Efficient Semiclassical Evaluation of Electronic Coherences in Polyatomic Molecules

Exposing a molecule to an intense light pulse can create a nonstationary quantum state, thus launching correlated dynamics of electrons and nuclei. Although much had been achieved in the understanding of fundamental physics behind the electron-nuclear interactions and dynamics, accurate numerical simulations of light-induced processes taking place in polyatomic molecules remain a formidable challenge. Here, we review a recently developed theoretical approach for evaluating electronic coherences in molecules, in which the ultrafast electronic dynamics is coupled to nuclear motion. The presented technique, which combines accurate ab initio on-the-fly simulations of electronic structure with efficient semiclassical procedure to compute the dynamics of nuclear wave packets, is not only computationally efficient, but also can help shed light on the underlying physical mechanisms of decoherence and revival of the electronic coherences driven by nuclear rearrangement.

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

Christoph Grosse

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of

EPFL2015

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

This thesis presents the first experimental results of a scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) investigation of rubrene at the supramolecular, molecular and submolecular level. Based on its semiconducting and fluorescent properties, this molecule is of particular interest in view of the emerging fields of molecular electronics and optoelectronics which could one day replace the conventional technology relying on semiconductors such as silicon and gallium arsenide. The goal is the substitution of these inorganic materials by cheap and flexible layers of semiconducting organic molecules for a new class of diodes and transistors, as well as the realization of electronic switches based on individual molecules. One fundamental approach is to take advantage of the molecular self-assembly behavior which results in the creation of well-ordered supramolecular structures. The investigations of the self-assembly of rubrene adsorbed on metal surfaces (Au(111), Au(100), Ag(111), and Ag(100)), which were carried out within the framework of this thesis, show a surprising diversity of supramolecular structures. Amongst other shapes, the molecules organize themselves into geometries of perfect hexagonal and pentagonal symmetry and create multifaceted patterns on the surface. A fascinating peculiarity consists in the spontaneous construction of nested structures which are built up by a hierarchical self-assembly of individual molecules into pentagonal supermolecules which form in a second step perfect supramolecular decagons. The geometric shape of rubrene is characterized by a structural asymmetry leading to the existence of two mirror imaged versions of the molecule which are not superimposable to each other, such as for instance our left and right hand or the helical DNA. The aspect of chirality is crucial for basic processes in living systems and calls for a fundamental understanding of the interaction mechanisms occurring between chiral molecules. The experiments on rubrene reveal that the intermolecular bonding differentiates between the two chiral types of the molecule (chiral recognition), yielding the self-organization into homochiral structures. These assemblies exhibit a geometry which is again chiral, demonstrating a propagation of chirality throughout the three stages of the supramolecular hierarchy. The semiconducting behavior of rubrene is furthermore probed by STS measurements detecting the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The experimental data uncover that different adsorption conformations exhibit characteristic HOMO energies and reveal adsorption conformations of rubrene which preserve the intrinsic electronic structure of the free molecule. Furthermore, a switching of the molecular conformation and the electronic structure of one rubrene conformer is induced with the STM.

EPFL2006

First-Principles Simulations of C-S Bond Cleavage in Rhenium Thioether Complexes

Ursula Röthlisberger

We present first-principles mol. dynamics studies of the reductive C-S bond cleavage reaction in hexathioether complexes of the form [Re(9S3)2]m+ (with 9S3 = 1,4,7 trithiacyclononane and m = 1,2). Our calcns. show that electron transfer and bond dissocn. take place as two distinct consecutive reaction steps. For the reduced complex, C-S bond fission and subsequent release of ethene can be obsd. directly at only slightly elevated temps. Car-Parrinello mol. dynamics of the reactive process demonstrate that for the dissocn. to occur two carbon-sulfur bonds have to be broken quasi simultaneously. For the oxidized form on the other hand, no release of ethene takes place at the same temp. within the limited time scale of our simulations. The activation energies of the dissocn. process calcd. at the gradient-cor. d. functional (BP) level are 21 and 10 kcal/mol for the oxidized and the reduced form, resp. A detailed anal. of the electronic structure in the transition states confirms the presence of a strong p-back-donation from rhenium d-orbitals into antibonding s*-orbitals of the C-S bonds that is responsible for the pronounced weakening of the carbon-sulfur bond upon redn. [on SciFinder (R)]

2004

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

EPFL2006

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

Christoph Grosse

EPFL2015