82 research outputs found
Simulation of angle-resolved photoemission spectra by approximating the final state by a plane wave: From graphene to polycyclic aromatic hydrocarbon molecules
AbstractWe present a computational study on the angular-resolved photoemission spectra (ARPES) from a number of polycyclic aromatic hydrocarbons and graphene. Our theoretical approach is based on ab-initio density functional theory and the one-step model where we greatly simplify the evaluation of the matrix element by assuming a plane wave for the final state. Before comparing our ARPES simulations with available experimental data, we discuss how typical approximations for the exchange-correlation energy affect orbital energies. In particular, we show that by employing a hybrid functional, considerable improvement can be obtained over semi-local functionals in terms of band widths and relative energies of π and σ states. Our ARPES simulations for graphene show that the plane wave final state approximation provides indeed an excellent description when compared to experimental band maps and constant binding energy maps. Furthermore, our ARPES simulations for a number of polycyclic aromatic molecules from the oligo-acene, oligo-phenylene, phen-anthrene families as well as for disc-shaped molecules nicely illustrate the evolution of the electronic structure from molecules with increasing size towards graphene
Photoemission Orbital Tomography for Excitons in Organic Molecules
Driven by recent developments in time-resolved photoemission spectroscopy, we
extend the successful method of photoemission orbital tomography (POT) to
excited states. Our theory retains the intuitive orbital picture of POT, while
respecting both the entangled character of the exciton wave function and the
energy conservation in the process. Analyzing results from three organic
molecules, we classify generic exciton structures and give a simple
interpretation in terms of natural transition orbitals. We validate our
findings by directly simulating pump-probe experiments with time-dependent
density functional theory.Comment: 28 pages, 8 figure
Band renormalization of a polymer physisorbed on graphene investigated by many-body perturbation theory
Many-body perturbation theory at the level is employed to study the
electronic properties of poly(\emph{para}-phenylene) (PPP) on graphene.
Analysis of the charge density and the electrostatic potential shows that the
polymer-surface interaction gives rise to the formation of only weak surface
dipoles with no charge transfer between the polymer and the surface. In the
local-density approximation (LDA) of density-functional theory, the band
structure of the combined system appears as a superposition of the eigenstates
of its constituents. Consequently, the LDA band gap of PPP remains unchanged
upon adsorption onto graphene. calculations, however, renormalize the
electronic levels of the weakly physisorbed polymer. Thereby, its band gap is
considerably reduced compared to that of the isolated PPP chain. This effect
can be understood in terms of image charges induced in the graphene layer,
which allows us to explain the quasi-particle gap of PPP versus
polymer-graphene distance by applying a classical image-potential model. For
distances below 4.5 {\AA}, however, deviations from this simple classical model
arise which we qualitatively explain by taking into account the polarizablity
of the adsorbate. For a quantitative description with predictive power,
however, we emphasize the need for an accurate ab-initio description of the
electronic structure for weakly coupled systems at equilibrium bonding
distances.Comment: 9 pages, 11 figure
kMap.py: A Python program for simulation and data analysis in photoemission tomography
For organic molecules adsorbed as well-oriented ultra-thin films on metallic
surfaces, angle-resolved photoemission spectroscopy has evolved into a
technique called photoemission tomography (PT). By approximating the final
state of the photoemitted electron as a free electron, PT uses the angular
dependence of the photocurrent, a so-called momentum map or k-map, and
interprets it as the Fourier transform of the initial state's molecular
orbital, thereby gains insights into the geometric and electronic structure of
organic/metal interfaces.
In this contribution, we present kMap.py which is a Python program that
enables the user, via a PyQt-based graphical user interface, to simulate
photoemission momentum maps of molecular orbitals and to perform a one-to-one
comparison between simulation and experiment. Based on the plane wave
approximation for the final state, simulated momentum maps are computed
numerically from a fast Fourier transform of real space molecular orbital
distributions, which are used as program input and taken from density
functional calculations. The program allows the user to vary a number of
simulation parameters such as the final state kinetic energy, the molecular
orientation or the polarization state of the incident light field. Moreover,
also experimental photoemission data can be loaded into the program enabling a
direct visual comparison as well as an automatic optimization procedure to
determine structural parameters of the molecules or weights of molecular
orbitals contributions. With an increasing number of experimental groups
employing photoemission tomography to study adsorbate layers, we expect kMap.py
to serve as an ideal analysis software to further extend the applicability of
PT
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
We present new H I imaging and spectroscopy of the 14 UV-selected
star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a
detailed study of the processes governing the production, propagation, and
escape of Ly photons. New H I spectroscopy, obtained with the 100m
Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the
14 observed LARS galaxies (although the profiles of two of the galaxies are
likely confused by other sources within the GBT beam); the three highest
redshift galaxies are not detected at our current sensitivity limits. The GBT
profiles are used to derive fundamental H I line properties of the LARS
galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS
galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging
localizes the H I gas and provides a measurement of the total H I mass in each
galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal
an enormous tidal structure that extends over 160 kpc from the main interacting
systems and that contains 10 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly escape fraction are coupled with the new direct
measurements of H I mass and significantly disturbed H I velocities. Our
robustly detected sample reveals that both total H I mass and linewidth are
tentatively correlated with key Ly tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
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