Biomedical subjects

Volkhard May

Publications and source records attributed to Volkhard May.

8 recordsLinked to original sources

PubMed2006

Self-trapping of the N-H vibrational mode in alpha-helical polypeptides.

Recent calculations on the formation of self-trapped amide group vibrational states in alpha-helical polypeptides [J. Chem. Phys. 124, 134907 (2006)] are extended to the amide N-H normal mode vibrations. First, the adiabatic N-H vibrational single- and two-exciton states are examined by treating the longitudinal chain coordinates as parameters. Then, in using the multiconfiguration time-dependent Hartree method coupled exciton-chain vibrational quantum dynamics are accounted for. Based on the respective exciton-chain vibrational wave function propagation the infrared transient absorption related to a sequential pump-probe experiment is calculated. The modulation of local amide vibrational energies by the longitudinal chain coordinates is found to have a pronounced effect on the broadening of absorption lines. Moreover, the ultrafast exciton transfer in the system is studied in order to characterize the dynamics of the self-trapped single-exciton states on a time scale below 10 ps.

Computer Simulation↗

PubMed2006

Femtosecond laser pulse control of multidimensional vibrational dynamics: Computational studies on the pyrazine molecule.

The multiconfiguration time-dependent Hartree (MCTDH) method is combined with the optimal control theory (OCT) to study femtosecond laser pulse control of multidimensional vibrational dynamics. Simulations are presented for the widely discussed three-electronic-level vibronic coupling model of pyrazine either in a three or four vibrational coordinate version. Thus, for the first time OCT is applied to a four-coordinate system. Different control tasks are investigated and also some general aspects of the OCT-MCTDH method combination are analyzed.

Journal Article↗

PubMed2006

Vibrational excitons in alpha-helical polypeptides: multiexciton self-trapping and related infrared transient absorption.

Based on the multiexciton expansion of a model Hamiltonian, an accurate quantum-dynamical description of vibrational states formed by amide modes in alpha-helical polypeptides is presented. Using the multiconfiguration time-dependent Hartree method, linear and pump-probe infrared absorption spectra are calculated by numerical time propagation of the exciton-chain vibrational wave function. The formation of self-trapped exciton states is discussed within the approximation of adiabatic excitons and within the full quantum description.

Journal Article↗

PubMed2006

Ultrafast heterogeneous electron transfer reactions: comparative theoretical studies on time- and frequency-domain data.

Recent theoretical studies on linear absorption spectra of dye-semiconductor systems [perylene attached to nanostructured TiO2, L. Wang et al., J. Phys. Chem. B 109, 9589 (2005)] are extended here in different respects. Since the systems show ultrafast photoinduced heterogeneous electron transfer the time-dependent formulation used to compute the absorbance is also applied to calculate the temporal evolution of the sub-100 fs charge injection dynamics after a 10 fs laser-pulse excitation. These studies complement our recent absorption spectra fit for two perylene bridge-anchor group TiO2 systems. Moreover, the time-dependent formulation of the absorbance is confronted with a frequency-domain description. The latter underlines the central importance of the self-energy caused by the coupling of the dye levels to the semiconductor band continuum. The used model is further applied to study the effect of different parameters such as (1) the dependence on the reorganization energies of the involved intramolecular transitions, (2) the effect of changing the transfer integral which couples the excited dye state with the band continuum, and (3) the effect of the concrete form of the semiconductor band density of states. Emphasis is also put on the case where the charge injection level of the dye is near or somewhat below the band edge. This nicely demonstrates the change from a structureless absorption to a well-resolved vibrational progression including characteristic shifts of the absorption lines which are a direct measure for the dye-semiconductor coupling.

Journal Article↗

PubMed2005

Absorption spectra related to heterogeneous electron transfer reactions: the perylene TiO2 system.

Linear absorption spectra of dye-semiconductor systems (perylene attached to nanostructured TiO2) are studied theoretically and experimentally. The systems show ultrafast photoinduced heterogeneous electron transfer (HET). By applying a time-dependent formulation of the absorbance, the theoretical analysis of the measured data is carried out. The respective electron-vibrational wave packet propagation fully accounts for the electronic coupling to the conduction band continuum of TiO2 and is based on a single-reaction-coordinate model (corresponding to a perylene in-plane C-C stretching vibration with a quantum energy of 1370 cm(-1)). By the insertion of different bridge-anchor groups, the electronic coupling responsible for HET is varied. The dye absorbance in a solvent and the trends in the line broadening of the vibrational progression due to the coupling to the conduction band continuum are reproduced for all investigated types of bridge-anchor groups. HET rates deduced from the calculations on the absorbance displaying line broadenings follow the qualitative trend obtained from transient absorption spectra.

Journal Article↗

PubMed2004

Laser pulse control of ultrafast heterogeneous electron transfer: a computational study.

Laser pulse control of the photoinduced 90 fs charge injection from perylene into the conduction band of TiO2 is studied theoretically. The approach accounts for the electronic-ground state of the dye, the first excited state, the ionized state formed after charge injection, and the continuum of the electronic states in the conduction band, all defined vs a single reaction coordinate. To address different control tasks optimal control theory is combined with a full quantum dynamical description of the electron-vibrational motion accompanying the charge injection process. First it is proved in which way the charge injection time can be changed by tailored laser pulses. In a second step a pump-dump scheme from the perylene ground state to the first excited electronic state and back to the ground state is discussed. Because of the strong coupling of the excited perylene state to the band continuum of TiO2 this control task is more suited to an experimental test than the direct control of the charge injection.

Journal Article↗

PubMed2004

Optimal control theory for a target state distributed in time: optimizing the probe-pulse signal of a pump-probe-scheme.

Optimal control theory (OCT) is formulated for the case of a two-color pump-probe experiment. The approach allows to calculate the pump-pulse shape in such a way that the probe-pulse absorption signal is maximized. Since the latter quantity is given by the time-averaged expectation value of a time dependent operator (the probe-pulse field-strength times the dipole operator) a version of OCT has to be used where the target state is distributed in time. The method is applied to a molecular three-level system with the pump-pulse driving the transition from the electronic ground state into the first-excited electronic state and the probe-pulse connecting the first-excited state with a higher lying electronic state. Depending on the probe-pulse duration, the vibrational wave packet becomes localized or at least highly concentrated in the Franck-Condon window for the transition into the higher-excited state. The dependence on the probe-pulse duration and on the delay time between the optimized pump-pulse and the probe-pulse is discussed in detail. The whole study demonstrates the feasibility of laser pulse induced temporal wave packet localization and the use of spectroscopic quantities as target states in experiments on femtosecond laser pulse control.

Journal Article↗

PubMed2004

Vibrational effects in laser-driven molecular wires.

The influence of an electron-vibrational coupling on the laser control of electron transport through a molecular wire that is attached to several electronic leads is investigated. These molecular vibrational modes induce an effective electron-electron interaction. In the regime where the wire electrons couple weakly to both the external leads and the vibrational modes, we derive within a Hartree-Fock approximation a nonlinear set of quantum kinetic equations. The quantum kinetic theory is then used to evaluate the laser driven, time-averaged electron current through the wire-leads contacts. This formalism is applied to two archetypical situations in the presence of electron-vibrational effects, namely, (i) the generation of a ratchet or pump current in a symmetrical molecule by a harmonic mixing field and (ii) the laser switching of the current through the molecule.

Journal Article↗