Download Book-Abstracts - The Fritz Haber Center for Molecular dynamics

Abstracts of Oral
Presentations
Ultrafast Charge Transfer and Localized Bond Breaking at Surfaces
Dietrich Menzel
Physik-Department E20, T.U. München, Garching b. München
Charge transfer between adsorbates and their substrates are of decisive importance for the strong
modification of photochemical processes at surfaces. Indirect arguments suggest that the timescales of
these processes at metal surfaces must lie in the range from under a femtosecond to a few femtoseconds;
they are therefore difficult to measure. Core electron excitations are localized and atom specific, and
possess known lifetimes which are often in the range of a few femtoseconds. The first property allows to
selectively excite certain atoms of a system, for instance of an adsorbed molecule; the second property can
be used as an internal clock to measure the rates of competing extremely fast processes. This is
accomplished by using narrow band synchrotron radiation (band width below the lifetime width of the
excitation, so called Auger-Raman conditions) to measure core excitation and deexcitation spectra. I will
describe the principle and the use of this method for the measurement of charge transfer times at metal
surfaces and discuss typical results. Then I will show how such atom specific core excitations can be used
to selectively break certain bonds in adsorbates. Using very simple model systems (molecularly adsorbed
N2 und CO) the mechanism of this selectivity can be understood. Potential further developments will be
briefly discussed.
Production and Detection of Chemically-Induced Hot Electrons in Surface Processes: xray edges, driven oscillators, friction
J. W. Gadzuk
NIST
Gaithersburg, MD 20899
The potential involvement of electron-hole pair excitations in atomic/molecular processes such as
sticking/adsorption/dissociation at metal surfaces has long been debated, particularly by those previously
involved with similar issues in electron spectroscopies of localized core levels in solids. Of special
relevance here are the fundamental studies of Muller-Hartmann et al. on the dynamic response of Fermi
systems to transient localized perturbations1 as subsequently applied to non-adiabatic surface dynamics2.
Recent experiments have detected hot electrons produced when various gases were adsorbed on a thin
metal film that formed a Schottky barrier with an n-type Si substrate upon which the film was deposited.3
Drawing on analogies with electron-hole pair shakeup in spectroscopic processes which lead to the x-ray
edge singularity, a theoretical model for the electronically non-adiabatic effects is presented that accounts
for the observed initial hot electron production, roughly 0.001-0.01 electrons per incident stronglyinteracting adsorbate such as O, H, and NO2 on Ag.4 Since the fundamental physical content of the x-ray
edge model is the Fermi-level phase shift associated with the localized perturbation and the rate at which it
is switched on, straightforward connections with friction-based models are easily established.5
1
E. Muller-Hartmann, T. V. Ramakrishnan, and G. Toulouse, Phys Rev B 3, 1102 (1971).
J. W. Gadzuk and H. Metiu, Phys Rev B 22, 2603 (1980).
3
B. Gergen, H. Nienhaus, W. H. Weinberg, and E. W. McFarland, Science 294, 2521 (2001); H.
Nienhaus, Surf Sci Rep 45, 3 (2002).
4
J. W. Gadzuk, J Phys Chem B 106, 8265 (2002).
5
K. Schonhammer and O. Gunnarsson, Phys Rev B 27, 5113 (1983); J. C. Tully, Annu Rev Phys Chem
51, 153 (2000); J. R. Trail, M. C. Graham, D. M. Bird, M. Persson, and S. Holloway, Phys Rev Lett
88,166802(2002).
2
Non-adabatic pathways in the dissociative adsorption of simple molecules on the Al(111)
surface
Eckart Hasselbrink
Fachbereich Chemie, Universität Duisburg-Essen
Abstract: Molecular beams have been used to study the interaction of molecules such as O2 and NO2 with
the Al(111) surface. Using REMPI laser spectrometry a search for the gas phase products of possible
abstraction reaction channels has been carried. In the case of O2 O-atoms and in the case on NO2 NOmolecules have been detected. The reaction products have carried with respect to their internal and
translational energies where possible. The abstraction reaction channels are seen as a manifestation of
non-adiabatic pathways on the incoming trajectory of the molecule.
Abstractive Dissociation of Oxygen Over Al(111): A Non Adiabatic Quantum
Model
Gil Katz, Yehuda Zeiri and Ronnie Kosloff
Department of Physical Chemistry and the Fritz Haber Research Center, the Hebrew University,
Jerusalem 91904, Israel
The dissociation of oxygen on a clean aluminum surface is studied theoretically. A non
adiabatic quantum dynamical model is used, based on four electronically distinct
potential energy surfaces characterized by the extent of charge transfer from the metal
to the adsorbate. A flat surface approximation is used to reduce the computation
complexity. The conservation of the helicopter angular momentum allows Boltzmann
averaging the outcome of the the propagation of a three degrees of freedom
wavefunctions. The dissociation event is simulated by solving the time dependent
Schrodinger equation for a period of 30 femtoseconds. As a function of incident kinetic
energy, the dissociation yield follows the experimental trend. An attempt at simulation
employing only the lowest adiabatic surface failed, qualitatively disagreeing with both
experiment and non-adiabatic calculations. The final products, adsorptive dissociation
and abstractive dissociation are obtained by carrying out a semiclassical molecular
dynamics simulation with surface hopping which describes the back charge transfer
from an oxygen atom negative ion to the surface. The final adsorbed oxygen pair
distribution compares well with experiment. By running the dynamical events backward
in time, a correlation is established between the products and the initial conditions
which lead to their production. Qualitative agreement is obtained with recent
experiments is thus obtained that show suppression of abstraction by rotational
excitation.
Hyperthermal ejection of halogen atoms from the reaction of diatomic halogens
on the Al(111) surface: Evidence of a vertical electron harpooning mechanism.
