Markus Oppel

Analysis and control of laser induced fragmentation processes in CpMn(CO)3

Abstract In this paper we present experimental and theoretical findings about the dynamics of ultrafast fragmentation processes which occur during resonant multiphoton ionization of CpMn(CO) 3 with femtosecond laser pulses. Employing a two-color pump and probe scheme, it was possible to retrieve lifetimes of the electronically excited parent molecule and its first fragment CpMn(CO) 2 . The observed time of 66 fs for the loss of the first CO-ligand is in good agreement with the results of one-dimensional quantum dynamical model simulations based on three relevant adiabatic ab initio potentials and the related components of the transition dipole matrix elements, in C s symmetry. Subsequently, smaller fragments appear somewhat later during approximately 100 fs. Based on these findings we performed feedback control experiments on the system in order to optimize individual fragmentation/ionization paths. With the routine a considerable increase of – for example – the CpMn(CO) + /CpMn(CO) + 3 intensity ratio was achieved. The obtained optimized laser pulses correlate well with the fast dynamics of the photoinduced preparation of CpMn(CO) + 3 versus CpMn(CO) + product ions, respectively.

Hydrogen adsorption on the tetragonal ZrO2(101) surface: a theoretical study of an important catalytic reactant

In order to understand the fundamental properties of the activation of zirconia by hydrogen in relation to the dehydrogenation of hexane, we have performed first-principles calculations using the density functional formalism (PW91 functional) and a plane wave basis set to describe the valence electronic wavefunctions. The interaction of hydrogen atoms and molecules with the thermodynamically most stable, stoichiometric (101) surface has been examined in detail. Three main stages of the hydrogen–ZrO2(101) interaction can be found: a weak interaction corresponding to molecular adsorption of H2 on the top of one surface Zr atom (Ead = −7.1 kJ mol−1), dissociative adsorption, which leaves H atoms on top of one Zr and one O atom (−17.8 kJ mol−1), and, finally, a repulsive interaction (+81.0 kJ mol−1) as precursor of water formation when hydrogen atoms are located above oxygen positions. Desorption of water (+179.9 kJ mol−1) forms a defect at the surface and creates therefore a zirconia suboxide. To separate these effects, atomic hydrogen adsorption has also been considered. Changes in the geometry and charge are discussed as well as the band structures of the adsorbates relative to the vacuum energy. The results are discussed and compared with the available experimental data.

IR + UV laser pulse control of momenta directed to specific products: Quantum model simulations for HOD* → H + OD versus HO + D

Selective bond breaking may be achieved in two steps. First, an intense ultrashort, i.e. few-cycle infrared (IR) laser pulse creates momentum along the bond to be broken. Another ultrashort few-cycle ultraviolet (UV) laser pulse then induces a Franck–Condon (FC)-type transition from the electronic ground to an excited state. The initial bond selective momentum is approximately conserved during this FC-type transition, thus causing a stretch and finally a break in the specific bond. Bond selectivity via few-cycle IR + UV laser pulses can be achieved even if the forces of the excited molecule are not bond selective in the domain of the FC-type transition. The mechanism is demonstrated by means of quantum simulations of IR + UV laser driven wavepackets of the model system, HOD* → H + OD versus HO + D.

Efficient and Flexible Computation of Many-Electron Wave Function Overlaps

A new algorithm for the computation of the overlap between many-electron wave functions is described. This algorithm allows for the extensive use of recurring intermediates and thus provides high computational efficiency. Because of the general formalism employed, overlaps can be computed for varying wave function types, molecular orbitals, basis sets, and molecular geometries. This paves the way for efficiently computing nonadiabatic interaction terms for dynamics simulations. In addition, other application areas can be envisaged, such as the comparison of wave functions constructed at different levels of theory. Aside from explaining the algorithm and evaluating the performance, a detailed analysis of the numerical stability of wave function overlaps is carried out, and strategies for overcoming potential severe pitfalls due to displaced atoms and truncated wave functions are presented.

