Volker Engel

Exciton Trapping in π-Conjugated Materials: A Quantum-Chemistry-Based Protocol Applied to Perylene Bisimide Dye Aggregates

Access to excited-state structures and dynamics of pi-chromophor aggregates is needed to understand their fluorescence behavior and the properties of related materials. A quantum-chemistry-based protocol that provides quantitative and qualitative insight into fluorescence spectra has been applied to perylene bisimide dimers and provides excellent agreement with measured fluorescence spectra. Both dispersion and dipol-dipole interactions determine the preferred relative arrangements of the chromophores in ground and excited states of the dimer. An exciton trapping mechanism is identified, which may limit the energy transfer properties of perylene bisimide and other dye materials.

The calculation of autocorrelation functions for spectroscopy

It is shown that the autocorrelation function, i.e. the scalar product 〈ϕ(0)¦ϕ(t)〉 of a wave packet at times zero and t, may be calculated from the wave packet propagated until a time 12t. This automatically allows the computation time in the calculation of absorption spectra to be reduced by a factor of two.

Femtosecond pump—probe study of the spreading and recurrence of a vibrational wave packet in Na2

Abstract The motion of a vibrational wave packet in the bound A (1Σu+) electronic state of the sodium dimer is detected in a femtosecond pump/probe molecular beam experiment. For short times harmonic motion is seen in the total ion yield of Na2+ as a function of delay time between the two laser pulses. The spreading of the wave packet results in the loss of the periodic variation of the ion signal. For longer delay times (47 ps) the wave packet regains its initial form which is reflected in the revival structure of the Na2+ signal. Time-dependent quantum calculations reproduce the measured effects.

Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates.

Femtosecond time-resolved experiments demonstrate that the photoexcited state of perylene tetracarboxylic acid bisimide (PBI) aggregates in solution decays nonradiatively on a time-scale of 215 fs. High-level electronic structure calculations on dimers point toward the importance of an excited state intermolecular geometry distortion along a reaction coordinate that induces energy shifts and couplings between various electronic states. Time-dependent wave packet calculations incorporating a simple dissipation mechanism indicate that the fast energy quenching results from a doorway state with a charge-transfer character that is only transiently populated. The identified relaxation mechanism corresponds to a possible exciton trap in molecular materials.

Vibronic energies and spectra of molecular dimers

We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipole-dipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the Jaynes-Cummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the time-dependent Schrodinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra.

Mapping of wave‐packet dynamics in a double‐well potential via femtosecond pump/probe photoelectron spectroscopy

The kinetic energy distribution of electrons obtained from femtosecond pump/probe ionization of Na2 is calculated. The pump laser pulse prepares a vibrational wave packet in the 1Σ+u double minimum potential of the molecule which serves as the initial state for subsequent ionization induced by absorption of another photon from the probe pulse. The time dependence of the electron kinetic energy distribution reflects details of the vibrational dynamics. In particular the splitting of the packet can be observed in the case when its mean energy equals the barrier height between the potential wells. The estimate of several molecular properties from a theoretical analysis of the electron spectra is carried through and discussed.

Excitation of molecules with ultrashort laser pulses: exact time-dependent quantum calculations

Abstract Excitation processes with ultrashort laser pulses are described within a time-dependent, quantum mechanical approach. First-order perturbation theory is employed. The predissociation of sodium iodine and methyl nitrite serve as numerical examples. Computational difficulties which arise for long propagation times are discussed. The calculated wavefunctions are used to compute excited-state populations, resonance lifetimes and to demonstrate the buildup of fragment distributions in time.

Femtosecond pump/probe experiments and ionization: the time dependence of the total ion signal

Abstract We investigate the pump/probe femtosecond spectroscopy of a diatomic molecule which is ionized by the absorption of a photon from the probe laser pulse. As a realistic example, an electronic transition of the sodium dimer is investigated. The dependence of the total ion signal on the delay time between the pump and probe pulse is studied for different frequencies of the ionizing laser. Also, non-Condon effects are shown to be important under certain conditions.

APPROXIMATIVE CALCULATION OF SHORT-PULSE PUMP-PROBE IONIZATION SIGNALS

Approximative calculations of pump–probe ionization signals induced by short laser pulses are compared to numerically exact calculations performed in first‐order perturbation theory. The motivation is to demonstrate that in cases where the wavepacket prepared by the pump pulse does not move over a region where the difference between the potentials of the involved electronic states changes significantly, the computational effort can be reduced dramatically. This is of great importance for quantum mechanical simulations of systems with several degrees of freedom.

A theoretical study of I2 vibrational motion after excitation with an ultrashort pulse

We calculate the population created by a short pump pulse exciting the I2 molecule to the bound region of the B state, followed by excitation with a short probe pulse to the state E (or F). The nuclear state produced by the pump oscillates in the well of the B state and the probe is absorbed to populate the E (or F) state only when the wave function passes through the Franck–Condon region of the B→E (or B→F) transition. Because of this, the population on the E (or F) state oscillates with the delay time between the pump and the probe. The calculations agree with the experiment in the case when the probe excites the E state. When the F state is excited the theory predicts a doublet structure which is not observed; moreover, in some cases the experiment and theory differ at the shortest delay times. We discuss the dependence of the LIF signal on the pulse width and the initial state, the long time behavior of the LIF signal, and illustrate the role of the population transients on the B states at early times...