Exciton Dissociation in a Model Organic Interface: Excitonic State-Based Surface Hopping versus Multiconfigurational Time-Dependent Hartree

Quantum dynamical simulations are essential for a molecular-level understanding of light-induced processes in optoelectronic materials, but they tend to be computationally demanding. We introduce an efficient mixed quantum-classical nonadiabatic molecular dynamics method termed eXcitonic state-based Surface Hopping (X-SH), which propagates the electronic Schrödinger equation in the space of local excitonic and charge-transfer electronic states, coupled to the thermal motion of the nuclear degrees of freedom. The method is applied to exciton decay in a 1D model of a fullerene-oligothiophene junction, and the results are compared to the ones from a fully quantum dynamical treatment at the level of the Multilayer Multiconfigurational Time-Dependent Hartree (ML-MCTDH) approach. Both methods predict that charge-separated states are formed on the 10-100 fs time scale via multiple "hot-exciton dissociation" pathways. The results demonstrate that X-SH is a promising tool advancing the simulation of photoexcited processes from the molecular to the true nanomaterials scale

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behaviour. In the higher temperature, higher symmetry phases of these materials, several complex structural features have been observed. The local structure can differ greatly from the average structure and there is evidence that dynamic two-dimensional structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI$_3$ is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, we reveal that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and we report the length and timescales of the motion. Finally, we investigate and visualise the spatial arrangement of these features and their interactions, providing a comprehensive picture of local structure in the higher symmetry phases

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases

Archetype analysis in sustainability research : meanings, motivations, and evidence-based policy making

Archetypes are increasingly used as a methodological approach to understand recurrent patterns in variables and processes that shape the sustainability of social-ecological systems. The rapid growth and diversification of archetype analyses has generated variations, inconsistencies, and confusion about the meanings, potential, and limitations of archetypes. Based on a systematic review, a survey, and a workshop series, we provide a consolidated perspective on the core features and diverse meanings of archetype analysis in sustainability research, the motivations behind it, and its policy relevance. We identify three core features of archetype analysis: recurrent patterns, multiple models, and intermediate abstraction. Two gradients help to apprehend the variety of meanings of archetype analysis that sustainability researchers have developed: (1) understanding archetypes as building blocks or as case typologies and (2) using archetypes for pattern recognition, diagnosis, or scenario development. We demonstrate how archetype analysis has been used to synthesize results from case studies, bridge the gap between global narratives and local realities, foster methodological interplay, and transfer knowledge about sustainability strategies across cases. We also critically examine the potential and limitations of archetype analysis in supporting evidence-based policy making through context-sensitive generalizations with case-level empirical validity. Finally, we identify future priorities, with a view to leveraging the full potential of archetype analysis for supporting sustainable development

Helium clusters doped with electronically excited alkali metal atoms

International audienc

Excited Li and Na in He(n): influence of the dimer potential energy curves.

International audienceThe X(2)Σ ground and the A(2)Π and B(2)Σ first two excited states of Li-He and Na-He are determined using high level complete active space self-consistent field-multireference configuration interaction ab initio method. The obtained potentials differ from the ones proposed by Pascale [Phys. Rev. A 28, 632 (1983)], more strongly for the ground than for the excited states. Quantum diffusion Monte Carlo studies of small Li(∗)He(n) and Na(∗)He(n) with n ≤ 5 are performed using a diatomics-in-molecule approach to model the non-pair additive interaction potential. The sensitivity of our results to the A(2)Π and B(2)Σ potentials used is assessed by an analysis of the structure and of the energetics of the clusters. For these small clusters, the physical conclusions are essentially independent of the diatomic curves employed

Structure et dynamique d'agrégats d'hélium dopés

National audienc

Excited alkali atoms in Hen: influence of the potential energy curves

International audienc

Structure and dynamics of doped helium clusters

Les agrégats quantiques, caractérisées par une forte délocalisation des particules dans l'état fondamental en raison des faibles interactions, des faibles masses, et de la statistique bosonique, ont attiré beaucoup d'intérêt depuis le début des années '90. Les agrégats et matrices d'hélium ou de parahydrogène sont les représentants principaux de ce type de systèmes aux effets quantiques collectifs prononcés. La faiblesse des interactions avec un dopant implanté apporte des perturbations minimales et rend possible l'exploitation de cet environnement quantique pour la spectroscopie matricielle et pour des études dynamiques. La nature quantique de l'ensemble du système demande l'utilisation des méthodes spécialisées, telles que les méthodes statistiques de MC quantique qui permettent de résoudre l'équation de Schrödinger en grande dimensionalité. La méthode Monte-Carlo nous permet non seulement d'évaluer les énergies de ces agrégats, mais aussi d'avoir accès à des informations structurelles grâce auxquelles on peut déduire la géométrie du cluster. En raison des grands effets d'énergie de point zéro dans les agrégats d'hélium, un scénario classique concernant le système AkHen doit être corrigé par des études de dynamique. Pour toutes les tailles étudiées, nous avons trouvé un potentiel chimique négatif qui souligne la stabilité des agrégats.In these last two decades, helium clusters have attracted considerable attention in many experimental and theoretical groups. These finite systems present unique and peculiar properties, like for example superfluidity. Helium nano-droplets also offer a quite unique and weakly perturbing environment, with cold temperature, quantum nature, high thermal conductivity and superfluidity, for the spectroscopic study of diverse species including unstable and transient systems.Given the large zero point energy effects in helium clusters, a classical scenario concerning alkali-helium clusters need to be corrected by dynamical studies. Within the Born-Oppenheimer approximation, a manifold of adiabatic potential energy surfaces represents the forces between the atoms in the possible molecular electronic states. Assuming we have such a multidimensional potential energy surface, it is impossible to solve the vibrational Schrödinger equation analytically, thus it is only possible to access physical properties from a theoretical model by computer simulation. Monte Carlo methods permit not only to evaluate the energies but provide geometrical informations on the system. For all sizes studied, we found negative chemical potential that underlines the stability of the clusters with respect to helium evaporation.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF