transient laser-induced fragmentation of photoexcited 1,3-cyclohexadiene undergoing isomerization have seen that some of the fragments have higher velocity at the pump-probe delay corresponding to the time of passage through a conical intersection (Coln). (http://arxiv.org/abs/1212.3728) Here we explore several possible explanations related to the electronic and ionic structure of molecules in the vicinity of Colns and suggest some experimental tests that could illuminate the mechanism and ultimately control this behaviour.

7.
J Chem Phys ; 136(5): 054303, 2012 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-22320738

RESUMO

We use spectral unmixing to determine the number of transient photoproducts and to track their evolution following the photo-excitation of 1,3-cyclohexadiene (CHD) to form 1,3,5-hexatriene (HT) in the gas phase. The ring opening is initiated with a 266 nm ultraviolet laser pulse and probed via fragmentation with a delayed intense infrared 800 nm laser pulse. The ion time-of-flight (TOF) spectra are analyzed with a simplex-based spectral unmixing technique. We find that at least three independent spectra are needed to model the transient TOF spectra. Guided by mathematical and physical constraints, we decompose the transient TOF spectra into three spectra associated with the presence of CHD, CHD(+), and HT, and show how these three species appear at different times during the ring opening.


Assuntos
Cicloexenos/química , Gases , Espectrometria de Massas por Ionização e Dessorção a Laser Assistida por Matriz , Fatores de Tempo , Raios Ultravioleta
8.
Phys Rev Lett ; 109(26): 263001, 2012 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-23368555

RESUMO

We investigate molecular dynamics of multiple ionization in N2 through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short x-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of the multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.

9.
Phys Rev Lett ; 107(23): 233001, 2011 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-22182083

RESUMO

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 photon-driven 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.

10.
Phys Rev Lett ; 106(8): 083002, 2011 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-21405568

RESUMO

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.

11.
Phys Rev Lett ; 104(25): 253002, 2010 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-20867372

RESUMO

Sequential multiple photoionization of the prototypical molecule N2 is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for single-pulse imaging applications.

12.
Phys Rev Lett ; 105(8): 083004, 2010 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-20868096

RESUMO

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.


Assuntos
Elétrons , Fenômenos Físicos , Luz , Nitrogênio/química , Teoria Quântica , Análise Espectral , Fatores de Tempo
13.
Phys Rev Lett ; 105(8): 083005, 2010 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-20868097

RESUMO

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.


Assuntos
Elétrons , Nitrogênio , Fenômenos Físicos , Lasers , Espectroscopia Fotoeletrônica , Teoria Quântica , Síncrotrons , Raios X
14.
Opt Express ; 18(17): 17620-30, 2010 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-20721148

RESUMO

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.


Assuntos
Elétrons , Lasers , Síncrotrons , Desenho de Equipamento , Fibras Ópticas , Fatores de Tempo , Raios X
15.
Nature ; 466(7302): 56-61, 2010 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-20596013

RESUMO

An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

16.
Opt Lett ; 35(12): 2028-30, 2010 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-20548375

RESUMO

We create a transient Bragg grating in a high-harmonic generation medium using two counterpropagating pulses. The Bragg grating disperses the harmonics in angle and can diffract a large bandwidth with temporal resolution limited only by the source size.

17.
Opt Express ; 17(17): 15134-44, 2009 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-19687991

RESUMO

We present a method that allows for a convenient switching between high harmonic generation (HHG) and accurate calibration of the vacuum ultraviolet (VUV) spectrometer used to analyze the harmonic spectrum. The accurate calibration of HHG spectra is becoming increasingly important for the determination of electronic structures. The wavelength of the laser harmonics themselves depend on the details of the harmonic geometry and phase matching, making them unsuitable for calibration purposes. In our calibration mode, the target resides directly at the focus of the laser, thereby enhancing plasma emission and suppressing harmonic generation. In HHG mode, the source medium resides in front or after the focus, showing enhanced HHG and no plasma emission lines. We analyze the plasma emission and use it for a direct calibration of our HHG spectra.


Assuntos
Lasers , Espectrofotometria Ultravioleta/métodos , Calibragem , Simulação por Computador , Eletrônica , Desenho de Equipamento , Luz , Óptica e Fotônica , Teoria Quântica , Espalhamento de Radiação
18.
Phys Rev Lett ; 103(2): 028104, 2009 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-19659250

RESUMO

Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.


Assuntos
Difração de Raios X/métodos , Processamento de Imagem Assistida por Computador/métodos , Lasers
19.
Phys Rev Lett ; 100(13): 135502, 2008 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-18517965

RESUMO

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

20.
Phys Rev Lett ; 98(12): 125501, 2007 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-17501133

RESUMO

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA
...