Pascal Krause

Nonadiabatic Events and Conical Intersections

Nonadiabatic events, in which the Born-Oppenheimer approximation breaks down, are ubiquitous in chemistry and biology. It is now widely accepted that they are facilitated by conical intersections (CIs), actual degeneracies between electronic states. We review the basic theory of CIs and how they can be studied using modern quantum chemistry and nuclear dynamics. We highlight their importance by presenting their role in radiationless decay pathways present in the building blocks of DNA and proteins. The presence of CIs may contribute to the photostability of these important biomolecules.

Time-dependent configuration-interaction calculations of laser-pulse-driven many-electron dynamics: Controlled dipole switching in lithium cyanide

We report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (doubles) approach. The method accounts for the correlation of ground and excited states, is capable of describing explicitly time-dependent, nonlinear phenomena, and is systematically improvable. Lithium cyanide serves as a molecular test system in which the charge distribution and hence the dipole moment are shown to be switchable, in a controlled fashion, by (a series of) laser pulses which induce selective, state-to-state electronic transitions. One focus of our time-dependent calculations is the question of how fast the transition from the ionic ground state to a specific excited state that is embedded in a multitude of other states can be made, without creating an electronic wave packet.

Molecular response properties from explicitly time-dependent configuration interaction methods

In this paper we report the calculation of molecular electric response properties with the help of explicitly time-dependent configuration interaction (TD-CI) methods. These methods have the advantage of being applicable (within the limitations of the time-dependent Schrodinger equation) to time-dependent perturbations of arbitrary shape and strength. Three variants are used to solve the time-dependent electronic Schrodinger equation, namely, the TD-CIS (inclusion of single excitations only), TD-CISD (inclusion of single and double excitations), and TD-CIS(D) (single excitations and perturbative treatment of double excitations) methods and applied for illustration to small molecules, H(2) and H(2)O. In the calculation, slowly varying off-resonant electric fields are applied to the molecules and linear (polarizabilities) and nonlinear (hyperpolarizabilities, harmonic generation) response properties are determined from the time-dependent dipole moments.

Strong field ionization rates simulated with time-dependent configuration interaction and an absorbing potential

Ionization rates of molecules have been modeled with time-dependent configuration interaction simulations using atom centered basis sets and a complex absorbing potential. The simulations agree with accurate grid-based calculations for the ionization of hydrogen atom as a function of field strength and for charge resonance enhanced ionization of H2(+) as the bond is elongated. Unlike grid-based methods, the present approach can be applied to simulate electron dynamics and ionization in multi-electron polyatomic molecules. Calculations on HCl(+) and HCO(+) demonstrate that these systems also show charge resonance enhanced ionization as the bonds are stretched.

Angle-Dependent Ionization of Small Molecules by Time-Dependent Configuration Interaction and an Absorbing Potential

The angle-dependence of strong field ionization of O2, N2, CO2, and CH2O has been studied theoretically using a time-dependent configuration interaction approach with a complex absorbing potential (TDCIS-CAP). Calculation of the ionization yields as a function of the direction of polarization of the laser pulse produces three-dimensional surfaces of the angle-dependent ionization probability. These three-dimensional shapes and their variation with laser intensity can be interpreted in terms of ionization from the highest occupied molecular orbital (HOMO) and lower lying orbitals, and the Dyson orbitals for the ground and excited states of the cations.

Dipole switching in large molecules described by explicitly time-dependent configuration interaction

In this paper, we report laser-driven charge transfer simulations for Li-(Ph)(n)-CN (n=1,2,3) using the time-dependent configuration interaction single approach. These molecules serve as systematically extendable model systems, in order to investigate the selectivity, and thus controllability, of an ultrashort laser-induced electronic excitation as a function of the molecular size. For example, such control would be needed if a small electronic molecular switch is connected to a larger molecular device. We demonstrate that for larger molecules, the selectivity of the electronic transition is considerably reduced even for rather long pulses due to dynamic polarizations of the molecules. We also show that these dynamic polarizations might be substantially underestimated in few state models.

Angle-Dependent Ionization of Hydrides AHn Calculated by Time-Dependent Configuration Interaction with an Absorbing Potential

The angle dependence of strong-field ionization was studied for a set of second period hydrides (BH(3), CH(4), NH(3), H(2)O, and HF) and third period hydrides (AlH(3), SiH(4), PH(3), H(2)S, and HCl). Time-dependent configuration interaction with a complex absorbing potential was used to model ionization by a seven cycle 800 nm cosine squared pulse. The ionization yields were calculated as a function of the laser polarization and plotted as three-dimensional surfaces. The general shapes of angular dependence can be understood in terms of ionization from the highest occupied orbitals. Variations in the shapes with laser intensity indicate that ionization occurs not just from the highest occupied orbitals, but also from lower-lying orbitals. These deductions are supported by variations in the population analysis with the intensity of the laser field and the direction of polarization.

Strong-field ionization rates of linear polyenes simulated with time-dependent configuration interaction with an absorbing potential.

The strong field ionization rates for ethylene, trans 1,3-butadiene, and trans,trans 1,3,5-hexatriene have been calculated using time-dependent configuration interaction with single excitations and a complex absorbing potential (TDCIS-CAP). The calculations used the aug-cc-pVTZ basis set with a large set of diffuse functions (3 s, 2 p, 3 d, and 1 f) on each atom. The absorbing boundary was placed 3.5 times the van der Waals radius from each atom. The simulations employed a seven-cycle cosine squared pulse with a wavelength of 800 nm. Ionization rates were calculated for intensities ranging from 0.3 × 10(14) W/cm(2) to 3.5 × 10(14) W/cm(2). Ionization rates along the molecular axis increased markedly with increasing conjugation length. By contrast, ionization rates perpendicular to the molecular axis were almost independent of the conjugation length.

Nuclear dynamics for a three-state Jahn–Teller model system

We report wavepacket dynamics on a model system with a three-state conical intersection. Quantum wavepacket dynamics using the multiconfigurational time-dependent Hartree method have been carried out for the T ⊗ (e + t(2)) Jahn-Teller problem, using a Jahn-Teller vibronic model Hamiltonian. The effects of the magnitude of the coupling parameters and of the initial position of the wavepacket on the dynamics around the three-state conical intersection have been considered. It was found that the effect of the coupling strength is not dramatic for the population transfer in most cases, but the details of the dynamics and the involvement of the different modes are affected by it.

The influence of excited state topology on wavepacket delocalization in the relaxation of photoexcited polyatomic molecules.

This paper compares the relaxation dynamics of several molecules that display internal conversion on ultrafast time scales. We find that the degree of wavefunction localization during relaxation is strongly correlated with the rate of relaxation. We discuss our experimental findings in terms of two-dimensional model simulations which try to capture the essential features of the potential energy landscapes relevant to the relaxation dynamics. Our model calculations show how relaxation can be local or nonlocal depending on basic features of the potential energy surface traversed by the wavepacket en route back to the ground state.