Professor (Research), Photon Science Directorate
Member, Stanford PULSE Institute
Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy.
2020; 10 (1): 16837
We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted 'probe-before-destroy' measurement strategy can be adopted for electronic-structure related XFEL experiments.
View details for DOI 10.1038/s41598-020-74003-1
View details for PubMedID 33033373
Compact hard x-ray split-delay system based on variable-gap channel-cut crystals
2019; 44 (10): 2582–85
We present the concept and a prototypical implementation of a compact x-ray split-delay system that is capable of performing continuous on-the-fly delay scans over a range of ∼10 ps with sub-100 nanoradian pointing stability. The system consists of four channel-cut silicon crystals, two of which have gradually varying gap sizes from intentional 5 deg asymmetric cuts. The delay adjustment is realized by linear motions of these two monolithic varying-gap channel cuts, where the x-ray beam experiences pairs of anti-parallel reflections, and thus becomes less sensitive in output beam pointing to motion imperfections of the translation stages. The beam splitting is accomplished by polished crystal edges. A high degree of mutual coherence between the two branches at the focus is observed by analyzing small-angle coherent x-ray scattering patterns. We envision a wide range of applications including single-shot x-ray pulse temporal diagnostics, studies of high-intensity x-ray-matter interactions, as well as measurement of dynamics in disordered material systems using split-pulse x-ray photon correlation spectroscopy.
View details for DOI 10.1364/OL.44.002582
View details for Web of Science ID 000467906400050
View details for PubMedID 31090737
- Higher-order modes at FELs: a machine interpretation SPIE-INT SOC OPTICAL ENGINEERING. 2019
- Characterization of defect clusters in ion-irradiated tungsten by X-Ray diffuse scattering JOURNAL OF NUCLEAR MATERIALS 2018; 510: 322–30
- Nonsequential two-photon absorption from the K shell in solid zirconium PHYSICAL REVIEW A 2016; 94 (4)
The phase-contrast imaging instrument at the matter in extreme conditions endstation at LCLS
REVIEW OF SCIENTIFIC INSTRUMENTS
2016; 87 (10)
We describe the phase-contrast imaging instrument at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. The instrument can image phenomena with a spatial resolution of a few hundreds of nanometers and at the same time reveal the atomic structure through X-ray diffraction, with a temporal resolution better than 100 fs. It was specifically designed for studies relevant to high-energy-density science and can monitor, e.g., shock fronts, phase transitions, or void collapses. This versatile instrument was commissioned last year and is now available to the MEC user community.
View details for DOI 10.1063/1.4963906
View details for Web of Science ID 000387661900033
View details for PubMedID 27802688
Femtosecond photodissociation dynamics of 1,4-diiodobenzene by gas-phase X-ray scattering and photoelectron spectroscopy.
We present a multifaceted investigation into the initial photodissociation dynamics of 1,4-diiodobenzene (DIB) following absorption of 267 nm radiation. We combine ultrafast time-resolved photoelectron spectroscopy and X-ray scattering experiments performed at the Linac Coherent Light Source (LCLS) to study the initial electronic excitation and subsequent rotational alignment, and interpret the experiments in light of Complete Active Space Self-Consistent Field (CASSCF) calculations of the excited electronic landscape. The initially excited state is found to be a bound (1)B1 surface, which undergoes ultrafast population transfer to a nearby state in 35 ± 10 fs. The internal conversion most likely leads to one or more singlet repulsive surfaces that initiate the dissociation. This initial study is an essential and prerequisite component of a comprehensive study of the complete photodissociation pathway(s) of DIB at 267 nm. Assignment of the initially excited electronic state as a bound state identifies the mechanism as predissociative, and measurement of its lifetime establishes the time between excitation and initiation of dissociation, which is crucial for direct comparison of photoelectron and scattering experiments.
View details for PubMedID 27711844
- Anomalous nonlinear X-ray Compton scattering NATURE PHYSICS 2015; 11 (11): 964-970
The linac coherent light source single particle imaging road map
2015; 2 (4)
Intense femtosecond x-ray pulses from free-electron laser sources allow the imaging of individual particles in a single shot. Early experiments at the Linac Coherent Light Source (LCLS) have led to rapid progress in the field and, so far, coherent diffractive images have been recorded from biological specimens, aerosols, and quantum systems with a few-tens-of-nanometers resolution. In March 2014, LCLS held a workshop to discuss the scientific and technical challenges for reaching the ultimate goal of atomic resolution with single-shot coherent diffractive imaging. This paper summarizes the workshop findings and presents the roadmap toward reaching atomic resolution, 3D imaging at free-electron laser sources.
