Physics Colloquium

Spring 2016

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
Colloquium organizer: Daniel Fischer
(Link to main colloquium page)

Green - open date
Yellow - tentative (reserved)
Red - firm commitment

Date Speaker/Title
Jan. 28 tba
Feb. 4 tba
Feb. 11 Artem Rudenko (Kansas State University, Manhattan)
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Title: Imaging light-induced dynamics of small quantum systems: From infrared to hard X-ray domain
Abstract: Ultrashort light pulses provide experimental tools capable of tracing in real time the motion of atomic nuclei or sometimes even electrons. This opens up a variety of new possibilities to study dynamics of different physical, chemical or biological processes in time domain, revealing the structure of transient states and reaction intermediates, which are often not accessible by energy (frequency) domain spectroscopies. The extension of femtosecond (or even sub-femtosecond) light sources from optical to XUV and X-ray wavelengths (nowadays down to ~ 1 Angstrom) allowed probing these dynamics with atomic spatial resolution and enabled studying a particular site in an extended system by element-specific inner shell excitations. For many light-induced reactions in relatively small systems it became possible to obtain a very intuitive picture of what happens after the initial photoexcitation by acquiring a sequence of timed snapshots of the nuclei positions and (in certain cases) evolving electronic structure. In this talk I will present an overview of our ongoing effort to image a broad range of ultrafast photo-induced processes in molecules employing the so-called "momentum microscopy" (i.e., three-dimensional mapping of momentum vectors of charged reaction products) to acquire the snapshots of molecular structure. This activity combines experiments using optical lasers and their high-order harmonics at the J.R. Macdonald Laboratory at KSU with measurements performed at accelerator-based free-electron laser facilities such as LCLS in Stanford and FLASH in Hamburg. A basic underlying idea is to exploit the availability of short-pulsed light sources in a very broad span of wavelength and parameters to ensure optimal probing conditions, and to obtain the most comprehensive (often complimentary) information for each particular reaction. The examples to be considered include imaging of light-induced wave packets in simple molecules, structural rearrangement reactions like isomerization or proton migration, and charge transfer dynamics after inner-shell photoabsorption.
Host: Daniel Fischer
Feb. 18 Richard Dawes (Missouri S&T)
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Title: Calculations of Molecular Spectroscopy and Scattering using Interpolated ab initio Potentials
Abstract: Beginning with the Coulomb Hamiltonian for molecules, the Born-Oppenheimer separation yields from the electronic Schrödinger equation, a potential energy surface or surfaces that govern the motion (states and dynamics) of the nuclei. Part of this talk describes the development of a PES generator (software code) which uses parallel processing on High-Performance Computing (HPC) clusters to construct PESs automatically. Thousands of ab initio data are computed at geometries chosen by the algorithm and fit to a functional form. The electronic structure of molecules is difficult to describe continuously across global reactive PESs since it changes qualitatively as bonds are formed and broken along reaction coordinates. I will discuss a high-level ab initio method (GDW-SA-CASSCF/MRCI) designed to allow the electronic wavefunction to smoothly evolve across the PES and provide an accurate and balanced description of the various regions. These methods are combined with numerical methods to solve the Schrödinger equation for the nuclei in order to study a number of small gas-phased molecules from the areas of atmospheric, combustion and interstellar chemistry. For example, the MCTDH method was used to compute vibrational states and photodissociation dynamics of HCO exploring the effect of non-adiabatic Renner-Teller coupling.
Host: Daniel Fischer
Feb. 25 Charles Adler (St. Mary's College, MD)
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Title: Where is the science in all that fiction?
Abstract: When you hear the words "science fiction", you feel that there should be some science in the fiction. However, a lot of books, TV shows and movies throw in "baloneyum" which looks and sounds "scientific", but is really nonsense. How can we sort through the baloney to find the good stuff? I'm going to talk about some basic scientific principles, especially the law of conservation of mass-energy, and review some fictional works that have good science, some that have bad, and one or two which are truly awful.