G.C. Poon, T.J. Grassman, A.C. Kummel
Dept. Chem 0358, Univ. of California, San Diego. La Jolla CA 92093-0358 USA
Resonantly enhanced multiphoton ionization (REMPI) and time-of-flight mass
spectroscopy (TOF-MS) have been used to demonstrate that the reaction of Cl2 on the
low work function Al(111) surface proceeds via a prompt, vertical electron harpooning
process. As Cl2 approaches Al(111), an electron harpoons from the surface, suddenly
converting Cl2 to Cl2-. This vertical transition places the molecule high on the repulsive
portion of the Cl2- potential curve, leading to rapid dissociation into Cl- and Cl
fragments. The Cl- proceeds toward the surface and sticks while the neutral Cl atom is
ejected into the gas phase. An experimentally observable signature of this harpooning
process would be a hyperthermal translational energy of the ejected fragment, whose
energy is determined by the vertical transition between Cl2 and Cl2- and should be
nearly independent of incident translational energy. Cl2 beams with translational
energies ranging from 0.11 eV to 0.65 eV were directed at the surface at three incident
angles: 0º, 20º, and 40º off the surface normal. The translational energy of the ejected
Cl was shown to be a weak function of the incident translational energy. For 0.11 eV
normal incidence Cl2, the ejected Cl had a translational energy of 0.22 eV, while 0.65
eV normal incidence Cl2 produced 0.19 eV Cl. In addition, for incident Cl2 with a
velocity of 535 m/sec, the ejected Cl atoms were accelerated to a velocity of 1036
m/sec.
To gain more information about the harpooning process, the angular distribution of the
ejected chlorine was measured. For direct chemisorption of Cl2 onto Al(111), the
maximum initial sticking probability is only 65%. This strongly suggests that molecular
orientation may be an important initial condition in determining the reaction
probability. For normal incident Cl2 on Al(111) at high surface temperature (500K) and
high incident energy (0.65 eV), we have observed that the angular distribution of
ejected Cl atoms is strongly peaked (cos6θ )c0 long the surface normal. In
contrast, the angular distributions for both the etch product and the inelastically
scattered Cl2 are broader. Glancing incidence Cl2 exhibits Cl abstraction product
focused again sharply along the surface normal with a cos7 distribution. Etch product
has been measured with the expected cos distribution. The data show that the electron
harpooning followed by abstractive chemisorption is most favorable for molecules
oriented along the surface normal independent of the incident angle of the molecular
beam.
Mechanism of low energy electron stimulated desorption of H- and charge trapping on
hydrogenated diamond surfaces
Alon Hoffman
Chemistry Department, Technion, Israel Institute of Technology.
In this lecture I will discuss the influence of low energy electron irradiation of hydrogenated diamond
surfaces. The mechanism of H- electron stimulated desorption (ESD) and surface charge trapping (SCT)
for incident electron energies in the 2-40 eV range were investigated. It was found that the H- ESD cross
section displays a resonance behavior displaying two well defined peaks at 9 and 22 eV and a monotonic
increase with a threshold at ~14 eV as function of incident electron energy. The 9 eV peak was
interpreted as due to dissociative electron attachment (DEA) via a single Feshbach anion resonance state.
For incident electron energies higher than ~14 eV H- ESD proceeds also via dipolar dissociation (DD)
processes. The 22 eV peak was suggested to be associated with multiple electron scattering or loss events
prior to electron attachment leading to DEA. We have showed that that electron trapping induced by low
energy irradiation of hydrogenated diamond surfaces is enhanced at ~9 eV suggesting the importance of
resonance electron attachment in surface charging.
From a general perspective, the interaction of slow electrons with surfaces plays a very important role in a
wide range of phenomena such as in upper atmosphere and outer space chemistry where it may be an
important factor affecting heterogeneous chemical processes. In biological systems the interaction of slow
electrons is also a most important aspect of radiation medicine as many very low energy secondary
electrons are created upon interaction of ionizing radiation such as UV, X-ray, etc. Charge trapping of
slow electrons on surfaces is another effect of large environmental and biological importance. Charge
trapping can strongly influence the interaction between microscopic particles and of central importance in
diverse phenomena like behavior of dust, electrical shook in biological systems and electrical breakdown
of materials. All these phenomena are poorly understood phenomena and most likely occur through REA
of slow electrons.
Beyond Born-Oppenheimer: Molecular Dynamics through a Conical
Intersection
Lorenz S. Cederbaum
Theoretische Chemie, Universität Heidelberg,
Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
Non-adiabatic effects play an important role in many areas of physics and chemistry. The
coupling between electrons and nuclei may, for example, lead to the formation of a conical
intersection between potential energy surfaces, which provide an efficient pathway for
radiationless decay between electronic states. At such intersections the Born-Oppenheimer
approximation breaks down, and unexpected dynamical processes result, which can be
observed spectroscopically. We review the basic theory required to understand and describe
conical, and related, intersections. A simple model is presented, which can be used to classify
the different types of intersections known. An example is also given using wavepacket
dynamics simulations to demonstrate the prototypical features of how a molecular system
passes through a conical intersection.
Non adiabatic effects in the light-induced desorption of O2 from the TiO2(110) surface
M. P. de Lara-Castells1, A. O. Mitrushenkov2, and Jeffrey L. Krause3
(1) Instituto de Matemáticas y Física Fundamental (CSIC), Serrano 123, E-28006-Madrid, Spain
(2) Dipartimento di Chimica Fisica ed Inorganica, University of Bologna, Viale Rigorgimento 4, 40136
Bologna, Italy
(3) Quantum Theory Project, University of Florida, Gainesville FL 32611, USA
A wealth of experimental data now exists on the quantum-state-selective, laser-induced physical
and chemical processes of molecules adsorbed on metal and metal-oxide surfaces. Most calculations to
date on these systems have used empirical potential energy surfaces, which fail to capture much of the
relevant physics. Our work addresses the study of the laser-induced reactive dynamics of molecules
adsorbed on surfaces by using ab-initio methods to characterize the important regions of the potential, and
the molecule-surface coupling, and quantum-mechanical treatments of the nuclear motions. The ultimate
goal is to use specially shaped, ultrafast laser pulses to control product state distributions.
