82 research outputs found
Coherent two-dimensional multiphoton photoelectron spectroscopy of metal surfaces
Light interacting with metals elicits an ultrafast coherent many-body
screening response on sub- to few-femtosecond time-scales, which makes its
experimental observation challenging. Here, we describe the coherent
two-dimensional (2D) multi-photon photoemission study of the Shockley surface
state (SS) of Ag(111) as a benchmark for spectroscopy of the coherent nonlinear
response of metals to an optical field in the perturbative regime. Employing
interferometrically time-resolved multi-photon photoemission spectroscopy
(ITR-mPP), we correlate the coherent polarizations and populations excited in
the sample with final photoelectron distributions where the interaction
terminates. By measuring the non-resonant 3- and 4-photon photoemission of the
SS state, as well as its replica structures in the above-threshold
photoemission (ATP), we record the coherent response of the Ag(111) surface by
2D photoemission spectroscopy and relate it to its band structure. We interpret
the mPP process by an optical Bloch equation (OBE) model, which reproduces the
main features of the surface state coherent polarization dynamics recorded in
ITR-mPP experiments: The spectroscopic components of the 2D photoelectron
spectra are shown to depend on the nonlinear orders of the coherent
photoemission process m as well as on the induced coherence n.Comment: 34 pages, 8 figures in main paper, pages 33 and 34: supplemental
material, 1 figur
Comparing quasiparticle HO level alignment on anatase and rutile TiO
Knowledge of the molecular frontier levels' alignment in the ground state can
be used to predict the photocatalytic activity of an interface. The position of
the adsorbate's highest occupied molecular orbital (HOMO) levels relative to
the substrate's valence band maximum (VBM) in the interface describes the
favorability of photogenerated hole transfer from the VBM to the adsorbed
molecule. This is a key quantity for assessing and comparing HO
photooxidation activities on two prototypical photocatalytic TiO surfaces:
anatase (A)-TiO(101) and rutile (R)-TiO(110). Using the projected
density of states (DOS) from state-of-the-art quasiparticle (QP)
calculations, we assess the relative photocatalytic activity of intact and
dissociated HO on coordinately unsaturated (Ti) sites of
idealized stoichiometric A-TiO(101)/R-TiO(110) and bridging O vacancies
(O) of defective
A-TiO(101)/R-TiO(110) surfaces ()
for various coverages. Such a many-body treatment is necessary to correctly
describe the anisotropic screening of electron-electron interactions at a
photocatalytic interface, and hence obtain accurate interfacial level
alignments. The more favorable ground state HOMO level alignment for
A-TiO(101) may explain why the anatase polymorph shows higher
photocatalytic activities than the rutile polymorph. Our results indicate that
(1) hole trapping is more favored on A-TiO(101) than R-TiO(110) and (2)
HO@Ti is more photocatalytically active than intact
HO@Ti
Level alignment of a prototypical photocatalytic system: Methanol on TiO2(110)
Photocatalytic and photovoltaic activity depends on the optimal alignment of
electronic levels at the molecule/semiconductor interface. Establishing level
alignment experimentally is complicated by the uncertain chemical identity of
the surface species. We address the assignment of the occupied and empty
electronic levels for the prototypical photocatalytic system of methanol on a
rutile TiO2 (110) surface. Using many-body quasiparticle (QP) techniques we
show that the frontier levels measured in ultraviolet photoelectron and two
photon photoemission spectroscopy experiments can be assigned with confidence
to the molecularly chemisorbed methanol, rather than its decomposition product,
the methoxy species. We find the highest occupied molecular orbital (HOMO) of
the methoxy species is much closer to the valence band maximum, suggesting why
it is more photocatalytically active than the methanol molecule. We develop a
general semi-quantitative model for predicting many-body QP energies based on
the appropriate description of electronic screening within the bulk, molecular
or vacuum regions of the wavefunctions at molecule/semiconductor interfaces.Comment: 5 pages, 5 figure
Coherent phonon induced optical modulation in semiconductors at terahertz frequencies
The coherent modulation of electronic and vibrational nonlinearities in atoms
and molecular gases by intense few-cycle pulses has been used for high-harmonic
generation in the soft X-ray and attosecond regime, as well as for Raman
