Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H41: Irving Langmuir Prize Session: Ultrafast Dynamics |
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Chair: Oliver Gessner, Lawrence Berkeley National Laboratory Room: A115/117 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H41.00001: Irving Langmuir Prize in Chemical Physics Talk: Attosecond Electron Dynamics Invited Speaker: Isolated attosecond pulses are produced by the process of high order harmonics, and these pulses are used as a soft X-ray probe in wavelength-dispersed transient absorption. Inner shell core-level spectroscopic transitions are thus used to analyze the chemical and electronic environment of specific atomic states as a function of time following ionization and dissociation. High field ionization processes, using 800 nm pulses, result in spin-orbit electronic state populations, alignment, and electronic wave packet superpositions, all of which are investigated by the spectrally-resolved X-ray probe. By using isolated attosecond pulses as the probe, high field ionization events on a subfemtosecond timescale are investigated. The generality of the transient absorption method for attosecond dyamics is described, as well as the challenges during the pump-probe pulse overlap time period. The results are compared to theoretical calculations by collaborators. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H41.00002: Time- and Angle-Resolved Photoemission Spectroscopy: Ultrafast Dynamics of Electronic Structure Jonathan A. Sobota, Patrick S. Kirchmann, Shuolong Yang, Zhi-Xun Shen Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental tool for condensed matter systems as it measures the single-particle spectral function. Using femtosecond laser pulses in a pump-probe scheme, ARPES can be extended into the time domain. Here we report the construction of a time-resolved ARPES (trARPES) system. We utilize a Ti:Sapphire oscillator to produce infrared pump pulses, while ultraviolet probe pulses are generated by frequency quadrupling. A hemispherical electron analyzer measures the photoemission spectrum as a function of pump-probe delay. We present results on Gallium Arsenide, which displays hot electron dynamics on two distinct timescales in the unoccupied states. Interestingly, the signal in the occupied states has high temporal contrast, resembling a step-function with dynamics at negative delays. These properties make Gallium Arsenide a versatile tool for trARPES system characterization, allowing for calibration of pump-probe temporal and spatial overlap, as well as determination of time resolution. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:24AM |
H41.00003: Beyond the Frontiers of Time-Resolved Spectroscopy Invited Speaker: Time-resolved spectroscopy experiments typically require the measurement of at least two (``pump'' and ``probe'') interactions of a field (e.g. a laser pulse) with the spectroscopic target system (e.g. atom, molecule) at variable but known temporal delays. It is often assumed that the shortest dynamics measurable with such techniques is on the order of the pulse duration of the pump and probe events. In this talk, it will be shown that attosecond electron wavefunction beating can, in principle, be resolved by employing a nonlinear interferometry concept with phase-stabilized femtosecond pulses that does not require attosecond pulses for pumping nor probing. The perfectly coherent and reproducible electric fields of the pump and probe pulses thus seem the ultimate technical goal to achieve the highest temporal resolution in science. By contrast, however, it will be shown that statistically varying (colored-noise) partially coherent pulses typically produced at free-electron lasers (FELs) can be beneficial in resolving dynamics beyond their average pulse duration. These findings may carry general implications for the future development of time-resolved spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H41.00004: Quantitative imaging of ultrashort photoelectron pulse dynamics Zhensheng Tao, He Zhang, Phillip Duxbury, Martin Berz, Chong-yu Ruan Understanding and mitigating the space charge effects is a pressing issue in the development of ultrafast electron diffraction and imaging. Using a novel ultrafast projection imaging technique, quantitative imaging of transient space charge effects in the generation of high density ultrashort electron pulses is performed, which offers a means to directly compare with multi-electron calculations. We establish that the pulse width exhibits a fractional power-law scaling with the sheet density of the emitted electron pulses. By comparing to multi-electron simulations, the initial longitudinal phase space of the photoelectrons is extracted, demonstrating a strong dependence of the initial momentum spread on the sheet density. Multielectron effects are treated using a simple extension of single electron photoemission theory yielding qualitatively correct estimates of the quantum efficiency. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H41.00005: Electron and ion dynamics in the melting of two-dimensional charge density waves Tzong-Ru Han, Christos Malliakas, S.D. Mahanti, Mercouri Kanatzidis, Chong-Yu Ruan The cause of local lattice distortion in the formation of charge density waves (CDW) in 1D materials is often attributed to the Peierls mechanism, while for 2D system, such as CeTe3, it is not precisely known, due to imperfect nesting of the Fermi surface and a rather large CDW gap observed. Using ultrafast electron crystallography, the femtosecond electronic melting and recrystallization of CDW is investigated by following the superlattice peaks (order parameter) originated from the long-range charge ordering and the accompanying lattice distortion. We find that the reconstitution of CDW is subject to a bottleneck effect that can be attributed to the distinctively separated dynamical properties of electrons and ions in the short time scale, revealing the complexity of 2D CDW formation. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:24AM |
H41.00006: Nonperturbative Rydberg excitations triggered by electrons or photons Invited Speaker: Recently investigated processes with autoionizing Rydberg atoms or molecules will be discussed. In one class of processes, two or more Rydberg state are dressed by a laser field that couples them nonperturbatively, after which the coupled states are subsequently probed by XUV photons in a transient absorption experiment. This class will be discussed in the context of two recent experiments involving doubly-excited autoionizing states of atomic helium. In the second class of processes, the Rydberg states are initially created when electrons collide with molecular ions in a plasma environment, then get trapped temporarily in a high Rydberg state after giving up part of their energy to vibrational or rotational degrees of freedom. The Rydberg molecules then have competitive decay pathways, via photon emission, autoionization, or dissociation. The theory will be discussed in the context of experiments that bear on this second class of dynamical processes, which have been performed in Berkeley and also in Prague. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H41.00007: Molecular structures studied using laser induced electron diffraction Yunfei Lin, Suk Kyoung Lee, Lu Yan, Wen Li In a strong laser field, the field ionized electrons from molecules can be returned to the parent molecules by the laser field. These electrons are then scattered off their parent ions. Such phenomena can be used to study molecular structures like the conventional electron diffraction technique, with much better temporal resolution (a few femtoseconds). In this study, we demonstrated its capability to retrieve static structures of molecules using a simple experimental setup. We obtained electron diffraction patterns from spatially aligned oxygen, nitrogen and carbon dioxide in a strong laser field with intensity around 7*10$^{13}$ W cm$^{-2}$. Excellent energy and angular resolutions were achieved by using velocity map imaging detection of electron momentum. The analysis shows that in order to extract the structure information, two kinds of interferences have to be considered: in the first kind, the electrons are ionized and scattered from the same atom; in the second kind, they are ionized from one atom but scattered off another atom in the molecule. We were able to account for the main features in the diffraction patterns of all three molecules and thus obtained the internuclear distances. In the future, we will apply this technique to retrieve structures of polyatomics and also plan to study molecular dynamics exploiting its superb temporal resolution. [Preview Abstract] |
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