T. E. Glover

Femtosecond X-ray Pulses at 0.4 Å Generated by 90° Thomson Scattering: A Tool for Probing the Structural Dynamics of Materials

Pulses of x-rays 300 femtoseconds in duration at a wavelength of 0.4 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly relativistic electrons from an accelerator. In the right-angle scattering geometry, the duration of the x-ray burst is determined by the transit time of the laser pulse across the ∼ 90-micrometer waist of the focused electron beam. The x-rays are highly directed (∼ 0.6° divergence) and can be tuned in energy. This source of femtosecond x-rays will make it possible to combine x-ray techniques with ultrafast time resolution to investigate structural dynamics in condensed matter.

A setup for ultrafast time-resolved x-ray absorption spectroscopy

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

Hydrodynamics of particle formation following femtosecond laser ablation

Ablation driven by intense, femtosecond laser pulses offers a novel route to fabrication of nanometer-sized particles. I model particle formation by considering the hydrodynamics of material expansion into vacuum. Modeling reveals rapid material dilution and cooling. Vacuum expansion is found to quench the ejected material 1–3 orders of magnitude more efficiently than thermal conduction quenches the residual bulk surface. Efficient quenching implies that solid-phase particles are produced rapidly (in ≪1 ns) following laser excitation; this may allow unique material states to be frozen within the ejected particles. Finally, the mean particle size is estimated to range from ∼1 to ∼10 nm for initial lattice temperatures ranging from 0.3 to 10 eV.

The Advanced Light Source (ALS) Slicing Undulator Beamline

A beamline optimized for the bunch slicing technique has been construction at the Advanced Light Source (ALS). This beamline includes an in‐vacuum undulator, soft and hard x‐ray beamlines and a femtosecond laser system. The soft x‐ray beamline may operate in spectrometer mode, where an entire absorption spectrum is accumulated at one time, or in monochromator mode. The femtosecond laser system has a high repetition rate of 20 kHz to improve the average slicing flux. The performance of the soft x‐ray branch of the ALS slicing undulator beamline will be presented.

Measurement of synchrotron pulse durations using surface photovoltage transients

MEASUREMENT OF SYNCHROTRON PULSE DURATIONS USING SURFACE PHOTOVOLTAGE TRANSIENTS T.E. Glover 1 , G.D. Ackermann 1 , A. Belkacem 2 , B. Feinberg 1 , P.A. Heimann 1 , Z. Hussain 1 , H.A. Padmore 1 , C. Ray 2 , R.W. Schoenlein 3 , W.F. Steele 1 . Advanced Light Source Division, 2 Chemical Sciences Division, 3 Materials Sciences Division Lawrence Berkeley National Laboratory 1 Cyclotron Road, MS 2-345 Berkeley, California 94720, USA Abstract We report results on experiments using combined laser and synchrotron radiation. Picosecond laser pulses at 800 nm are used to induce surface photovoltage transients in p-type Si samples. A two-component decay is observed. The fast component of decay provides a direct measure of synchrotron soft x-ray pulse durations. PACS : 42.50.Hz, 32.80.Wr Keywords : Combined laser-synchrotron spectroscopy; x-ray pulse measurement. Correspondence author : T. Ernest Glover MS 2-345, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA. 94720, USA. phone : 510-486-6556 fax : 510-486-5530 email : teglover @lbl.gov

Ultrafast X-ray diffraction of laser-irradiated crystals

Abstract Coherent acoustic phonons have been observed in the X-ray diffraction of a laser-excited InSb crystal. Modeling based on time-dependent dynamical diffraction theory has allowed the extraction of fundamental constants, such as the electron-acoustic phonon coupling time. A dedicated beamline for time-resolved studies has been developed at the Advanced Light Source with special considerations toward high transmission, low scattering and a wide photon energy range. The facility combines a bend magnet beamline, time-resolved detectors and a femtosecond laser system.

Glass-like recovery of antiferromagnetic spin ordering in a photo-excited manganite Pr0.7Ca0.3MnO3

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr0.7Ca0.3MnO3 following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.

Laser pump and X-ray probe surface photovoltage spectroscopy on Si(111)

Abstract Laser pump and X-ray probe core-level photoemission experiments probe surface photovoltage transients on p-type Si(111) surfaces. The data are consistent with a picture where the dynamics of mobile surface charge dominate the photovoltage shift, with changes in the surface-states charge density of only secondary importance. A value for the equilibrium band bending is determined, which suggests that a residual oxide layer reduces the density of surface states.

Ultrafast X-ray science at the advanced light source

Our scientific understanding of the static or time-averaged structure of condensed matter on the atomic scale has been dramatically advanced by direct structural measurements using x-ray techniques and modern synchrotron sources. Of course the structure of condensed matter is not static, and to understanding the behavior of condensed matter at the most fundamental level requires structural measurements on the time scale on which atoms move. The evolution of condensed-matter structure, via the making and breaking of chemical bonds and the rearrangement of atoms, occurs on the fundamental time scale of a vibrational period, ~;100 fs. Atomic motion and structural dynamics on this time scale ultimately determine the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes. The integration of x-ray measurement techniques, a high-brightness femtosecond x-ray source, femtosecond lasers, and stroboscopic pump-probe techniques will provide the unique capability to address fundamental scientific questions in solid-state physics, chemistry, AMO physics, and biology involving structural dynamics. In this paper, we review recent work in ultrafast x-ray science at the ALS including time-resolved diffraction measurements and efforts to develop dedicated beamlines for femtosecond x-ray experiments.

Scattering bottleneck for spin dynamics in metallic helical antiferromagnetic dysprosium

We have performed time-resolved resonant x-ray scattering studies in the Lanthanide metal Dy to reveal the dynamic response of the helical order exchange coupling to injection of unpolarized spins. The observed spin dynamics are significantly slower than that exhi bited by the ferromagnetic phase in Lanthanide metals and are strongly dependent on temperature and excita tion fluence. This unique behavior results from transient changes in the shape of the conduction electron Fe rmi surface and subsequent scattering events that transfer the excitation to the core spin. PACS numbers: 71.20.Eh, 78.70.Ck