View details for DOI 10.1063/1.4918726
View details for Web of Science ID 000360649200003
View details for PubMedCentralID PMC4711616
Imaging Molecular Motion: Femtosecond X-Ray Scattering of an Electrocyclic Chemical Reaction
PHYSICAL REVIEW LETTERS
2015; 114 (25)
Structural rearrangements within single molecules occur on ultrafast time scales. Many aspects of molecular dynamics, such as the energy flow through excited states, have been studied using spectroscopic techniques, yet the goal to watch molecules evolve their geometrical structure in real time remains challenging. By mapping nuclear motions using femtosecond x-ray pulses, we have created real-space representations of the evolving dynamics during a well-known chemical reaction and show a series of time-sorted structural snapshots produced by ultrafast time-resolved hard x-ray scattering. A computational analysis optimally matches the series of scattering patterns produced by the x rays to a multitude of potential reaction paths. In so doing, we have made a critical step toward the goal of viewing chemical reactions on femtosecond time scales, opening a new direction in studies of ultrafast chemical reactions in the gas phase.
View details for DOI 10.1103/PhysRevLett.114.255501
View details for Web of Science ID 000356586100002
View details for PubMedID 26197134
- Ultrabright X-ray laser scattering for dynamic warm dense matter physics NATURE PHOTONICS 2015; 9 (4): 274-279
Demonstration of Single-Crystal Self-Seeded Two-Color X-Ray Free-Electron Lasers
PHYSICAL REVIEW LETTERS
2014; 113 (25)
A scheme for generating two simultaneous hard-x-ray free-electron laser pulses with a controllable difference in photon energy is described and then demonstrated using the self-seeding setup at the Linac Coherent Light Source (LCLS). The scheme takes advantage of the existing LCLS equipment, which allows two independent rotations of the self-seeding diamond crystal. The two degrees of freedom are used to select two nearby crystal reflections, causing two wavelengths to be present in the forward transmitted seeding x-ray pulse. The free-electron laser system must support amplification at both desired wavelengths.
View details for DOI 10.1103/PhysRevLett.113.254801
View details for Web of Science ID 000349416600005
View details for PubMedID 25554887
New experimental platform to study high density laser-compressed matter
REVIEW OF SCIENTIFIC INSTRUMENTS
2014; 85 (11)
We have developed a new experimental platform at the Linac Coherent Light Source (LCLS) which combines simultaneous angularly and spectrally resolved x-ray scattering measurements. This technique offers a new insights on the structural and thermodynamic properties of warm dense matter. The < 50 fs temporal duration of the x-ray pulse provides near instantaneous snapshots of the dynamics of the compression. We present a proof of principle experiment for this platform to characterize a shock-compressed plastic foil. We observe the disappearance of the plastic semi-crystal structure and the formation of a compressed liquid ion-ion correlation peak. The plasma parameters of shock-compressed plastic can be measured as well, but requires an averaging over a few tens of shots.
View details for DOI 10.1063/1.4896175
View details for Web of Science ID 000345646000203
- X-Ray Second Harmonic Generation PHYSICAL REVIEW LETTERS 2014; 112 (16)
Toward structural femtosecond chemical dynamics: imaging chemistry in space and time
2014; 171: 81-91
We aim to observe a chemical reaction in real time using gas-phase X-ray diffraction. In our initial experiment at the Linac Coherent Light Source (LCLS), we investigated the model system 1,3-cyclohexadiene (CHD) at very low vapor pressures. This reaction serves as a benchmark for numerous transformations in organic synthesis and natural product biology. Excitation of CHD by an ultraviolet optical pulse initiates an electrocyclic reaction that transforms the closed ring system into the open-chain structure of 1,3,5-hexatriene. We describe technical points of the experimental method and present first results. We also outline an approach to analyze the data involving nonlinear least-square optimization routines that match the experimental observations with predicted diffraction patterns calculated from trajectories for nonadiabatic vibronic wave packets.