This talk focuses on the study of the adsorption and light-induced desorption of molecular oxygen on a
reduced TiO2 (110) surface [1-2]. This system is known to play a fundamental role in the area of
heterogeneous photocatalysis. The electronic non-adiabatic coupling between the ground and the first
excited state of O2 adsorbed on the catalyst is calculated directly from correlated ab-initio wave functions
by using a novel optimal orbital approach and the transformation to bi-orthonormal (dual) orbital sets [2].
In particular, the role of the coupling on the light-induced desorption dynamics is discussed.
1. M. P. de Lara-Castells and Jeffrey L. Krause, J. Chem. Phys. 115, 4798 (2001); Chem. Phys. Lett. 354
485 (2002); J. Chem. Phys. 118, 5098 (2003).
2. M. P. de Lara-Castells, A. O. Mitrushenkov, and Jeffrey L. Krause, in preparation.
Photodesorption from oxides and deposited metal clusters: REMPI and 2PPE
Investigations
Chr. Rakete, F. Evers, K. Watanabe, S. Borowski, T. Klüner, W. Drachsel, H.-J. Freund
Department Chemical Physics, Fritz-Haber-Institut der Max-PlanckGesellschaft, Faradayweg 4-6, 14195 Berlin-Dahlem
NO-desorption has been investigated from NiO(100) and Cr2O3(0001) surfaces and from Ag clusters
grown on alumina thin films. Both experimental as well as theoretical studies have been undertaken on the
oxide surfaces and a quantitative picture of the processes involved has been developed for scalar
quantities. Vectorial quantities have been measured for CO desorbing from Cr2O3(0001) and the results
are compared with theoretical attempts to calculate corresponding quantities. Desorption and reaction
from deposited metal clusters has been investigated for NO and CH4 for Pd and Ag particles deposited on
alumina. As far as Pd particles are concerned CH4 dissociation into CH3 and H and also NO desorption
has been investigated. For Ag clusters we have exited into the so called plasmon resonances and study the
influence of such excitations onto NO desorption dynamics.
Electronic processes and transitions on surfaces – a time dependent density functional
approach
Roi Baer
Department of Physical Chemistry and the Fritz Haber Research Center, the Hebrew University,
Jerusalem 91904, Israel
Methods are developed for studying the electronic properties of metallic clusters, modeled as jellium
slabs, with adsorbates on their surface (or within). The generality of the method allows many types of
processes to be studied. We will describe several applications. First, a study of the way molecules conduct
electricity: we describe the time dependent flow of charge through molecules connected to two metallic
leads under a voltage bias. Next, we discuss thin metallic shells. The ratio between the thickness of the
shell and its radius determines the plasma frequency of the shell. The huge cross section for absorption in
this frequency can be used to enhance and alter the photo-activity of a semiconducting core. Finally, the
first steps in studying adsorbate reaction mechanisms on metallic surfaces will be described.
Theoretical Investigation of Photochemical Reactions on Surfaces
from first principles
Dr. Thorsten Klüner
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Chemical Physics, Faradayweg 4-6,
14195 Berlin, Germany
Surface photochemistry occurs in many instances, including photocatalysis, laser induced desorption, and
solar energy conversion. Despite its ubiquitous nature, a microscopic understanding of the underlying
basic processes and elementary reactions remains a great challenge. In this talk, I will focus on the
theoretical description of the simplest photochemical phenomenon on surfaces: the laser-induced
desorption of small molecules.
Ab initio calculations of high dimensional potential energy surfaces (PES) for ground and excited states of
the CO/Cr2O3(0001) will be presented. The surface model consists of a small cluster embedded in a semiinfinite array of point charges to simulate the electrostatic field above the surface. Based on Configuration
Interaction (CI) calculations a reliable construction of global potential energy surfaces for the electronic
ground state as well as for excited states of the adsorbate substrate system becomes feasible.
Subsequent high-dimensional time-dependent wave packet studies using the calculated potential surfaces
enable us to simulate quantum state resolved experiments without using empirical information.
In addition to the calculation of excited states of molecules on insulator surfaces we recently extended our
studies to adsorbates on metal surfaces using a new ab initio embedded cluster approach. In this
embedding scheme, a local region in space is described by accurate quantum chemical cluster calculations
(CI), whereas the infinite solid is taken into account by an effective embedding operator obtained from
periodic density functional theory (DFT). I will present first results for adsorption energies and vertical
excitation energies for the benchmark system CO/Pd(111).
PHOTO PHYSICS AND PHOTO CHEMISTRY OF ICE FILMS ON GRAPHITE
Dinko Chakarov
Department of Applied Physics, Chalmers University of Technology and Göteborg University,
412 96 Göteborg, Sweden
The extensive interest in the physics and chemistry of ice is motivated by its universal appearance; in
particular by the special role of ice films in ozone depletion chemistry, cometary science, and
astrophysics. In these examples, the structure of ice and its chemical properties are strongly influenced and
altered by incoming photons. Our model system consists of ice films with submonolayer to several
hundred monolayers thickness deposited at ultrahigh vacuum and low (from ~25 K) temperatures on
atomically clean graphite (0001) surface. The films are characterized before, during, and after cw or
pulsed photon irradiation in broad wavelength and photon flux ranges. Main analytical tools are
temperature programmed desorption - isothermal desorption spectroscopy (TPD - ITD), photodesorption
spectroscopy (PID), and high-resolution electron energy loss spectroscopy (HREELS).
