Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates

We theoretically demonstrate a strong dependence of the annihilation rate between (singlet) excitons on the sign of dipole-dipole couplings between molecules. For molecular H-aggregates, where this sign is positive, the phase relation of the delocalized two-exciton wavefunctions causes a destructive interference in the annihilation probability. For J-aggregates, where this sign is negative, the interference is constructive instead, as a result of which no such coherent suppression of the annihilation rate occurs. As a consequence, room temperature annihilation rates of typical H- and J-aggregates differ by a factor of ~3, while an order of magnitude difference is found for low-temperature aggregates with a low degree of disorder. These findings, which explain experimental observations, reveal a fundamental principle underlying exciton-exciton annihilation, with major implications for technological devices and experimental studies involving high excitation densities

Overcoming positivity violations for density matrices in surface hopping

Fewest-switches surface hopping (FSSH) has emerged as one of the leading methods for modeling the quantum dynamics of molecular systems. While its original formulation was limited to adiabatic populations, the growing interest in the application of FSSH to coherent phenomena prompts the question how one should construct a complete density matrix based on FSSH trajectories. A straightforward solution is to define adiabatic coherences based on wavefunction coefficients. In this Paper, we demonstrate that inconsistencies introduced in the density matrix through such treatment may lead to a violation of positivity. We furthermore show that a recently proposed coherent generalization of FSSH results in density matrices that satisfy positivity, while yielding an improved accuracy throughout much (but not all) of parameter space

Spectroscopic signatures of excited state dynamics in organic materials

In our quest for a green energy supply, the sun is arguably the most promising option. In natural photosynthesis, solar light harvesting has been optimized through a long time of evolution. Understanding the physics of this phenomenon opens avenues to improve man-made solar cells in order to maximize efficiencies. For this reason, research on photosynthesis has blossomed for several decades already. A potential optimization principles relies on quantum mechanics according to which energy can be transported swiftly as a wave. Recent experiments using ultrashort laser pulses have provided indications that wavelike transport is present in photosynthetic complexes. Nevertheless, the organic molecules constituting such complexes are soft and disordered, as a result of which waves are expected to die out fast. How wavelike behavior can still be retained is therefore an intriguing question. In this thesis, quantum behavior of energy is considered in a synthetic molecule. By looking at a small molecule, quantum effects can be studied more tractably than in a large photosynthetic complex. Our study shows that wavelike behavior is maintained for longer times due to vibrations of the molecule. Hence, surprisingly, the soft character of organic materials actually have a beneficial impact on wavelike energy transport. This thesis concludes with a proposal of an ultrafast laser experiment in which quantum effects can be identified with a much higher certainty than the experiments used to date. This helps us to determine the possibility of engineering solar cells with optimized quantum transport