Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B27: Focus Session: Emerging Ultrafast Technologies II |
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Sponsoring Units: DCP Chair: Munira Khali, University of Washington Room: 204B |
Monday, March 2, 2015 11:15AM - 11:51AM |
B27.00001: Tracking the charge and spin dynamics of electronic excited states in inorganic complexes Invited Speaker: Kelly Gaffney Inorganic complexes have many advantageous properties for solar energy applications, including strong visible absorption and photocatalytic activity. Whether used as a photocatalyst or a photosensitizer, the lifetime of electronic excited states and the earth abundance of the molecular components represent a key property for solar energy applications. These dual needs have undermined the usefulness of many coordination compounds. Isoelectronic iron and ruthenium based complexes represent a clear example. Ru-polypyridal based molecules have been the workhorse of solar energy related research and dye sensitized solar cells for decades, but the replacement of low abundance Ru with Fe leads to million-fold reductions in metal to ligand charge transfer (MLCT) excited state lifetimes. Understanding the origin of this million-fold reduction in lifetime and how to control excited state relaxation in 3d-metal complexes motivates the work I will discuss. We have used the spin sensitivity of hard x-ray fluorescence spectroscopy and the intense femtosecond duration pulses generated by the LCLS x-ray laser to probe the spin dynamics in a series of electronically excited [Fe(CN)$_{6-2N}$(2,2'-bipyridine)$_{N}$]$^{2N-4}$ complexes, with N $=$ 1-3. These femtosecond resolution measurements demonstrate that modification of the solvent and ligand environment can lengthen the MLCT excited state lifetime by more than two orders of magnitude. They also verify the role of triplet ligand field excited states in the spin crossover dynamics from singlet to quintet spin configurations. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B27.00002: Time-resolved X-Ray Absorption Spectroscopy of a Cobalt-Based Hydrogen Evolution System for Artificial Photosynthesis Dooshaye Moonshiram, Carolina Gimbert, Carl Lehmann, Stephen Southworth, Antoni Llobet Production of cost-effective hydrogen gas through solar power is an important challenge of the Department of Energy among other global industry initiatives. In natural photosynthesis, the oxygen evolving complex(OEC) can carry out four-electron water splitting to hydrogen with an efficiency of around 60{\%}. Although, much progress has been carried out in determining mechanistic pathways of the OEC, biomimetic approaches have not duplicated Nature's efficiency in function. Over the past years, we have witnessed progress in developments of light harvesting modules, so called chromophore/catalytic assemblies. In spite of reportedly high catalytic activity of these systems, quantum yields of hydrogen production are below 40 {\%} when using monochromatic light. Proper understanding of kinetics and bond making/breaking steps has to be achieved to improve efficiency of hydrogen evolution systems. This project shows the timing implementation of ultrafast X-ray absorption spectroscopy to visualize in ``real time'' the photo-induced kinetics accompanying a sequence of redox reactions in a cobalt-based molecular photocatalytic system. Formation of a Co(I) species followed by a Co(III) hydride species all the way towards hydrogen evolution is shown through time-resolved XANES. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B27.00003: Spatiotemporal Imaging of Chemical Reactions: Making Molecular Movies with Femtosecond X-Ray Scattering J.M. Budarz, M.P. Minitti, A. Kirrander, J.B. Hastings, P.M. Weber The study of ultrafast reaction dynamics of molecular systems has benefited from the rapid development of spectroscopic and imaging techniques that follow their temporal evolution on a sub-picosecond time scale. More complete understanding of molecular behavior, however, is expected to arise from a full observation of electronic and nuclear motions during reactions. Our recent experiments at the Linac Coherent Light Source (LCLS) have allowed us to develop and implement a method wherein the ultrafast reaction dynamics of molecules in dilute gases (4 Torr) are captured by time-resolved X-ray scattering. Using a pump-probe scheme with 267 nm excitation laser and 8.3 keV X-ray probe pulses, we performed a series of measurements on the interatomic positions at variable delay times to produce a `molecular movie.' In our experiments, 1,3-cyclohexadiene (CHD) is prepared on an excited state surface, causing the molecule to accelerate down several potential energy surfaces coupled by conical intersections, to open into 1,3,5-hexatriene within 80 fs. The resulting `movie' has been supplemented with molecular trajectory calculations to separate the multiple pathways the excited molecule takes toward the open ring. In this talk, the experimental methods and designs that made these experiments possible will be presented together with the first results describing the photochemical reaction dynamics of CHD. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:51PM |
