Directional States of Symmetric-Top Molecules Produced by Combined Static and Radiative Electric Fields

We show that combined electrostatic and radiative fields can greatly amplify the directional properties, such as axis orientation and alignment, of symmetric top molecules. In our computational study, we consider all four symmetry combinations of the prolate and oblate inertia and polarizability tensors, as well as the collinear and perpendicular (or tilted) geometries of the two fields. In, respectively, the collinear or perpendicular fields, the oblate or prolate polarizability interaction due to the radiative field forces the permanent dipole into alignment with the static field. Two mechanisms are found to be responsible for the amplification of the molecules' orientation, which ensues once the static field is turned on: (a) permanent-dipole coupling of the opposite-parity tunneling doublets created by the oblate polarizability interaction in collinear static and radiative fields; (b) hybridization of the opposite parity states via the polarizability interaction and their coupling by the permanent dipole interaction to the collinear or perpendicular static field. In perpendicular fields, the oblate polarizability interaction, along with the loss of cylindrical symmetry, is found to preclude the wrong-way orientation, causing all states to become high-field seeking with respect to the static field. The adiabatic labels of the states in the tilted fields depend on the adiabatic path taken through the parameter space comprised of the permanent and induced-dipole interaction parameters and the tilt angle between the two field vectors

Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory

In this work, the structures of cationic SinNb+ (n = 4-12) clusters are determined using the combination of infrared multiple photon dissociation (IR-MPD) and density functional theory (DFT) calculations. The experimental IR-MPD spectra of the argon complexes of SinNb+ are assigned by comparison to the calculated IR spectra of low-energy structures of SinNb+ that are identified using the stochastic 'random kick' algorithm in conjunction with the BP86 GGA functional. It is found that the Nb dopant tends to bind in an apex position of the Si-n framework for n = 4-9 and in surface positions with high coordination numbers for n = 10-12. For the larger doped clusters, it is suggested that multiple isomers coexist and contribute to the experimental spectra. The structural evolution of SinNb+ clusters is similar to V-doped silicon clusters (J. Am. Chem. Soc., 2010, 132, 15589-15602), except for the largest size investigated (n = 12), since V takes an endohedral position in Si12V+. The interaction with a Nb atom, with its partially unfilled 4d orbitals leads to a significant stability enhancement of the Si-n framework as reflected, e.g. by high binding energies and large HOMO-LUMO gaps.EC/FP7/226716/EU/European Light Sources Activities - Synchrotrons and Free Electron Lasers/ELISADFG, FOR 1282, Controlling the electronic structure of semiconductor nanoparticles by doping and hybrid formatio

Theoretical description of adiabatic laser alignment and mixed-field orientation: the need for a non-adiabatic model

We present a theoretical study of recent laser-alignment and mixed-field-orientation experiments of asymmetric top molecules. In these experiments, pendular states were created using linearly polarized strong ac electric fields from pulsed lasers in combination with weak electrostatic fields. We compare the outcome of our calculations with experimental results obtained for the prototypical large molecule benzonitrile (C$_7$H$_5$N) [J.L. Hansen et al, Phys. Rev. A, 83, 023406 (2011)] and explore the directional properties of the molecular ensemble for several field configurations, i.e., for various field strengths and angles between ac and dc fields. For perpendicular fields one obtains pure alignment, which is well reproduced by the simulations. For tilted fields, we show that a fully adiabatic description of the process does not reproduce the experimentally observed orientation, and it is mandatory to use a diabatic model for population transfer between rotational states. We develop such a model and compare its outcome to the experimental data confirming the importance of non-adiabatic processes in the field-dressed molecular dynamics.Comment: 11 pages, 9 figure