frequency combs that span multiple octaves from the Terahertz to Petahertz
frequency regions. In principle, similar high-order nonlinear processes can be
excited efficiently in solids and liquids on account of their high nonlinear
polarizability densities. In this paper, we demonstrate the phononic modulation
of the optical index of Si and GaAs for excitation and probing near their
direct band gaps, respectively at ~3.4 eV and ~3.0 eV. The large amplitude
coherent longitudinal optical polarization due to the excitation of
longitudinal optical (LO) phonon of Si (001) and LO phonon-plasmon coupled
modes in GaAs (001) excited by 10-fs laser pulses induces effective amplitude
and phase modulation of the reflected probe light. The combined action of the
amplitude and phase modulation in Si and GaAs generates phonon frequency combs
with more than 100 and 60 THz bandwidth, respectively.Comment: 15 pages, 11 figure
Quasiparticle level alignment for photocatalytic interfaces
arXiv:1404.5166v1Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic materials. Standard density functional theory (DFT)-based methods have proven unable to provide a quantitative description of this level alignment. This requires a proper treatment of the anisotropic screening, necessitating the use of quasiparticle (QP) techniques. However, the computational complexity of QP algorithms has meant a quantitative description of interfacial levels has remained elusive. We provide a systematic study of a prototypical interface, bare and methanol-covered rutile TiO2(110) surfaces, to determine the type of many-body theory required to obtain an accurate description of the level alignment. This is accomplished via a direct comparison with metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and two-photon photoemission (2PP) spectroscopy. We consider GGA DFT, hybrid DFT, and G0W0, scQPGW1, scQPGW0, and scQPGW QP calculations. Our results demonstrate that G0W0, or our recently introduced scQPGW1 approach, are required to obtain the correct alignment of both the highest occupied and lowest unoccupied interfacial molecular levels (HOMO/LUMO). These calculations set a new standard in the interpretation of electronic structure probe experiments of complex organic molecule/semiconductor interfaces.We acknowledge funding from the European Projects DYNamo (No. ERC-2010-
AdG-267374), and CRONOS (No. 280879-2 CRONOS CPFP7); Spanish Grants (Nos. FIS2012-37549-C05-02, FIS2010-21282-C02-01, PIB2010US-00652, RYC-2011-09582, JAE DOC, JCI-2010-08156); Grupos Consolidados UPV/EHU del Gobierno Vasco (No. IT-319-07); NSFC (Nos. 21003113 and 21121003); MOST (No. 2011CB921404); and NSF Grant No. CHE-1213189.Peer Reviewe
Ultrafast electronic response of Ag(111) and Cu(111) surfaces: From early excitonic transients to saturated image potential
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We investigate the evolution of attosecond to femtosecond screening and emergent potentials that govern the dynamics and energetics of electrons and holes excited in the various stages of multiphoton photoemission processes and control the photoelectron yield in recently reported experiments [X. Cui, C. Wang, A. Argondizzo, S. Garrett-Roe, B. Gumhalter, and H. Petek, Nat. Phys. 10, 505 (2014)1745-247310.1038/nphys2981]. The study is focused on the dynamical screening of holes created in preexistent quasi-two-dimensional Shockley state bands on Ag(111) and Cu(111) surfaces and of electrons excited to the intermediate and emerging screened states. Using the formalism of self-consistent electronic response, we analyze first the effects of screening on the dynamics of photoexcited electrons and holes and then of the Coulomb correlated photoexcited pair. Special attention is paid to the correlated primary electron-hole states, which commence as transient surface excitons and develop in the course of screening into uncorrelated electrons and holes propagating in the image potential and surface state bands, respectively. The obtained results enable to establish a consistent picture of transient electron dynamics at Ag(111) and Cu(111) surfaces that are becoming accessible by the time-, energy-, and momentum-resolved pump-probe multiphoton photoelectron spectroscopies.V.M.S. acknowledges partial support from the Basque Departamento de Educacion, UPV/EHU (Grant No. IT-756-13) and the Spanish Ministry od Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P). N.D. acknowledges the support of the Unity Through Knowledge Fund (UKF B1). H.P. was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-09ER 16056.Peer Reviewe
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