View details for DOI 10.1039/c4fd00030g
View details for Web of Science ID 000345529900004
View details for PubMedID 25415842
Dovetailing biology and chemistry: integrating the Gene Ontology with the ChEBI chemical ontology
The Gene Ontology (GO) facilitates the description of the action of gene products in a biological context. Many GO terms refer to chemical entities that participate in biological processes. To facilitate accurate and consistent systems-wide biological representation, it is necessary to integrate the chemical view of these entities with the biological view of GO functions and processes. We describe a collaborative effort between the GO and the Chemical Entities of Biological Interest (ChEBI) ontology developers to ensure that the representation of chemicals in the GO is both internally consistent and in alignment with the chemical expertise captured in ChEBI.We have examined and integrated the ChEBI structural hierarchy into the GO resource through computationally-assisted manual curation of both GO and ChEBI. Our work has resulted in the creation of computable definitions of GO terms that contain fully defined semantic relationships to corresponding chemical terms in ChEBI.The set of logical definitions using both the GO and ChEBI has already been used to automate aspects of GO development and has the potential to allow the integration of data across the domains of biology and chemistry. These logical definitions are available as an extended version of the ontology from http://purl.obolibrary.org/obo/go/extensions/go-plus.owl.
View details for DOI 10.1186/1471-2164-14-513
View details for Web of Science ID 000322660400001
View details for PubMedID 23895341
View details for PubMedCentralID PMC3733925
- Recent development of thin diamond crystals for X-ray FEL beam-sharing Conference on Advances in X-ray Free-Electron Lasers II - Instrumentation SPIE-INT SOC OPTICAL ENGINEERING. 2013
- Demonstration of self-seeding in a hard-X-ray free-electron laser NATURE PHOTONICS 2012; 6 (10): 693-698
X-ray and optical wave mixing
2012; 488 (7413): 603-?
Light-matter interactions are ubiquitous, and underpin a wide range of basic research fields and applied technologies. Although optical interactions have been intensively studied, their microscopic details are often poorly understood and have so far not been directly measurable. X-ray and optical wave mixing was proposed nearly half a century ago as an atomic-scale probe of optical interactions but has not yet been observed owing to a lack of sufficiently intense X-ray sources. Here we use an X-ray laser to demonstrate X-ray and optical sum-frequency generation. The underlying nonlinearity is a reciprocal-space probe of the optically induced charges and associated microscopic fields that arise in an illuminated material. To within the experimental errors, the measured efficiency is consistent with first-principles calculations of microscopic optical polarization in diamond. The ability to probe optical interactions on the atomic scale offers new opportunities in both basic and applied areas of science.
View details for DOI 10.1038/nature11340
View details for Web of Science ID 000308095100046
View details for PubMedID 22932384
- A single-shot transmissive spectrometer for hard x-ray free electron lasers APPLIED PHYSICS LETTERS 2012; 101 (3)
X-Ray Parametric Down-Conversion in the Langevin Regime
PHYSICAL REVIEW LETTERS
2012; 109 (1)
We experimentally and theoretically study the coincidence count rate for down-converted x-ray photons. Because of photoionization, parametric down-conversion at x-ray wavelengths generally involves loss and the theoretical description requires a Langevin approach. By working in a transmission geometry (Laue) rather than in the Bragg geometry of previous experiments, we obtain an improvement in the signal-to-noise ratio of 12.5, and find agreement between experiment and theory.
View details for DOI 10.1103/PhysRevLett.109.013602
View details for Web of Science ID 000306323900005
View details for PubMedID 23031104
- A hard X-ray transmissive single-shot spectrometer for FEL sources Conference on X-Ray Free-Electron Lasers - Beam Diagnostics, Beamline Instrumentation, and Applications SPIE-INT SOC OPTICAL ENGINEERING. 2012
- Photon beamlines and diagnostics at LCLS NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 2011; 635: S6-S11
- First lasing and operation of an angstrom-wavelength free-electron laser NATURE PHOTONICS 2010; 4 (9): 641-647
- High-energy X-ray diffuse scattering JOURNAL OF APPLIED CRYSTALLOGRAPHY 2009; 42: 392-400
- Gas detectors for x-ray lasers JOURNAL OF APPLIED PHYSICS 2008; 103 (9)
X-ray diffuse scattering measurements of nucleation dynamics at femtosecond resolution
PHYSICAL REVIEW LETTERS
2008; 100 (13)
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.
View details for DOI 10.1103/PhysRevLett.100.135502
View details for Web of Science ID 000254670300046
View details for PubMedID 18517965
- Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses JOURNAL OF APPLIED PHYSICS 2008; 103 (6)
- Femtosecond x-ray diffuse scattering measurements of semiconductor ablation dynamics Conference on High-Power Laser Ablation VII SPIE-INT SOC OPTICAL ENGINEERING. 2008
Carrier-density-dependent lattice stability in InSb
PHYSICAL REVIEW LETTERS
2007; 98 (12)
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.