We will discuss the phenomena of “photon annealing” of pure amorphous films and their structureselective laser ablation: A new non-thermal mechanism by which submonolayer and multilayer pure
amorphous ice films, crystallize due to UV radiation [1, 4]. In the case of ice doped with alkali metals [2,
3], we will report on the observations of photoreactions leading to formation of H2, CH4, CO, and CO2.
Introduction of metal clusters (Na and Ag) and coadsorption with other simple molecules [5] leads to
wavelength specific enchantments of the photoyeild. In all cases the primary interest will be directed on
revealing the operating mechanism of observed transformations and reactions.
[1] Phys. Rev. Lett., 81 (1998) 5181
[2] Surface Sci., 420 (1999) 174
[3] J. Chem. Phys., 115 (2001)
[4] Langmuir, 19 265 (2003)
[5] J. Chem. Phys., in press (2003)
Steric Effect in Electron-Molecule Interaction
Yigal Lilach and Micha Asscher
Department of Physical Chemistry and The Farkas Center for Light Induced Processes,
The Hebrew University, Jerusalem 91904, Israel
Steric effect has been identified and measured in the interaction of electrons with oriented molecules, for
the first time. Photo-electron mediated reactivity has been determined, with a cross section of à = 3.0x1019
cm2 (methyl down) for an CD3Br adsorbed on O/Ru(001). Three times smaller cross section was
obtained for the bromine down configuration, controlled by oxygen pre-coverage. Qualitatively similar
molecular orientation dependent effect was measured by direct bombardment with 10eV electrons,
however at much larger cross sections. The significance of the steric effect reported here for electron
transfer processes in general is discussed.
Trapping of electrons due to nonadiabatic processes
Nimrod Moiseyev
Department of Chemistry, Technion – Israel Institute of Technology 32000 Haifa, Israel
This work has been motivated by the following puzzle;. How comes that sharp peaks have been observed
in electron scattering experiments from hydrogen molecule whereas all known theoretical adiabatic
calculations show that the hydrogen molecular ion has an extremely short lifetime (~ 10-15 sec).
Due to the complexity of the problem it was hard/impossible to carry out nonadiabatic calculations of
e– /H2 scattering cross-sections. The method we proposed is as follows: first scaled the electron
coordinates by a complex factor, exp(i). Upon complex scaling the adiabatic Hamiltonian becomes nonHermitian. The complex eigenvalues which are -independent provide the complex adiabatic surfaces. The
imaginary parts are the nuclear dependent ionization rate of decay (inverse lifetimes). By solving the
nuclear equation of motion with complex uncoupled adiabatic PES we obtained approximated solutions
for the problem. In the specific case of electron scattering from hydrogen molecule we have shown (Refs.
(1-4)) that the solutions obtained for the uncoupled complex PES are sufficient to provide an ab-initio
cross section that is in complete agreement with the experimental results. It has been shown that the sharp
peaks in the cross section result from interference between adjacent broad overlapping resonances, in spite
of the fact that there is a common believe that they are not observable. There are cases, however, where
even upon CS, the complex scaled adiabatic states do not describe the resonance phenomena and the
coupling between the complex scaled adiabatic PES cannot be neglected.
Recently we introduced a method that enables the calculation of resonance energies, widths/lifetimes, and
cross sections for such cases (see Ref. (5)). The key point is to use a resonance diabatic representation
over the adiabatic representation. The “frozen” diabatic states in the Hermitian approach, ,
are defined as states for which the matrix elements , where indices j and j’ refer to diabatic channel
quantum numbers. This definition of diabatic states provides in our case bound states in the continuum
(i.e., zero widths). The question is, how diabatic resonance states can be calculated in non-Hermitian
quantum mechanic? Here we first introduce a method that enables the calculations of resonance diabatic
states (i.e., metastable states) which serve as an effective basis set in the solution of the full problem where
the coupling between the electron and the nuclear motions are taken into consideration. Using this
method, we calculate the autodetachment rates of protonium negative ion, Pn-(n) resonances that form in
collision of a slow antiproton, , and hydrogen atom. Although we have chosen this exotic example for the
first application of the method we believe it can be applied to gas/surface scattering experiments or when
adsorbed molecules are ionized due to the interaction with a laser.
1. E. Narevicius and N. Moiseyev, “Fingerprints of Broad Overlapping Resonances in
the e+H2 Cross section”, Phys. Rev. Lett., 81, 2221-2224 (1998).
2. E. Narevicius and N. Moiseyev, “Trapping of an Electron due to Molecular
Vibrations”, Phys. Rev. Lett., 84, 1681-1684 (2000).
3. E. Narevicius and N. Moiseyev, “Non-Hermitian Formulation of Interference Effect
in Scattering Experiments”, J. Chem. Phys., 113, 6088-6095 (2000).
4. H. Barkay, E. Narevicius and N. Moiseyev, “Non-Hermitian Scattering Theory:
Resonant Tunneling Probability Amplitude in a Quantum Dot”, Phys. Rev. B, 67,
045322 (2003).
5. P. R. Zdanska, H. R. Sadghpour and N. Moiseyev, “The Non-Hermitian Quantum
Mechanics of Antiproton Collisions: Protonium Formation”, submitted (2003).
