Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J38: Focus Session: Ultrafast Dynamics and Imaging II |
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Sponsoring Units: DCP Chair: Markus Guehr, Stanford University Room: A130/131 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J38.00001: Probing Ultrafast Solution-Phase Chemistry in the X-ray Water Window and beyond via Soft X-ray Spectroscopy Invited Speaker: A prerequisite for a microscopic understanding of chemical reactions is knowledge of the ultrafast interplay of valence charge distributions, spin states, and nuclear degrees of freedom. These degrees of freedom are often intricately coupled, leading to very complex dynamics. Femtosecond core-level spectroscopy is very well suited to study such dynamics via x-ray absorption near-edge structure (for information on changes in valence charge distribution / spin-state) and via the extended x-ray absorption fine structure (for information in nuclear arrangements) due to the very localized nature of the initial states of well-defined symmetry and the high chemical specificity of core-level excitations. We have employed femtosecond core-level spectroscopy to study metal-ligand interactions in solvated transition metal complexes as an important class of model systems to demonstrate the feasibility and merit of ultrafast solution-phase soft X-ray spectroscopy.\footnote{N. Huse, T. K. Kim, L. Jamula, J. K. McCusker, F. M. F. de Groot, R. W. Schoenlein, {\it J. Am. Chem. Soc.}, {\bf 132}, 6809.}$^,$\footnote{H. Wen, N. Huse, R. W. Schoenlein, A. M. Lindenberg, {\it J. Chem. Phys.}, {\bf 131}, 234505.} Laser-induced charge transfer reactions on sub-100 fs time scales trigger structural dynamics in first-row transition-metal complexes that display multiple spin-state changes within 300 fs upon photo-excitation. The combined analysis of vibrational, optical, and core-level spectroscopy reveals a complex interplay of nuclear, electronic, and spin degrees of freedom in these systems that leads to detailed insights into the underlying reaction mechanisms. These are prototypical in nature for a variety of organometallic systems. The chemical specificity of core-level spectroscopy is exploited by probing metal-centered transitions to elucidate the ``metallic view.'' We have very recently succeeded in also following the ``ligand view'' via soft X-ray spectroscopy in the X-ray water window. The later experiment has far-reaching consequences as it demonstrates the feasibility of studying ultrafast processes and short-lived species of solvated organic compounds via Nitrogen K-edge spectroscopy to deliver a detailed picture of the evolving valence charge density in chemical reactions. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J38.00002: Ultrafast Dynamics in Helium Nanodroplets Probed by Femtosecond Time-Resolved EUV Photoelectron and Ion Imaging Oleg Kornilov, Oliver B\"unermann, Ali Eftekhari-Bafrooei, Stephen R. Leone, Daniel M. Neumark, Oliver Gessner Femtosecond time-resolved EUV photoelectron and ion imaging are employed to study the relaxation dynamics of electronically excited helium nanodroplets. Excitation into a broad droplet absorption band ($\sim $23.8 eV) is followed by ionization with a delayed IR pulse. The transient photoelectron spectra and angular distributions indicate that electronically excited helium atoms are predominantly emitted in either an aligned 1s4p Rydberg state within less than $\sim $100~fs or in a non-aligned 1s3d state within $\sim $200~fs. The transient ion imaging results suggest that different Rydberg atoms are emitted with significantly different kinetic energy distributions that closely resemble Maxwell-Boltzmann distributions with temperatures of 2700~K (1s3d) and 490~K (1s4p). The results are interpreted in terms of a dynamic model that is based on the local density dependent blue shift of atomic Rydberg states in the droplet environment. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:39PM |
J38.00003: ``Molecular spectrometers'' in the condensed phase: local THz-FIR response from femtosecond fluorescence Invited Speaker: We examine dye molecules whose color depends on the polarity of the environment. Following fast optical excitation, their fluorescence band typically red-shifts by 0.5 eV on femtosecond to nanosecond time scales. This ``dynamic Stokes shift'' reflects the joint molecular and environmental reorganisation of the system. Solvation dynamics has been studied for decades in the hope that the dynamics of the environment itself can be extracted. We contribute with two research lines: (1) development of rigid polar solvation probes whose vibrational response is removed from that of water, for example, and (2) fluorescence techniques which measure the dynamic Stokes shifts more precisely. Two results will be shown. The frequency-dependent permittivity $\varepsilon (\omega )$ of water surrounding N-Methyl-6-Quinolone is extracted up to about 100 cm$^{-1}$ from the time-resolved fluorescence shift R(t). The key consists in an analytical connection $\varepsilon (\omega ) \quad \to $ R(t) which is needed for data fitting. Measurements with the cryoprotectant disaccharide trehalose in water serve to establish the method. Its unique feature is locality, $i.e.