B27.00004: The orbital-based view on reaction dynamics: ligand exchange of Fe(CO)$_{5}$ in solution Invited Speaker: Alexander F\"ohlisch Time resolved soft X-ray spectroscopy has proven recently, that it can beat the complexity of dynamics in materials and chemical processes by its high selectivity towards elemental, chemical, and magnetic properties. Changes in chemical bonding, in particular bond breaking and bond creation seem conceptually simple, but as a result of coherent wave packet motion it is difficult to catch the dynamic pathways in a multidimensional potential energy landscape. In this contribution we exploit the unique approach of femtosecond time resolved resonant inelastic X-ray scattering at LCLS to derive how ultrafast spin-crossover and ligation determines the pathways of ligand exchange of Ironpentacarbonyl (Fe(CO)$_{5}$) in solution. As an outlook, it will be discussed, how non-linear X-ray processes can push time resolved soft X-ray spectroscopy in a new phase. In particular, stimulated Raman scattering and amplified spontaneous emission can overcome the weak scattering cross-sections of spontaneous processes, help to suppress sample damage and increase spectral resolution and excited state selectivity through the exploitation of Anti-Stokes Raman Scattering. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B27.00005: Tracking picosecond molecular dynamics in solution using a suite of synchrotron-x-ray spectroscopic tools Anne Marie March, Gilles Doumy, Elliot P. Kanter, Stefan Lehmann, Dooshaye Moonshiram, Stephen H. Southworth, Linda Young, Tadesse A. Assefa, Christian Bressler, Wojciech Gawelda, Zolt\'{a}n N\'{e}meth, Gy\"{o}rgy Vank\'{o} Laser-pump, X-ray-probe techniques are powerful tools for exploring molecular structural changes that occur in complex environments such as solutions, during a photo-initiated reaction. We are developing such methods using hard x-rays from the Advanced Photon Source, combining x-ray emission spectroscopy and x-ray absorption spectroscopy as probes of electronic and geometric structure and using high-power, MHz lasers as pumps. The high-duty-cycle pump-probe measurements efficiently utilize the synchrotron x-ray flux and enable high-fidelity measurements of the structures of transient intermediates. We present measurements on the model system [Fe(II)(CN)$_6$]$^{4-}$ (ferrocyanide) in an aqueous solution after excitation with 355 nm and 266 nm laser light. The system undergoes two wavelength dependent reactions: photooxidation and photoaquation. Iron K-edge absorption spectra were obtained along with iron emission spectra. Our data support the presence of a previously unobserved pentacoordinated intermediate species in the photoaquation reaction. Its lifetime has been measured to be 4.6 ns and details of its structure will be discussed. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B27.00006: Ultrafast Coherent Photoelectron Spectroscopy of Electronic States on a Cu (111) Surface Adra Carr, Cong Chen, Zhensheng Tao, Margaret Murnane, Henry Kapteyn, Piotr Matyba, Sebastian Emmerich, Martin Aeschlimann, Ulrich Hoefer We use laser-assisted high-harmonic time- and angle-resolved photoemission to directly observe coherent photoemission from a Cu(111) metal surface and the interferences between the emitted photoelectron wavepackets. A comb of high harmonics in combination with interferometrically timed infrared pulses enable a powerful combination of attosecond time resolution and high energy resolution, making it possible to extract phase information about the emitted photoelectron wavepackets and the distinct electronic states from which they emerge. By comparing photoemission from the well-known Shockley surface state to the sp and d bulk bands of Cu(111), we can observe non-negligible phase shifts in the emitted wavepackets, which cannot be attributed to time delays resulting from classical electron transport to the surface. Rather, we interpret these phase shifts as due to an intrinsic photoemission phase that is different for the sp and d band wavefunctions, thus providing a physical interpretation of temporal delays observed in photoemission from surfaces. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B27.00007: Circularly polarized attosecond pulses for molecular atto-magnetism Andre D. Bandrauk Circularly polarized molecular high order harmonic generation, MHOHG, is modelled from numerical solutions of the time-dependent Schroedinger equation, TDSE, for the one-electron H2$+$ in the nonlinear nonperturbative regime of laser-molecule interaction. It is shown that molecules due to their nonspherical symmetry are the preferred medium for producing circularly polarized harmonics by few cycle intense IR (800,400nm) circularly polarized laser pulses. An intense TeraHz(4um) pulse is combined to force recollision of the ionized electron with the parent ion thus enhancing the efficiency of the circularly polarized MHOHG process through single recollision [1]. Superposition of these harmonics allows for the synthesis of single circularly polarized attosecond (10$^{-18}$s) pulses.Such new ultrashort pulses allow for controlling electrons on their natural time scale [2]. In particular the TDSE simulations illustrate the generation with such new pulses coherent quantum electronic currents inside molecules for the creation of attosecond magnetic field pulses of intensity \textgreater 10 Teslas [3].\\[4pt] [1] KJ Yuan, AD Bandrauk, Phys Rev Lett 110,023003(2013)\\[0pt] [2] KJ Yuan, AD Bandrauk, Chem Phys Lett 592,334(2014)\\[0pt] [3] KJ Yuan, AD Bandrauk, Phys Rev A 88,013417(2013) [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B27.00008: Coherent Chemistry with THz Pulses: TDDFT-Ehrenfest Simulations of Field-Induced LiNC Isomerization Lenson Pellouchoud, Evan Reed The ability to coherently rearrange molecular structures is among the grand challenges of physical chemistry. Some of the primary obstacles are non-adiabatic increases in energy, such as intramolecular vibrational relaxation (IVR) and electronic excitations. Motivated by recent advances in the generation and control of strong terahertz (THz) pulses, we have investigated their potential to circumvent these obstacles. THz pulses are promising because their spectral content is well separated from electronic excitation frequencies, yet they may be fast enough to add and remove energy from the ionic system without allowing IVR to take place. In this work, we utilize simulations to discover that LiNC can be isomerized to two distinct metastable conformations with very low residual heating and ionization rates, pointing out a new route towards THz coherent control of chemical bonds and materials. We use time-dependent DFT (TDDFT)-based Ehrenfest molecular dynamics simulations to test a variety of strong time-varying THz pulses applied to the molecule. We find the limits of how quickly an activation barrier can be surmounted before the driving pulse becomes strong enough to ionize the molecule, and how well the target must be aligned in order for the final configuration to be stable. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 2:15PM |
B27.00009: Ultrafast high harmonics for probing the fastest spin and charge dynamics in magnetic materials Invited Speaker: Patrick Grychtol Ultrafast light based on the high-harmonic up-conversion of femtosecond laser pulses have been successfully employed to access resonantly enhanced magnetic contrast at the $M $absorption edges of the 3d ferromagnets Fe, Co and Ni in a table-top setup. Thus, it has been possible to study element-specific dynamics in magnetic materials at femtosecond time scales in a laboratory environment, providing a wealth of opportunities for a greater fundamental understanding of correlated phenomena in solid-state matter. However, these investigations have so far been limited to linear polarized harmonics, since most techniques by which circular soft x-rays can be generated are highly inefficient reducing the photon flux to a level unfit for scientific applications. Besides presenting key findings of our ultrafast studies on charge and spin dynamics, we introduce a simple setup which allows for the efficient generation of circular harmonics bright enough for XMCD experiments. Our work thus represents a critical advance that enables element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution on the tabletop.\\[4pt] In collboration with Ronny Knut, Emrah Turgut, Dmitriy Zusin, Christian Gentry, Henry Kapteyn, Margaret Murnane, JILA, University of Colorado, Boulder; Justin Shaw, Hans Nembach, Tom Silva, Electromagnetics Division, NIST, Boulder, CO; and Ofer Kfir, Avner Fleischer, Oren Cohen, Extreme Nonlinear Optics Group, Solid State Institute, Technion, Israel. [Preview Abstract] |
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