Time-resolved two-photon photoemission of Ar/Cu-interface states
Ulrich Hoefer, Marcus Rohleder, Klaus Duncker, Wolfram Berthold, and Jens
Guedde
Fachbereich Physik, Philipps-Universitaet Marburg, Renthof 5, D-35032 Marburg, Germany
We demonstrate for the system Ar/Cu(100) how in favourable cases the dynamics of electronic
states located at the interface between a metal and an insulator can be investigated by means of
time-resolved two-photon photoemission (2PPE). The interface states of Ar/Cu are located above
the vacuum level in the band gaps of both the Cu(100) surface and the Ar films which were as
thick as 70 monolayers (Figure). A first laser pulse excites an electron into the normally
unoccupied state. A second laser pulse lifts the electron into the Ar conduction band, from where it
can escape into the vacuum without any appreciable loss of energy or momentum.
It is shown that with increasing layer thickness the states evolve from quantum-well-like
resonances into well-defined bound states which arise from the screened image-potential of the
metal. Electrons excited into these states decay on timescales between 100 fs and 250 fs by
electron-hole pair excitation in the metal and - for thin Ar layers (< 20-30 ML) - also by electron
transfer through the insulating Ar layers into the vacuum. In comparison to the image-potential
states of the clean surface, the interface states are modified by the potential inside the dielectric
layers. Screening reduces their binding energies with respect to the Ar conduction band. It also
results in longer lifetimes, as the wavefunctions become more extended. The dispersion of the
states is found to be parabolic near the band minimum with an effective mass of only 0.5 me. It is
further shown that these states are a sensitive probe for the electronic structure of the interface. In
particular, modification of the interface by preadsorption of 1 ML Xe results in a lifetime decrease
by a factor of 2. Numerical simulations based on a one-dimensional atomic potential provide a
good quantitative description of the physical properties of these interface states.
Photoinduced Electron Transfer Chemistry and Dissociation of Adsorbed CO2:
Harnessing Å–Scale Molecular Acceleration Towards a Surface
R. Zehr, T. Wagner, I. Samanta, I. Harrison
Chemistry Department, University of Virginia, Charlottesville, Va 22904-4319
Activated dissociation of molecules on a metal surface is essential to many catalytic syntheses (e.g.
N2 dissociation in NH3 synthesis) and a firm scientific understanding of this process is important to
advancing the field of heterogeneous catalysis. In commercial catalysis, activation energy barriers are
invariably surmounted by random thermal energy and not through a more directed use of the energy in
light, despite the ubiquitous example of photosynthesis in nature. Potential advantages of using light to
overcome a rate-limiting dissociative adsorption step in catalysis include better selectivity towards a
chosen reaction pathway, the ability to work at much reduced reaction temperatures, and the opportunity
to exploit solar energy. Here, we present evidence that photoinduced electron transfer from a low
temperature Pt(111) surface to physically adsorbed CO2 leads to rapid acceleration of the newly formed
negative ion towards the surface, neutralization, and a high energy collision with the surface that
efficiently dissociates (ca. 36%) and desorbs CO2. Importantly, this photochemical activation mechanism
constitutes an “Å-scale molecular accelerator” that may be applicable to other hard-to-activate adsorbates.
The ability to photochemically induce an adsorbate/surface collision at chemically significant energies (up
to ~2.5 eV), after an acceleration over a distance of no more than a few Å from an initial configuration
prescribed by the physisorption binding potential, provides novel opportunities to drive energetic
dissociation and desorption processes at low temperatures and to examine the reaction dynamics of
catalysis.
Ultrafast dynamics of non-adiabatic processes at interfaces: From surface
femtochemistry to coherent phonon excitation
Martin Wolf
Freie Universität Berlin, Institut für Experimentalphysik, Arnimallee 14, 14195 Berlin, Germany
One of the key goals in surface physics is to obtain a microscopic understanding of chemical reactions.
Their dynamics are governed by ultrafast charge and energy transfer between adsorbed species and the
underlying substrate, which leads to non-adiabatic coupling between electronic and nuclear degrees of
freedom. In this talk we discuss recent progress in the application of femtosecond laser excitation and
time-resolved spectroscopy to investigate non-adiabatic routes in surface femtochemistry as well as the
dynamics of electronic excitations and charge transfer processes and their coupling into nuclear degrees of
freedom.
Dynamics of the spin transition in the adsorption of hydrogen atoms on metals
S. Holloway, D. Bird, M. Persson and J. Trail
Surf. Sci IRC, University of Liverpool, UK
The time-dependent, mean field Newns-Anderson model is solved in the wide band limit. Equations for
the time-evolution of the occupation of the spin-dependent adsorbate levels and for the rate of nonadiabatic energy transfer from adsorbate to substrate are derived. Numerical solutions are obtained in the
region of space close to the transition point between spin-polarized and non-polarized ground states, for
model parameters that correspond to an H-atom incident on the Cu(111) surface. Away from the spin
transition, the non-adiabatic energy transfer is in close agreement with the nearly adiabatic limit. Near the
transition, non-adiabatic effects are large and the nearly-adiabatic limit approximation fails.
Timescale considerations in electron transfer and electron transmission
A Nitzan
School of Chemistry, Tel Aviv University, Israel.
Electron transfer between electron donor and acceptor species in a molecular system and electron
conduction through molecules connecting metal interfaces are closely related molecular processes that are
affected differently by environmental degrees of freedom. This talk will focus on the characteristic
electronic and nuclear timescales involved in these processes and on the way they affect the outcome of
the observed electronic transition.
Understanding electron transfer at molecule-metal
junctions: a spectroscopic approach
Xiaoyang Zhu
Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN
Electronic interaction between a molecule and a metal surface is one of the most important
and difficult problems in surface science. The recent surge of research interests in moleculebased electronic devices has necessitated a quantitative understanding of this difficult
problem. In molecule-based conventional electronic devices, such as organic light emitting
diodes and organic field-effect transistors, the metal-molecule interface often determines to a
great extent the operation of the device. The importance of the interface only increases as
device dimension shrinks, e.g., to the scale of a single molecule or a small group of molecules.