$ the possibility to measure $\varepsilon (\omega)$ around a supramolecular structure with a covalently connected or embedded probe. THz vibrational activity of a biopolymer is thus measured locally, on the effective length scale for polar solvation, with an embedded molecular probe. For this purpose 2-hydroxy-7-nitro-fluorene was linked into a 13mer duplex opposite an abasic site. The NMR solution structure shows that the fluorene moiety occupies a well-defined position in place of a base-pair. The dynamic Stokes shifts for solution in H$_{2}$O and D$_{2}$O are quantified. Their difference is much larger than expected for free water, suggesting that only bound water is observed. A weak 26 cm$^{-1}$ spectral oscillation of the emission band is observed which is not present when the probe is free in solution, and is therefore caused by the supramolecular structure (DNA and hydration water). [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J38.00004: Tunable acoustic terahertz generation in InGaN quantum wells effected by metal nanocrystals Meg Mahat, Antonio Liopis, Sergio Periera, Ian Watson, Tae Choi, Arkadii Krokhin, Arup Neogi The strained semiconductor multiple quantum wells have the capability to generate acoustic terahertz emission via coherent acoustic phonon oscillations. The frequency of the THz emission is usually limited by the periodicity of the quantum wells or superlattice structures. We propose a novel technique to modify the frequency and amplitude of THz oscillations by the inclusion of the metal nanocrystals (NCs) within InGaN/GaN multiple quantum wells via the self-assembled inverted hexagonal pits. Time resolved differential transmission measurements demonstrate a four-five folds decrease in the THz frequency under band edge excitation conditions. A theoretical model predicts a strong dependence of the amplitude and period of the oscillations on the radius of the metal NCs. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:27PM |
J38.00005: ``Making the Molecular Movie'': First Frames Invited Speaker: Femtosecond Electron Diffraction has enabled atomic resolution to structural changes as they occur, essentially watching atoms move in real time--directly observe transition states. This experiment has been referred to as ``making the molecular movie'' and has been previously discussed in the context of a gedanken experiment. With the recent development of femtosecond electron pulses with sufficient number density to execute single shot structure determinations, this experiment has been finally realized. A new concept in electron pulse generation was developed based on a solution to the N-body electron propagation problem involving up to 10,000 interacting electrons that has led to a new generation of extremely bright electron pulsed sources that minimizes space charge broadening effects. Previously thought intractable problems of determining t=0 and fully characterizing electron pulses on the femtosecond time scale have now been solved through the use of the laser pondermotive potential to provide a time dependent scattering source. Synchronization of electron probe and laser excitation pulses is now possible with an accuracy of 10 femtoseconds to follow even the fastest nuclear motions. The camera for the ``molecular movie'' is well in hand based on high bunch charge electron sources. Several movies depicting atomic motions during passage through structural transitions will be shown. Atomic level views of the simplest possible structural transition, melting, will be presented for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated to the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron systems will be presented in which an exceptionally cooperative phase transitions has been observed. The primitive origin of molecular cooperativity has also been discovered in recent studies of molecular crystals. These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules--the fundamental molecular basis of biological systems. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J38.00006: Two-dimensional dynamical reconstruction of the valence exciton in LiF Xiaoqian M. Chen, Yu Gan, Peter Abbamonte, Chen-Lin Yeh, Diego M. Casa, Wei Ku The structure and dynamics of excitons are interesting because excitons are model many-body excitations with technological relevance, e.g. to the behavior of photocells. In a previous study, we used inelastic X-ray scattering, together with inversion techniques, to reconstruct one-dimensional projection images of exciton propagation in LiF in real space and time, and showed that the exciton in LiF is of the Frenkel type. Here we generalize our previous work to a two dimensional plane in LiF. Our new images of exciton propagation show intricate shape changes arising from scattering off of the crystal lattice. Our results are compared to model Wannier function calculations for a more detailed test of the Frenkel model. [Preview Abstract] |
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