Host: Daniel Fischer
Mar. 3 tba
Mar. 10 tba
Mar. 24 tba
Apr. 7 Fabien Gatti (University of Montpellier, France)
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Title: The Multi-Configuration Time-Dependent Hartree(MCTDH) method. Applications to the control of ultrafast phenomena by lasers
Abstract: Many molecular processes, ranging from fundamental to applied problems, are known today that are impacted by strong nuclear quantum mechanical effects, including phenomena like tunneling, zero point energy effects, or non-adiabatic transitions. Recent success in helping to understand experimental observations in fields like heterogeneous catalysis, photochemistry, reactive scattering, optical spectroscopy, or femto- and attosecond chemistry and spectroscopy underline that nuclear quantum mechanical effects affect many areas of chemical and physical research. The correct theory to describe the corresponding dynamics is Molecular Quantum Dynamics. In contrast to standard quantum chemistry calculations, where the nuclei are treated classically, molecular quantum dynamics can cover quantum mechanical effects in their motion. Although the calculation of large systems still presents a challenge - despite the considerable power of modern computers - new strategies have been developed to extend the studies to systems of increasing size. In particular, we present here several applications of the Multi-Configuration Time-Dependent Hartree method (MCTDH) to the understanding and the control of molecular processes involving quantum effects. MCTDH can be seen as a time-dependent MCSCF approach for the nuclei where wavepackets are propagated on one or several potential energy surfaces. As regards the applications, we focus on the control of ultrafast molecular phenomena by lasers.
Host: Daniel Fischer/Richard Dawes
Apr. 14 Stephanie Law Toner (University of Delaware, Newark)
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Title: excitations in unusual materials: doped semiconductors and topological insulators
Abstract: The fields of plasmonics and metamaterials have seen significant growth in recent years, due to the interest in confining light to subwavelength volumes both for fundamental physics studies as well as novel device architectures. Much of this work has been done in the visible spectral range with traditional metals such as gold and silver. In this talk, I will discuss my recent work using new materials, specifically heavily-doped InAs grown by molecular beam epitaxy, for mid-infrared plasmonic and metamaterial devices. I will explain the advantages of these new materials over traditional plasmonic materials in the infrared and demonstrate that they act as near-perfect Drude metals with tunable optical properties which can also be integrated with existing semiconductor optoelectronic devices. I will then show new results on semiconductor infrared metamaterials, which exhibit negative refraction. I will close by discussing some recent work using other materials, including topological insulators for the far-infrared.
Host: Cihan Kurter
Apr. 21 Irma Kuljanishvili (St. Louis University)
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Title: tba
Abstract: tbd
Host: Cihan Kurter
Apr. 28 Erik Henriksen (Washington University, St. Louis)
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Title: Electronic transport in osmium-decorated graphene
Abstract: The electronic system in graphene is unprotected from the environment and thus readily disordered by numerous extrinsic sources. Generally such disorder is a nuisance, and helps drives the search for higher mobility devices in order to reveal e.g. correlated electron physics in graphene. Yet this sensitivity to the environment may be turned to advantage as suggested by several theoretical predictions that a nonnegligible spin-orbit interaction can be generated in graphene, by the presence of transition metal element adatoms or by proximity to certain substrates. In particular In, Tl, Os and Ir are candidate elements that, as dilute coatings (~ few % of a monolayer), are predicted to induce the Kane-Mele spin-orbit coupling in graphene that is required to realize a quantum spin Hall topological insulator.
Toward this end we have studied In adatoms on graphene, and are presently working on Os. Adatoms are deposited in situ by thermal or ebeam evaporation, and electronic transport measurements are used to probe the sample response. We will report our work to date, including the first steps in experiments toward inducing a spin-orbit interaction in graphene by proximity to the supporting substrate.
Host: Julia Medvedeva
May 5 45nd Annual Harold Q. Fuller Prize Colloquium