This talk will take an experimentalist view and discuss recent progress in understanding
electronic structure and dynamics at molecule/metal interfaces using two-photon
photoemission (2PPE) spectroscopy. In this approach, the first photon excites an electron from
an occupied molecular or metal state to the unoccupied state. The second photon ionizes the
transiently populated state for detection. This experiment not only probes the unoccupied state
in energy and momentum spaces, but also gives femtosecond resolution. The latter is
particularly important because the transport of an electron across a molecule-metal interface is
inherently a dynamic process in both electronic and nuclear coordinates. A time-resolved
2PPE experiment allows us to probe the rate of interfacial electron transfer, which is directly
proportional to the strength of electronic coupling between a localized molecular orbital and
the delocalized metal band structure. The combination of momentum and time resolution also
enables the direct probe in the time domain of polaron or polaron-exciton formation in the
molecular layer as well as their coupling to the metal surface. In this lecture, I will
demonstrate some of the physical concepts we have learnt from 2PPE measurements in model
systems. These experiments allow us to quantify the following concepts critical to the
understanding of interfacial effects in molecule based electronics: (1) the alignment of
unoccupied molecular orbitals to the metal Fermi level; (2) charge redistribution and the
presence of interfacial dipole; (3) the formation of localized electronic states resulting from
chemical bonding at the interface; (4) the strength and distance dependence of electronic
coupling between unoccupied molecular orbitals/bands and the metal substrate; (5) intermolecular interaction and the formation of molecular conduction band, and (6) the dynamic
localization of electronic excitation in the molecular layer to form a polaron or polaronexciton. Attempts will be made to correlate the spectroscopic measurements to concepts in
transport measurements and to theoretical studies.
Surprising electronic properties of two dimensional chemical systems
Ron Naaman
Department of Chemical Physics, Weizmann Institute, Rehovot 76100, Israel
It is usually assumed that the electronic properties molecules organized in two dimensional structure as a
monolayer are similar to that of the single adsorbed molecule. The weak coupling between the molecules
in a monolayer seems to support this notion.
Applying the photoelectron transmission technique we found that this assumption is generally not justified
and that properties of molecules can vary significantly upon formation of a close packed layer. An
electronic band structure was found in organized organic films and by studying well-characterized
monolayers of polyalanine and other self-assembled monolayers we observed new magnetic properties
never found before.
The new properties will be described and theoretical models will be presented.
Andrey Kaplan
Electron transfer processes in ion/atom scattering on surfaces
V.A. Esaulov
Laboratoire des Collisions Atomiques et Moléculaires, (Unité Mixte de Recherche N° 8625), bât.351,
Université de Paris-Sud, Orsay, France
Electron transfer processes play an important role in particle surface interactions. In this talk I will give an
overview of results of some experiments which deal with a study of electron transfer processes that occur
between an ion and an atom and a clean and adsorbate covered metal surface. Results of an investigation
of surface specific Auger neutralization on the example of the interaction of He ions with Ag(111) and
Ag(110) surfaces will be presented. Experiments here show an order of magnitude difference in ion
survival for scattering in a random (non channeling) direction. This large difference is observed even
though calculations indicate that the ion trajectories are similar above both surfaces. This difference could
be accounted for theoretically and this will be discussed. The second part of the talk will deal with
negative ion (H - or F - ) formation by resonant tunneling and on the effect of the addition of
electronegative adsorbates such as chlorine or oxygen. Adsorbates induce a strong coverage dependent
change in negative ion production. Qualitative explanations which account for these effects will be
proposed.
Theoretical study of reactions of benzene and dibromobenzene at a Si(111) surface.
Ioannis D. Petsalakisa, John C. Polanyib, and Giannoula Theodorakopoulosa
a
Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vass.
Constantinou Ave., Athens 116 35, Greece
bDepartment of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada
The AM1 method and to a lesser extent the DFT method have been employed to study the adsorption of
benzene as well as the molecular dynamics of the halogenation reaction of 1,2- and 1,4-dibromobenzene at
Si(111). The incentive for this study is the interpretation of recent experimental STM results from the
Toronto laboratory on a new electron- induced attachment process for benzene on Si(111), and
particularly experimental results related to the thermal dissociative reactions of 1,2- and 1,4dibromobenzene on a Si(111)7X7 surface. The central objective is to relate the reagent geometry in 1,2dibromobenzene and 1,4-dibromobenzene to the Br--Br pair distance of dibrominated Si(111)7X7. For
benzene we propose a possible path for the conversion from the normal strained di-sigma bound state (S)
at Si(111) to a more strongly bound state (B) consisting of a phenyl plus an H-atom adsorbed species. For
1,2- and 1,4-dibromobenzene evidence has been found for two mechanisms of reaction. One reaction
pathway involves intermediate binding of the organic molecule on the Si surface through C-Si bonds,
analogous to the benzene S structure. This we term ‘benzene-mediated reaction’. The second dynamical
pathway involves intermediate binding through weak Br..Si attachment followed by formation of pairs of
covalently-bound Br-Si. The outcomes from the two dynamical pathways are consistent with the observed
STM patterns for Br-Si at Si(111) 7X7 due to the reaction of 1,2- and 1,4- dibromobenzene.
The evanescent - wave mirror for cold atoms
Bilha Segev
Department of Chemistry, Ben Gurion University of the Negev, POB 653, Beer Sheva 84105, ISRAEL
The evanescent - wave mirror for cold atoms essentially combines laser - atom interaction, atom - surface
interaction, and vacuum fluctuations. As such, it is an interesting system with which to study quantum
dynamics of the reflected atoms and quantum electrodynamics (QED) effects. I will discuss the
engineering of such mirrors for fundamental research.
