No abstract

A method for solving the time-dependent Schrödinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

The ionization yield of molecular hydrogen exposed to intense laser pulses is studied as a function of the internuclear distance R. The results obtained by means of the full solution of the three-dimensional time-dependent Schrödinger equation (TDSE) describing electronic motion are compared with those predicted by the quasi-static approximation (QSA). It is found that for laser pulses with a wavelength of 800 nm and peak intensities between 3.5 × 1013 W cm−2 and 1 × 1014 W cm−2 the QSA predicts an R dependence that is in qualitative agreement with the TDSE results, provided a vertical ionization potential is used in the QSA model. The quantitative agreement depends, however, strongly on the laser intensity. While the yields for 800 nm vary smoothly with R, this is not the case for 266 nm (and 3.5 × 1013 W cm−2). As expected, for these parameters the ionization dynamics is better described as a multi-photon process; it is strongly influenced by channel closing and the appearance of resonantly enhanced multi-photon ionization.

Extended all-sp 2 -carbon macromolecules have the potential to replace silicon in integrated nanometer-scale devices. Up to now, studies on the electronic properties of such structures, for example, graphene nanoribbons, have been focused mostly on the infinitely long limit, which is inadequate when approaching future devices with sub-10 nm control. Moreover, their electronic variation has not been systematically assessed as a function of chemically diverse edge termini. Such knowledge is central when prototyping potential allcarbon circuits. Here, we present a graphene nanoribbon energy gap atlas based on density functional tight-binding spin-polarized calculations of nearly ten thousand randomly generated nanoribbons with a maximal nominal width of 1 nm and an armchair long edge. We classify ribbon families and show that their energy levels are strongly dependant on their termini edge states. We notably reveal modulation of the bulk energy gap by 0.3 eV through minimal edge modifications and put forward simple rules for inducing antiferromagnetic edge states.

Ionization, excitation, and de-excitation to the ground state is studied theoretically for the first excited singlet state B 1 Σ + u of H2 exposed to intense laser fields with photon energies in between about 3 eV and 13 eV. A parallel orientation of a linear polarized laser and the molecular axis is considered. Within the dipole and the fixed-nuclei approximations the time-dependent Schrödinger equation describing the electronic motion is solved in full dimensionality and compared to simpler models. A dramatic break-down of the single-active-electron approximation is found and explained to be due to the inadequate description of the final continuum states.