Agenda‎ > ‎


Near-edge spectroscopies using the NIST Bethe-Salpeter Equation Solver (NBSE)
Eric L. Shirley, NIST, Gaithersburg, Maryland
The NBSE program has been of use in several spectroscopies, including x-ray absorption (XAS) non-resonant inelastic x-ray scattering (NRIXS), electron energy-loss (EELS), in a variety of systems.  These include semiconductors, wide-gap insulators, perovskites of varying stoichiometries and phases, a wide range lithium compounds, materials under extreme conditions, etc.  The program has matured significant and been packaged so that it can be used with very straightforward input to obtain spectra in a fairly automatic and routine fashion.  As a survey on how to exploit this program, we shall review the physical approximations used with the NBSE program, discuss the required input that users should provide, and mention the interface developed by Vinson et al. for use with the ABINIT program, dubbed Obtaining Core Excitations with ABINIT and NBSE (OCEAN) [J.T. Vinson, J.J. Kas, J.J. Rehr, and E.L. Shirley, Phys. Rev. B 83, 115106 (2011)].

Wave function methods for molecular excited states of large molecules: What lies beyond time-dependent density functional theory?
Martin Head-Gordon, Chemistry Dept, UC Berkeley and JCAP

A summary of wave-function based methods for molecular excited state calculations on medium to large molecules will be given, together with a discussion of their performance strengths and weaknesses, both for conventional single electron excitations, as well as more exotic states where more than one electron is promoted.  Examples from energy conversion problems will be given.

Shaul Mukamel, Jun Jiang, Daniel Healion, Yu Zhang, and Jason Biggs
Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025, USA

Two dimensional ultraviolet (2DUV) spectra of protein backbone and side chains are presented. The
signals provide new insights into the protein structures, dynamics and functions. Simulated chirality-
induced 2DUV spectra reveal characteristic patterns of protein secondary structures, and explore the
structure and aggregation mechanism of amyloid fibrils which are associated with over 20 diseases related
to protein misfolding. Signatures of aggregation propensity of peptides are identified.

Time-domain experiments that employ sequences of attosecond x-ray pulses in order to probe electronic
and nuclear dynamics in molecules are made possible by newly developed bright coherent ultrafast
sources of soft and hard x-rays. By creating multiple core holes at selected atoms and controlled times it
is possible to study the dynamics and correlations of valence electrons as they respond to these
perturbations. Electron motions can thus be directly probed by detecting x-ray photons or photoelectrons.
Two-dimensional stimulated x-ray resonant Raman spectra of core excitons are predicted. The stimulated
x-ray Raman spectrum of trans-N-methylacetamide at the nitrogen and oxygen K-edges in response to
two soft x-ray pulses is calculated by treating the core excitations at the Hartree–Fock static-exchange
level. The signal is interpreted in terms of the dynamics of valence electronic wave packets prepared and
detected in the vicinity of (either the nitrogen or the oxygen) atom. The evolving electronic charge
density, as well as, electronic coherence of the doorway, and the window created by the two pulses is
visualized using a basis set of time-dependent natural orbital's, which reveals that the wave packets
consist of several entangled valence particle–hole pairs. Effects of orbital relaxation upon core excitations
are resolved. Extensions to spectroscopy of single molecules with time and frequency gated fluorescence
and to photoelectron detection are proposed.

1 "Two-dimensional ultraviolet (2DUV) spectroscopic tools for identifying fibrillation propensity of protein
residue sequences", J. Jiang and S. Mukamel, Angew. Chem. In. Ed. 49, 9666 (2010).
2 "Simulation and Visualization of Attosecond Stimulated X-ray Raman Spectroscopy (SXRS) Signals in trans-NMA
at the Nitrogen & Oxygen K-Edges ", D. Healion, H. Wang and S. Mukamel, J. Chem. Phys. 134, 124101 (2011).
3 “Coherent Multidimensional Optical Probes for Electron Correlations and Exciton Dynamics: From NMR to X-
rays”, S. Mukamel, D. Abramavicius, LJ Yang, et al. Accts Chem Res. 42, 553-562, (2009).
4 "Multidimensional Attosecond Photoelectron Spectroscopy with Shaped Pulses and Quantum Optical Fields",
S. Rahav and S. Mukamel, Phys. Rev. A. 81, 063810 (2010).

Real-time Calculations of X-ray and Optical Spectra*
J. J. Rehr, Dept. of Physics, Univ. of Washington, Seattle, Wa 98195

Real-time approaches are becoming increasingly important in understanding photon spectroscopies ranging from linear and non-linear optical response to x-ray absorption spectra (XAS).  In this talk I will discuss several methods based on a real-time approaches.  In particular I discuss how one might calculate the optical and x-ray response in a prototypical pump-probe experiment. For the pump, I discuss an approach using real-time, time-dependent density functional theory (RT-TDDFT) approach for calculations of the frequency-dependent linear and non-linear optical response. This approach is based on calculations of the time-dependent response using a real-time generalization of SIESTA and an explicit Crank-Nicholson time-evolution of the wave-functions, [1] starting from a given quasi-monochromatic initial pulse. For the x-ray response, I discuss two methods. One is based on finite-temperature density functional theory/molecular dynamics together with the real-space Green's function approach in the FEFF9 x-ray spectroscopy code [2,3].  As an alternative, I discuss a generalization of our real-time approach for core-level x-ray response based on time-correlation function techniques.

*Supported by DOE Grant DE-FG03-97ER45623 and NSF Grant PHY-0835543.

[1] Y. Takimoto et al., J. Rehr, J. Chem. Phys. 127, 154114 (2007).

[2] F. Vila et al., Phys. Rev. B 78, 121404(R), (2008).

[3] John J. Rehr et al., Comptes Rendus Physique, 10, 548 (2009).

Studies of core and valence ionization using the all-electron MCTDHF method
Daniel Haxton

The Multiconfiguration Time-Dependent Hartree Fock (MCTDHF) method provides a systematic expansion of a molecular wave function in terms of time-dependent linear combinations of time-dependent orbitals. The method is adaptive and "black-box" in that it requires no knowledge of relevant excited and final states. Since 2006, various groups have worked to implement the method; however, most implementations have only succeeded in treating models in reduced dimensionality. We have developed a code that appears capable of propagating a wave function for first row atoms and diatoms, including a rigorous treatment of ionization. The implementation is also capable of treating diatomic nuclear dynamics in a full nonadiabatic framework, including dissociation. Thus far we have focused on validating the method by calculating first order quantities, namely ionization cross sections for fixed nuclei, whereas the method is more suited to nonlinear processes such as STIRAP and stimulated Raman transitions. I will present our results on ionization of Be and H2, including the core electrons of Be, and preliminary work on auger spectra of Be and O2.

Simulations of x-ray spectroscopies in correlated materials in the frequency and time-domains
Tom Devereaux, SLAC and Stanford

In this talk I present an overview of recent progress on numerical simulations of x-ray spectroscopies in correlated materials, focusing on angle-resolved photoemission and resonant inelastic x-ray scattering (RIXS). I will talk about connecting RIXS to simpler density-density response functions, and present three examples of examining time-domain spectroscopies connecting to excitations involving charge and lattice degrees of freedom.

Ab initio calculations of photoemission and absorption spectra  within many body perturbation theory: applications to solids, clusters and nanostructures
Giulia Galli, University of California, Davis

We will discuss approaches recently developed to compute optical  absorption [1] and photoemission [2] spectra of molecules and solids, which are suitable for the  study of large systems and give access to spectra within a wide energy range. Our techniques are based on Density Functional perturbation theory: explicit calculations of single particle excited states and inversion and storage of dielectric matrices are avoided using recently developed algorithms [3].  Applications to clusters [1,2], solids [4] and nanowires [5] will be presented.

[1] D.Rocca, D. Lu, and G. Galli, J. Chem. Phys. 133, 164109 (2010).
[2] H.-V. Nguyen, T.A. Pham, D.Rocca and G.Galli 2011 (preprint).
[3] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B, 78,113303(2008); H. Wilson, D. Lu, F. 
Gygi and G. Galli, Phys. Rev. B., 79, 245106(2009).
[4] D.Rocca, Y.Ping and G.Galli 2011 (submitted).
[5] Y.Ping, D.Rocca, D.Lu and G.Galli 2011 (preprint).

Many-electron effects on optical absorption spectra of doped and strained graphene
Li Yang, Washington University, St Louis

We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime. Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect. If time allows, I will present the electronic structure and optical response of graphene under uniaxial strain, in which the whole optical absorption spectrum is substantially modified and the infrared optical absorbance is no longer a constant because of the broken symmetry of electrons and holes around the Dirac point.

Time domain simulations for thousand atom organic systems
Lin-wang Wang

Time domain simulation is to follow the time dependent Schrodinger's equation for electron movements and
Newton's equation for nuclei molecular dynamics (MD). The time-domain simulation can be useful to study carrier
transport in organic systems, chemical reactions involving charge transfer, or ultrafast electron dynamic processes.
Due to the small mass of the electron, the time step used for the numerical integration in time-domain simulation
can be a thousand times smaller than the 1 fs time step usually used for ab initio MD. This makes
the time-domain simulation extremely expensive. We have developed a method which uses the 1 fs time step to
carrier out time-domain simulations. Together with the charge patching method, and classical force field MD, we have
simulated the carrier transport in a layer of D5TBA organic molecules using the time-domain simulation.

Photo-excited states of condensed matter: The GW approach and beyond
Steven G. Louie
Department of Physics, University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory

We discuss some recent progress in using and extending the GW approach to address many-particle effects in the photo-excited state properties of condensed matter. In particular, we will present results on three topics: 1) the plasmon satellite structures in the spectral weight function of valence states, 2) the optical spin initialization process of the NV- center in diamond, and 3) the bulk and surface states of topological insulators.

This work is supported by the NSF and DOE.