Probing Nonadiabatic Coupling in Molecular Interactions with Surfaces
Jason White1, Daniel Matsiev1, Jun Chen1, Alec Wodtke1, Daniel Auerbach2
1
Department of Chemistry, University of California, Santa Barbara
2
Hitachi Global Storage Technologies
There is a growing body of indirect evidence that nonadiabatic processes play an important role in
vibrational energy transfer and chemical reactions on metal surfaces. In this talk we explore new direct
experimental probes of nonadiabatic processes. We detect electron emission from a low work function
(~1.3-1.6 eV) metal surface resulting from collisions of vibrationally excited molecules. The experiment
involves preparation of NO in vibrational states up to v=18 by stimulated emission pumping, state
selection in a hexapole field, collision of the molecules with a Cs / Au(111) surface, and detection of
electron emission. Many checks confirm that the observed particles are prompt emission of electrons after
the collision of vibrationally excited NO. Initial information concerning the vibrational state dependence
suggests a threshold close to the work function of the surface. In oxygen exposure studies, the electron
emission probability maximum corresponds roughly with the surface work function minimum. These
results support previous inferences that multi-quantum vibrational relaxation at metal surfaces results from
electron transfer between the surface and the molecule. We discuss possible mechanisms for vibration
induced electron emission. We believe this new technique has great promise to enable a new level of
understanding of nonadiabatic effects in chemical dynamics at metal surfaces and form the basis for a
more complete theoretical understanding.
Qunatum Dynamics of Non-Adiabatic Processes at Surfaces.
Peter Saalfrank
Chemistry Department, University of Podsdam, Potsdam Germany.
In this presentation, we review recent quantum theoretical work of our group towards the modeling and
understanding of non-adiabatic processes at surfaces and interfaces. Specific examples are:

The laser-electron-or STM-induced manipulation of adsorbates at surfaces [1]

The voltage pr laser driven electron transport across molecular or metal-insulator-metal junctions
[2].

The photoelectron spectroscopy of surface states [3].
[1] M. Nest and Peter Saalfrank, J. Chem. Phys. 116, 7189 (2001); M. Nest and Peter Saalfrank,
Submitted (2003); C. Corriol et. al. J. Chem. Phys. 117, 4489 (2002); A Abe. et. al Phys Rev. B 64
035420 (2001).
[2] A Thon et. al. Appl. Phys. A (in press 2003); A. Kopal and P. Saalfrank, Submitted (2003).
[3] T. Klamroth, U. Hofer and P. Saalfrank, Phys. Rev. B 64 035420 (2001).
Multidimensional mixed quantum-classical description of the laser-induced desorption of
molecules
Axel Groß
Physik-Department T30, Technische Universität München, 85747 Garching, Germany
A mixed quantum-classical method for the simulation of laser-induced desorption processes at surfaces
has been implemented [1-3]. In this method, the nuclear motion is described classically while the electrons
are treated quantum mechanically. Still the feedback between nuclei and electrons is taken into account
self-consistently. The computational efficiency of this method allows a more realistic multi-dimensional
treatment of desorption processes. We have applied this method to the laser-induced desorption of NO
from NiO(100) using a two-state two-dimensional potential energy surface derived from ab initio quantum
chemical calculations; we have extended this potential energy surface to seven dimensions employing a
physically reasonable model potential [3].
By comparing our method to jumping wave-packet calculations on exactly the same potential energy
surface we verify the validity of our method. We focus on the velocity, rotational and vibrational
distributions of the desorbing NO molecules. Furthermore, we model the energy transfer to the substrate
by a surface oscillator. Including recoil processes in the simulation has a decisive influence on the
desorption dynamics, as far as the velocity and rotational distribution is concerned. We also model the
excitation process in order to address pump-probe experiments. Furthermore, we employ the efficiency of
the mixed quantum-classical method in order to determine the dependence of the desorption probability in
laser-induced desorption on features of the ground-state and excited state potentials.
[1] J.C. Tully, J. Chem. Phys. 93, 1061 (1990).
[2] C. Bach and A. Groß, J. Chem. Phys. 114, 6396 (2001).
[3] C. Bach, T. Klüner and A. Groß, Chem. Phys. Lett. 376, 424 (2003).
Modulations of Electronic Tunneling Rates Through Flexible Molecular Bridges by a
Dissipative Superexchange Mechanism*
Musa Abu-Hilu and Uri Peskin
Department of Chemistry and The Lise Meitner Center for Computational Quantum Chemistry, TechnionIsrael Institute of Technology, Haifa 32000, Israel1
Coherent long-range electron tunneling is often assisted by intermediate molecular bridges. The effect of
electronic-nuclear coupling on the electronic tunneling rate and mechanism is analyzed using a dissipative
McConnell (superexchange) model. The electron donor and the acceptor are coupled through a dissipative
nuclear bath associated with the nuclear modes of the molecular bridge. At zero temperature and when the
electron tunneling is slower than the nuclear motion, the main effect of electronic-nuclear coupling is the
dissipation of electronic energy into nuclear vibrations at the bridge. At small coupling intensities, the
electronic tunneling rate increases due to this dissipative mechanism. This acceleration is shown to
increase with the “rigidity” of the molecular bridge (i.e., with increasing nuclear frequencies). For large
electronic-nuclear coupling intensities, the tunneling into the acceptor is suppressed and efficient
dissipation leads to electronic trapping (solvation) at the bridge. A Langevin-Schroedinger equation, based
on a mean field approximation, is applied in order to study the corresponding many-body dynamics, and
the results are supported by numerically exact calculations for a single nuclear bridge mode. The analysis
emphasizes the role of nuclear dynamics in controlling ET through molecular bridges (molecular wires)
and it agrees with numerous experimental and theoretical studies demonstrating the effect of the nuclear
bridge conformation and the bridge flexibility on the ET rate.
