Biomedical subjects

A M Lindenberg

Publications and source records attributed to A M Lindenberg.

7 recordsLinked to original sources

PubMed2005

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

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 [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

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PubMed2005

Time-resolved measurements of the structure of water at constant density.

Dynamical changes in the structure factor of liquid water, S(Q,t), are measured using time-resolved x-ray diffraction techniques with 100 ps resolution. On short time scales following femtosecond optical excitation, we observe temperature-induced changes associated with rearrangements of the hydrogen-bonded structure at constant volume, before the system has had time to expand. We invert this data to extract transient changes in the pair correlation function associated with isochoric heating effects, and interpret these in terms of a decrease in the local tetrahedral ordering.

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PubMed2005

Atomic-scale visualization of inertial dynamics.

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.

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PubMed2005

Clocking femtosecond X rays.

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.

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PubMed2005

Bonding in liquid carbon studied by time-resolved x-ray absorption spectroscopy.

Even the most basic properties of liquid carbon have long been debated due to the challenge of studying the material at the required high temperature and pressure. Liquid carbon is volatile and thus inherently transient in an unconstrained environment. In this paper we use a new technique of picosecond time-resolved x-ray absorption spectroscopy to study the bonding of liquid carbon at densities near that of the solid. As the density of the liquid increases, we see a change from predominantly sp-bonded atomic sites to a mixture of sp, sp2, and sp3 sites and compare these observations with molecular dynamics simulations.

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PubMed2003

Transient strain driven by a dense electron-hole plasma.

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

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PubMed2003

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy.

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.

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