arXiv subjects

Roel Tempelaar

Publications and source records attributed to Roel Tempelaar.

27 records · Page 2Linked to original sources

arXiv2020

Acene:spacer blends show exothermic singlet fission to proceed coherently, while endothermic fission proceeds incoherently

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet energy alignments. For exothermic fission, as found for pentacene, coherent mixing between the photoexcited singlet and triplet pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. For endothermic materials, such as tetracene, the impossibility of such vibronic resonances renders fission fully incoherent; a process that is shown to slow down with vacancy concentration.

cond-mat.mtrl-sci↗

arXiv2019

Removing instabilities in the hierarchical equations of motion: exact and approximate projection approaches

The hierarchical equations of motion (HEOM) provide a numerically exact approach for computing the reduced dynamics of a quantum system linearly coupled to a bath. We have found that HEOM contains temperature-dependent instabilities that grow exponentially in time. In the case of continuous-bath models, these instabilities may be delayed to later times by increasing the hierarchy dimension; however, for systems coupled to discrete, non-dispersive modes, increasing the hierarchy dimension does little to alleviate the problem. We show that these instabilities can also be removed completely at a potentially much lower cost via projection onto the space of stable eigenmodes; furthermore, we find that for discrete-bath models at zero temperature, the remaining projected dynamics computed with few hierarchy levels are essentially identical to the exact dynamics that otherwise might require an intractably large number of hierarchy levels for convergence. Recognizing that computation of the eigenmodes might be prohibitive, e.g. for large or strongly-coupled models, we present a Prony filtration algorithm that may be useful as an alternative for accomplishing this projection when diagonalization is too costly. We present results demonstrating the efficacy of HEOM projected via diagonalization and Prony filtration. We also discuss issues associated with the nonnormality of HEOM.

physics.chem-ph↗

arXiv2018

Multi-set matrix product state calculations reveal mobile Franck-Condon excitations under strong Holstein-type coupling

We show that the dynamics of (vertical) Franck-Condon excitations in the regime where Holstein-coupled vibrational modes mix strongly with electronic degrees of freedom sharply contrasts with the known self-localized behavior of vibrationally relaxed excitations. Instead, the strongly-coupled modes are found to periodically induce resonances between interacting electronic sites, during which effective excitation transfer occurs, allowing Franck-Condon excitations to attain substantial mean square displacements under conditions where relaxed excitations are essentially trapped to a single site. In demonstrating this behavior, we employ a multi-set matrix product state formalism. We find this numerically exact technique to be a remarkably efficient approach to the notoriously difficult problem posed by the Holstein model in the regime where the electronic coupling, the vibrational quantum, and the vibrational reorganization energy are comparable in magnitude.

physics.chem-ph↗

arXiv2018

Many-Body Simulation of Two-Dimensional Electronic Spectroscopy of Excitons and Trions in Monolayer Transition-Metal Dichalcogenides

We present a many-body formalism for the simulation of time-resolved nonlinear spectroscopy and apply it to study the coherent interaction between excitons and trions in doped transition-metal dichalcogenides. Although the formalism can be straightforwardly applied in a first-principles manner, for simplicity we use a parameterized band structure and a static model dielectric function, both of which can be obtained from a calculation using the $GW$ approximation. Our simulation results shed light on the interplay between singlet and triplet trions in molybdenum- and tungsten-based compounds. Our two-dimensional electronic spectra are in excellent agreement with recent experiments and we accurately reproduce the beating of a cross-peak signal indicative of quantum coherence between excitons and trions. Although we confirm that the quantum beats in molybdenum-based monolayers unambigously reflect the exciton-trion coherence time, they are shown here to provide a lower-bound to the coherence time of tungsten analogues due to a destructive interference emerging from coexisting singlet and triplet trions.

cond-mat.mtrl-sci↗

arXiv2018

Vibronic exciton theory of singlet fission. III. How vibronic coupling and thermodynamics promote rapid triplet generation in pentacene crystals

We extend the vibronic exciton theory introduced in our previous work to study singlet fission dynamics, in particular addressing recent indications of the importance of vibronic coupling in this process. A microscopic and non-perturbative treatment of electronic and selected vibrational degrees of freedom in combination with Redfield theory allows us to dynamically consider clusters of molecules under conditions close to those in molecular crystals that exhibit fission. Using bulk pentacene as a concrete example, our results identify a number of factors that render fission rapid and effective. Strong coupling to high-frequency Holstein modes generates resonances between the photo-prepared singlet and product triplet states. We furthermore find the large number of triplet combinations associated with bulk periodic systems to be critical to the fission process under such vibronically resonant conditions. In addition, we present results including, in an approximate manner, the effects of Peierls coupling, indicating that this factor can both enhance and suppress fission depending on its interplay with vibronic resonance and thermodynamics.

cond-mat.mtrl-sci↗

arXiv2017

Unravelling coherences in the FMO complex

The idea that excitonic state (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when, a decade ago, slowly dephasing quantum beats were observed in the two-dimensional electronic spectra of the Fenna-Matthews-Olson complex at 77 K. These were assigned to quantum superpositions of excitonic states; a controversial interpretation, as the spectral linewidths suggested fast dephasing arising from strong interactions with the environment. While it has been pointed out that vibrational motion produces similar spectral signatures, concrete assignment of these coherences to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the Fenna-Matthews-Olson complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived quantum beats originate exclusively from vibrational coherences, whereas electronic coherences dephase entirely within 240 fs even at 77 K - a timescale too short to play a significant role in light harvesting. Additionally, we demonstrate that specific vibrational coherences are excited via vibronically coupled states. The detection of vibronic coupling indicates the relevance of this phenomenon for photosynthetic energy transfer.

physics.chem-ph↗

arXiv2017

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.

cond-mat.mtrl-sci↗

arXiv2017

Vibronic exciton theory of singlet fission. II. Two-dimensional spectroscopic detection of the correlated triplet pair state

Singlet fission, the molecular process through which photons are effectively converted into pairs of lower energy triplet excitons, holds promise as a means of boosting photovoltaic device efficiencies. In the preceding article of this series, we formulated a vibronic theory of singlet fission, inspired by previous experimental and theoretical studies suggesting that vibronic coupling plays an important role in fission dynamics. Here, we extend our model in order to simulate two-dimensional electronic spectra, through which the theory is further validated based on a comparison to recent measurements on pentacene crystals. Moreover, by means of such spectral simulations, we provide new insights into the nature of the correlated triplet pair state, the first product intermediate in the fission process. In particular, we address a disagreement in the literature regarding the identification, energies, and transition dipole moments of its optical transitions towards higher-lying triplet states.

cond-mat.mtrl-sci↗

arXiv2017

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

Recent time-resolved spectroscopic experiments have indicated that vibronic coupling plays a vital role in facilitating the process of singlet fission. In this work, which forms the first article of a series, we set out to unravel the mechanisms underlying singlet fission through a vibronic exciton theory. We formulate a model in which both electronic and vibrational degrees of freedom are treated microscopically and non-perturbatively. Using pentacene as a prototypical material for singlet fission, we subject our theory to comparison with measurements on polarization-resolved absorption of single crystals, and employ our model to characterize the excited states underlying the absorption band. Special attention is given to convergence of photophysical observables with respect to the basis size employed, through which we determine the optimal basis for more expensive calculations to be presented in subsequent work. We furthermore evaluate the energetic separation between the optically prepared singlet excited state and the correlated triplet pair state, as well as provide a real-space characterization of the latter, both of which are of key importance in the discussion of fission dynamics. We discuss our results in the context of recent experimental studies.

cond-mat.mtrl-sci↗