Communication: On the calculation of time-dependent electron flux within the Born-Oppenheimer approximation: A flux-flux reflection principle.

It is commonly assumed that the time-dependent electron flux calculated within the Born-Oppenheimer (BO) approximation vanishes. This is not necessarily true if the flux is directly determined from the continuity equation obeyed by the electron density. This finding is illustrated for a one-dimensional model of coupled electronic-nuclear dynamics.

Mapping of exciton-exciton annihilation in MEH-PPV by time-resolved spectroscopy: experiment and microscopic theory.

Transient absorption traces taken on samples of the polymer MEH-PPV are measured as a function of the laser intensity. In increasing the laser power, different decay dynamics of the signal are obtained. This suggests that effective exciton-exciton annihilation takes place.

Coupled electron-nuclear quantum dynamics through and around a conical intersection.

In solving the time-dependent Schrödinger equation for a coupled electron-nuclear system, we study the motion of wave packets in a model which exhibits a conical intersection (CoIn) of adiabatic potential energy surfaces. Three different situations are studied. In the first case, an efficient non-adiabatic transition takes place while the wave packet passes the region of the CoIn.

Electron-nuclear wave-packet dynamics through a conical intersection.

We investigate the coupled electron-nuclear dynamics in a model system showing a conical intersection (CoIn) between two excited state potential energy surfaces. Within the model, a single electron and nucleus move in two dimensions in an external static field. It is demonstrated that the nuclear density conserves its initial Gaussian shape when directly passing the CoIn, whereas the electronic density remains approximately constant.

Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.

Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers.

H2CN+ and H2CNH+: new insight into the structure and dynamics from mass-selected threshold photoelectron spectra.

In this paper, we reinvestigate the photoionization of nitrogen containing reactive intermediates of the composition H2CN and H2CNH, molecules of importance in astrochemistry and biofuel combustion. In particular, H2CN is also of considerable interest to theory, because of its complicated potential energy surface. The species were generated by flash pyrolysis, ionized with vacuum ultraviolet synchrotron radiation, and studied by mass-selected threshold photoelectron (TPE) spectroscopy.

Weak-field, multiple-cycle carrier envelope phase effects in laser excitation.

Although the absolute or carrier envelope phase (CEP) of a laser pulse is usually assumed to be effective for ultrashort and/or ultrastrong pulses only, it is demonstrated that these limitations can eventually be removed. Therefore, the excitation of a model positively charged homonuclear diatomic molecule, in which four electronic states are coupled by the laser field, is studied. In an initial step, nuclear wave packets in two dissociative states are prepared.

Fragment momentum distributions obtained from coupled electron-nuclear dynamics.

We theoretically investigate fragmentation processes induced by femtosecond laser pulses within a model which incorporates electronic and nuclear motion. Single-pulse excitation leads to diffraction patterns in the electron momentum distribution which depend on the nature of the electronic state and also on the nuclear charge distribution. Additional structures appear in the nuclear momentum distribution if two time-delayed pulses produce fragments in the same dissociation channel.