View details for DOI 10.1103/PhysRevLett.98.125501
View details for PubMedID 17501133
Ultrafast bond softening in bismuth: Mapping a solid's interatomic potential with X-rays
2007; 315 (5812): 633-636
Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.
View details for DOI 10.1126/science.1135009
View details for Web of Science ID 000243909400039
View details for PubMedID 17272718
Detectors for ultrafast X-ray experiments at SPPS
SYNCHROTRON RADIATION INSTRUMENTATION, PTS 1 AND 2
2007; 879: 1176-?
View details for Web of Science ID 000244647900282
- Ultrafast time-resolved electron diffraction with megavolt electron beams APPLIED PHYSICS LETTERS 2006; 89 (18)
Single-shot spectrometry for x-ray free-electron lasers
PHYSICAL REVIEW LETTERS
2006; 97 (8)
An experimental scheme to realize single-shot spectrometry for the diagnostics of x-ray free-electron lasers (XFELs) is presented. The combination of an ultraprecisely figured mirror and a perfect crystal form a simple, high-precision spectrometer that can cover an energy range from a few eV to a hundred eV with high resolution. The application of the spectrometer to determine XFEL pulse widths was investigated theoretically and experimentally. It has been shown that the present system can determine pulse widths from sub-fs to ps in a single shot even for spontaneous radiation. The system can be easily extended to even shorter pulses.
View details for DOI 10.1103/PhysRevLett.97.084802
View details for Web of Science ID 000240043800029
View details for PubMedID 17026309
Megavolt electron beams for ultrafast time-resolved electron diffraction
Conference of the American-Physical-Society-Topical-Group-on-Shock-Compression-of-Condensed-Matter
AMER INST PHYSICS. 2006: 1287–1292
View details for Web of Science ID 000240034600303
Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb
PHYSICAL REVIEW LETTERS
2005; 95 (12)
The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the  direction increases faster than in the  direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.
View details for DOI 10.1103/PhysRevLett.95.125701
View details for Web of Science ID 000231908200033
View details for PubMedID 16197085
- X-ray free-electron lasers JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 2005; 38 (9): S799-S819
Atomic-scale visualization of inertial dynamics
2005; 308 (5720): 392-395
The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.
View details for DOI 10.1126/science.1107996
View details for Web of Science ID 000228492000046
View details for PubMedID 15831753
Clocking femtosecond x rays
PHYSICAL REVIEW LETTERS
2005; 94 (11)
Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.
View details for DOI 10.1103/PhysRevLett.94.114801
View details for Web of Science ID 000227923200034
View details for PubMedID 15903864
- Opportunities and challenges using short-pulse X-ray sources. 2nd International Conference on Photo-Induced Phase Transitions IOP PUBLISHING LTD. 2005: 87–94
Future possibilities of the Linac Coherent Light Source
JOURNAL OF SYNCHROTRON RADIATION
2004; 11: 227-238
A study of the potential for the development of the Linac Coherent Light Source (LCLS) beyond the specifications of the baseline design is presented. These future developments include delivery of X-ray pulses in the 1 fs regime, extension of the spectral range, increase of the FEL power, exploitation of the spontaneous emission, and a more flexible time structure. As this potential is exploited, the LCLS can maintain its role as a world-leading instrument for many years beyond its commissioning in 2008 and initial operation as the world's first X-ray free-electron laser.
View details for DOI 10.1107/S090904950400370X
View details for Web of Science ID 000220974600001
View details for PubMedID 15103109
- FEL research and development at the SLAC sub-picosecond photon source, SPPS 24th International Free Electron Laser Conference/9th Free Electron Laser Users Workshop ELSEVIER SCIENCE BV. 2003: 205–9
X-RAY OPTICS AND MONOCHROMATORS FOR SYNCHROTRON RADIATION
JOURNAL OF APPLIED PHYSICS
1977; 48 (4): 1576-1584
View details for Web of Science ID A1977DB88700034
MEASUREMENTS OF INTEGRATED INTENSITY NEAR ABSORPTION-EDGE WITH SYNCHROTON RADIATION
JOURNAL OF APPLIED CRYSTALLOGRAPHY
1977; 10 (AUG1): 321-324
View details for Web of Science ID A1977DS17900016
STUDY OF SPEAR AS A DEDICATED SOURCE OF SYNCHROTRON RADIATION
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1977; 24 (3): 1003-1005
View details for Web of Science ID A1977DL67400011