(*) M. A-Hilu and U. Peskin, Chem. Phys. 2003 (in press).
Nonadiabatic vibronic dynamics as a tool: From surface nanochemistry to new forms of
molecular machines
Tamar Seideman
Department of Chemistry, Northwestern University, Evanston, IL
Resonances are ubiquitous in molecular hetero-junctions and in STM experiments. In the former
environment, resonance tunneling is essential for favorable (non-exponential) wire-length-dependence of
the conductance and is often the mechanism underlying conductance enhancement though application of a
gate voltage. In the latter environment, resonance tunneling has served to develop a powerful vibrational
spectroscopy.
Resonance conductance is often strongly nonadiabatic; in the course of the tunneling event, electron
energy is channeled into vibrational modes and triggers molecular dynamics. The qualitative physics
underlying current-driven resonance-mediated dynamics in molecular electronics is very simple and, to
the audience of this meeting, is familiar from analogous phenomena such as gas phase electron-molecule
scattering and photochemistry on conducting surfaces. Equilibrium displacement between the initial and
resonant states translates into vibronic coupling in the language of the Marcus theory of electron transfer;
it produces a non-stationary superposition in the nuclear subspace that evolves during the resonance
lifetime. Upon relaxation the system is internally excited and interesting dynamics is likely to ensue.
While the underlying physics is very general, the single-molecule STM and molecular heterojunction
environments open unique and exciting opportunities. The former introduces the possibility of
determining resonance lifetimes through fit of experimental voltage dependencies to a quantum
mechanical theory. The latter introduces the possibility of developing coherently driven molecular
machines, a new form of nanolithography, and a new means of manipulating the conductivity of molecular
scale devices.
In the talk I will briefly outline the theory of current-driven dynamics in molecular scale devices, discuss
the results of ongoing research on surface nanochemistry and molecular machines, and mention several of
our dreams and plans in these areas.
Spontaneous and photo-induced, non-adiabatic processes on extended and nanoscale
surfaces
Bengt Kassemo
Titles and Abstracts of Poster
Presentations
Galilean-invariant exchange-correlation functional with quantum memory
Yair Kurzweil and Roi Baer
The Institute of Chemistry and the Lise Meitner Minerva-Center for Computational Quantum Chemistry,
the Hebrew University of Jerusalem, Jerusalem 91904 Israel.
We develop a complete scheme to derive non-adiabatic exchange correlation (XC) potentials for timedependent current-density functional theory that have memory and go beyond the linear response regime
(LR). These are derived from an appropriate action, depending on the electron density and fluid velocity
field, that observes Vignale’s “Galilean invariance” (GI) principle (Phys. Rev. Lett. 74, 3233 (1995)). As a
result from GI, the derived XC potential obeys Newton's third law and the harmonic potential theorem.
We introduce a Lagrangian coordinates system which moves with the electron fluid parcels. GI of the
action is obtained while the density and current density are replaced by their corresponding Lagrangian
quantities. By formulating the action using the Keldysh contour, violation of causality is avoided. The
resulting functional incorporates memory effects within Lagrangian framework. The parameterization of
the action is based on the linear response properties of the homogenous electron gas (HEG). Applications
with improved accuracy of numerical results, like excitations energies, optical spectra and molecular
electronics, of many electrons systems, are predicted.
Electron Transmission through Organized Organic Thin Films of DNA
Supratim Guha Ray, Shirley Daube and Ron Naaman
Department of Chemical Physics, Weizmann Institute, Rehovot, Israel
The electronic properties of DNA are important both for understanding the radiation damage occurring in
living organisms and because of the potential use of DNA in futuristic molecular electronics. We used
photoelectron spectroscopy to study the interaction of low energy electrons with DNA molecules rich with
guanine (G) bases. Organized monolayer of DNA is self-assembled on gold surface. Low energy
electrons (< 2 eV), ejected from gold surface by a laser pulse are allowed to pass through the DNA film
and the electrons transmission yield are measured by a Time of flight spectrometer.
We have found a clear inverse-correlation between the number of G bases in the DNA strand and the
transmission yield. The transmission through the double stranded DNA layer is 2 to 4 times more efficient
than through single stranded DNA. Guanine shows the highest electrons capture cross-section. A model is
proposed to explain the observation. The results obtained provide new information on the processes
occurring in DNA when it is exposed to ionizing radiation.
Adsorbate orientational dependent photoinduced desorption of CH3Br on O2/Ru(001)
Solvejg Jorgensen, Yehuda Zeiri, Ronnie Kosloff, Igal Lilach and Mich Asscher
The Fritz-Haber Research Center for Molecular Dynamics, Hebrew University, Jerusalem, Israel
Methyl bromide has two adsorption configurations on oxygen covered Ru(001) surface. It can adsorb with
the bromine group pointing down or up, denoted as Br-down and Br-up respectively. Upon electronic
excitation of the surface the molecule may desorb or dissociate. The cross section for desorption was
measured [1]. It was found that the probability for desorption depends strongly on the adsorbate
orientation on the surface. For Br-up the cross section for desorption is three times larger than that for the
Br-down. In this poster we analyze theoretically the dynamics leading to desorption for the two different
adsorption configurations.
Dissociative adsorption of X2 molecules on metal surfaces: Non-adiabatic and adiabatic
approaches
E.D. German, I. Efremenko, A.M. Kuznetsov and M. Sheintuch
Influence of the relative phase on the photo-induced dynamics of atoms in strong
bichromatic laser fields
I. Gilhar
The dynamics of endothedral complex formation in surface pick-up scattering as probed
by kinetic energy distributions: Experiment and model calculation for Cs/C60+
Y. Manor
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Participants