Aktionsspektroskopie von stark gebundenen Clustern in der Gasphase

Ziel der Doktorarbeit ist es, spektroskopische Informationen über die Geometrie von stark gebundenen Clustern in der Gasphase als Funktion ihrer Größe und ihres Ladungszustandes experimentell zu ermitteln. Der Schwerpunkt liegt auf der Entwicklung von Methoden zur Infrarot-Spektroskopie von Clustern unter Verwendung von weniger üblichen Lichtquellen, Infrarot Freie-Elektronen-Lasern. Dabei werden Verfahren mit breiter Anwendbarkeit präsentiert, welche die Untersuchung bisher unzugänglicher Systeme erlauben. Diese neuen Techniken werden mit etablierteren Methoden kombiniert, um tiefere Einblicke in ein einzelnes System (Magnesiumoxid) zu erlangen. Mittels abstimmbarer IR-UV-Zwei-Farben-Ionisation werden die IR-Spektren von neutralen Clustern bestimmt. Halbleiter und Metalloxid Cluster werden mit ihr im fernen und mittleren Infrarotbereich untersucht. Dabei kommt der Freie-Elektronen-Laser für Infrarot-Experimente (FELIX) zum Einsatz. Für anionische (Übergangsmetall-) Cluster können IR-Spektren mittels IR-Resonanz-Verstärkter-Mehr-Photonen-Elektronenablösung aufgenommen werden. Diese Technik wurde erst kürzlich durch die Verfügbarkeit einer intensiveren Infrarot Strahlungsquelle, dem Freie-Elektronen-Laser für Intra-Cavity-Experimente (FELICE), ermöglicht. Die hier vorgestellten Experimente zählen zu den ersten, die diesen neuen Laser verwenden. Die Instrumentenentwicklung und der Nachweis des Funktionsprinzips der Messmethode erfolgen durch Messungen an Metallkarbiden. Diese wurden zusätzlich mit Anionen-Photoelektronen-Spektroskopie untersucht, welche nützliche Informationen über die Energetik des Elektronenauslöseprozesses liefert. Die Methode wird anschließend dazu genutzt, IR-Spektren von Übergangsmetall-Clustern aus Niob zu gewinnen. Magnesiumoxid-Cluster mit unterschiedlicher Größe, Ladungszustand oder Stoichiometrie werden mittels verschiedenster IR Spektroskopiemethoden untersucht. Diese Cluster dienen als Modellsysteme für einen der vielversprechendsten Katalysatoren für die Aktivierung von Methan. Die strukturellen Unterschiede zwischen verschieden geladenen Gasphasen-Clustern und der Festkörperstruktur, sowie der Einfluss von zusätzlichem Sauerstoff und/oder Wasserstoff, werden erforscht. Zusätzlich wird die Reaktivität der Cluster mit verschiedenen Liganden, wie Wasser, Methan und Kohlenstoffmonoxid analysiert.The work presented in this thesis aims to provide experimental spectroscopic data to determine the geometric structures of strongly bound gas-phase clusters as function of their size and charge state. The development of spectroscopic methods which allow the study of the infrared spectra of clusters while using less common IR sources, infrared free electron lasers, was among the major objectives. Very generally applicable methods are presented, which allow previously inaccessible systems to be investigated. These new techniques are combined with more established methods to obtain a deeper insight into the properties of a single system, magnesium oxide clusters. Tunable IR-UV two-color ionization allows the investigation of the IR spectra of neutral clusters. It is used to study semiconductor and metal oxide clusters in the mid and far-IR with the Free Electron Laser for Infrared eXperiments (FELIX) as the IR light source. For anionic (transition metal) clusters IR spectra are obtained using IR resonance enhanced multiple photon electron detachment spectroscopy, a technique that recently became possible with the availability of a more intense IR light source, the Free Electron Laser for Intra-Cavity Experiments (FELICE). The experiments are among the first utilizing this new laser. Instrument development and proof-of-principle experiments are performed for metal carbide clusters, which are also studied by anion photoelectron spectroscopy to provide useful information about the energetics of the electron detachment process. The method is then applied for the IR spectroscopy of the transition metal cluster anions of niobium. Magnesium oxide clusters of different size, charge state, and stoichiometry are studied by different IR spectroscopy methods. These cluster systems serve as models for a promising catalyst for the activation of methane. The structural differences between differently charged gas-phase clusters and the bulk structure are investigated, as well as the effects of the addition of oxygen and/or hydrogen to the clusters. Furthermore, reactions of the clusters with different ligands, such as water, methane, or carbon monoxide are examined