Enhancing Intersystem Crossing in Phenotiazinium Dyes by Intercalation into DNA

Phenothiazinium dyes are used as photosensitizers in photodynamic therapy. Their mode of action is related to the generation of triplet excited states by intersystem crossing. Therefore, rationalizing the factors that influence intersystem crossing is crucial to improve the efficacy of photodynamic therapy. Here we employ quantum mechanics/molecular mechanics calculations to investigate the effect of aqueous and nucleic acid environments on the intersystem crossing mechanism in methylene blue. We find that the mechanism by which the triplet states are generated depends strongly on the environment. While intersystem crossing in water is mediated exclusively by vibronic spin-orbit coupling, it is enhanced in DNA due to a second pathway driven by electronic spin-orbit coupling. Competing charge-transfer processes, which are also possible in the presence of DNA, can therefore be suppressed by a suitable structural functionalization, thereby increasing the efficacy of photodynamic therapy.

Theory of ultrafast nonresonant multiphoton transitions in polyatomic molecules: basics and application to optimal control theory.

A systematic approach is presented to describe nonresonant multiphoton transitions, i.e., transitions between two electronic states without the presence of additional intermediate states resonant with the single-photon energy. The method is well suited to describe femtosecond spectroscopic experiments and, in particular, attempts to achieve laser pulse control of molecular dynamics. The obtained effective time-dependent Schrodinger equation includes effective couplings to the radiation field which combine powers of the field strength and effective transition dipole operators between the initial and final states. To arrive at time-local equations our derivation combines the well-known rotating wave approximation with the approximation of slowly varying amplitudes. Under these terms, the optimal control formalism can be readily extended to also account for nonresonant multiphoton events. Exemplary, nonresonant two- and three-photon processes, similar to those occurring in the recent femtosecond pulse-shaping experiments on CpMn(CO)(3), are treated using related ab initio potential energy surfaces.

Excited state photoelectron spectroscopy on molecular aggregates containing aromatic molecules

Abstract The photoelectron spectra of anisole and its van der Waals aggregates with argon (1:1 and 1:2) or carbon dioxide (1:1) have been measured using resonant two-photon laser ionization and a magnetic bottle spectrometer. The influence of the aggregation toward the intramolecular vibrations is discussed. The evaluation of the photoelectron spectra suggests that two different isomers of the anisole–argon (1:2) aggregate exist in the supersonic beam.

Ultrafast laser control of vibrational dynamics for a two-dimensional model of HONO2 in the ground electronic state: separation of conformers, control of the bond length, selective preparation of the discrete and the continuum states

Abstract Selective excitation of the vibrational bound and the continuum states, controlled by subpicosecond infrared (IR) laser pulses, is simulated within the Schrodinger wave function formalism for a two-dimensional model of the HONO2 molecule in the ground electronic state. State-selective excitation of the OH bond is achieved by single optimal laser pulses, with the probability being 97% for the bound states and more than 91% for the resonances. Stable, long-living continuum states are prepared with more than 96% probability by two optimal laser pulses, with the expectation energy of the molecule being well above the dissociation threshold of the ON single bond, and its life-time being at least 100 ps. The length of the ON single bond can be controlled selectively: stretching and contraction by about 45% of its equilibrium length are demonstrated. Laser separation of spatial conformers of HONO2 in inhomogeneous conditions occurring on an anisotropic surface or created by a direct current (DC) electric field is analysed. The relative yields of target conformers may be very high, ranging from 10 to 108, and the absolute yields of up to 40% and more are calculated.

Nuclear Spin Selective Torsional States: Implications of Molecular Symmetry

By Steffen Belz, Omar Deeb, Leticia González, Thomas Grohmann1,∗, Daniel Kinzel, Monika Leibscher,, Jörn Manz,, Rana Obaid2,3,∗, Markus Oppel3,∗, George Densingh Xavier,, and Shmuel Zilberg 1 Institut für Chemie and Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany 2 Faculty of Pharmacy, Al-Quds University, Abu Dis, Palestine 3 Institut für Theoretische Chemie, Universität Wien, Währinger Straße 17, 1090 Vienna, Austria 4 Institut für Theoretische Physik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany 5 Laser Spectroscopy Laboratory, Shanxi University, Wucheng Road 92, 030006 Taiyuan, China 6 Institute of Chemistry, The Edmond Safra Campus, Givat Ram, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel 7 Department of Chemistry, Indian Institute of Technology, 600 032 Madras, India

Discrimination of nuclear spin isomers exploiting the excited state dynamics of a quinodimethane derivative

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.