physics.atm-clus

The ab initio calculation of photoelectron angular distributions (PADs) for negative ions remains a significant theoretical challenge. In this work, we report a joint experimental and theoretical investigation of PADs for a series of molecular anions with varying polarities, including the nonpolar O2-, the weakly polar NO-, and the strongly polar AsO- and SbO-. To accurately describe the long-range electronic wavefunctions -- where photodetachment contributes most strongly -- we modified the standard Gaussian-type orbitals (GTOs) by augmenting them with a correct exponential Slater-tail basis set (~e^(-{\xi}r)). This simple yet effective approach significantly improves the agreement between the experimental and theoretical PADs for O2- and NO-. However, notable discrepancies persist for NO- for transitions to the v = 0 and v = 1 vibrational levels of neutral NO even after this correction. Given that our methodology successfully reproduced PADs for strongly polar anions (e.g., AsO- and SbO-), these residual discrepancies are unlikely to stem from "exit-channel scattering" induced by long-range dipole fields. Instead, we tentatively attribute the failure for NO- to the breakdown of the Born-Oppenheimer approximation or the frozen orbital approximation, arising from the extremely weak binding of the excess electron.

The size-dependent strong-field ionization and dissociation dynamics of (H$_2$O)$_n$ (n=1-4) are investigated using real-time time-dependent density functional theory (RT-TDDFT) coupled to Ehrenfest molecular dynamics under a common few-cycle near-infrared laser pulse. It is found that the net ionization per monomer varies only weakly on cluster size, whereas the protonic and oxygen response is changed much more strongly once the cluster size grows beyond the dimer. In particular, H-ejection activity is observed to rise sharply from the dimer to the trimer/tetramer regime, while stable H-transfer is essentially absent in the dimer under the present criterion but becomes substantial in the trimer and is further amplified in the tetramer. Through timing analyses, it is shown that the dimer exhibits a weak and temporally broad response, whereas the larger clusters display a much stronger early-time protonic response concentrated within and immediately after the laser pulse window. By endpoint oxygen statistics, a systematic increase in dissociation propensity with cluster size is likewise shown. For a clean subset of direct two-body dimer breakup trajectories, the asymptotic kinetic energy release is estimated to be 4.47 $\pm$ 1.03 eV, in reasonably good agreement with the experimental value for the unprotonated two-body Coulomb-explosion channel. Overall, it is shown by the results that increasing water-cluster size primarily reshapes the strong-field response through proton-mediated and topology-level nuclear dynamics rather than through a large change in net ionization alone.

Optical forces in focused-beam traps are generally nonconservative, yet the controlled use of this nonconservative component for airborne single-particle dynamics remains limited. We demonstrate a dual-beam optical trap in which a single aerosol can be switched between localized confinement and sustained orbital motion by tuning the relative positions of two counter-propagating foci. The axial separation controls the onset of nonconservative circulation, while the lateral offset tunes the projected orbit size and causes a monotonic change in the rotation frequency. T-matrix optical force calculations and Langevin simulations support this interpretation by showing that finite axial misalignment activates a circulating force component, whereas near-zero axial separation gives a confinement-dominated force field. Experiments confirm the predicted switching behavior through mean-square displacement and frequency measurements. We further show that the projected orbit geometry provides a particle-dependent observable, with the orbit anisotropy Ay/Ax varying systematically with aerosol diameter. The results provide a compact, low-power platform for controlled orbital dynamics of single airborne particles and for future aerosol measurements based on nonequilibrium trajectory observables.

We present a time-dependent wave-packet study of dissociative recombination (DR) and resonant ion-pair (RIP) formation in $\mathrm{HeH^+}$ isotopologues. Nuclear dynamics are treated on a manifold of 23 coupled electronic states of $^2\Sigma$, $^2\Pi$, and $^2\Delta$ symmetries, including rotational couplings between different symmetries. The results reveal that inclusion of a large manifold of resonant states and rotational couplings significantly enhances the DR cross section relative to earlier theoretical studies. In the diabatic representation, $^2\Sigma$ states dominate the recombination dynamics, while in the adiabatic representation, $^2\Pi$ and $^2\Delta$ states contribute significantly at low collision energies. For RIP formation, two different diabatization schemes yield systematically larger cross sections than previous models, highlighting the sensitivity of ion-pair production to electronic coupling structure. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. Thermal rate coefficients are computed over $10^{2}$ to $2\times 10^4$ K thermal electron temperatures. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. The results are compared with rotational-state-resolved experimental and theoretical results. The present results highlight the importance of multistate coupling and rotational interactions in electron-driven fragmentation processes relevant to primordial and astrophysical plasmas.

Attosecond pulse trains interacting with a resonantly dressed atom generate a pronounced intra-sideband phase structure that remains hidden in conventional spectrally integrated RABBITT measurements. Using \textit{ab initio} time-dependent Schr\"odinger equation calculations for lithium near the resonant $2s\to2p$ transition, we show that the phase extracted within a single sideband can vary by nearly $\pi$ across its spectral width. The resulting intra-sideband phase dispersion exhibits a characteristic dependence on the IR detuning, pulse duration, intensity, and sideband order. Most strikingly, exact resonant Rabi flopping flattens the intra-sideband phase dispersion, whereas a small detuning generates a pronounced phase modulation despite weaker population transfer. This counterintuitive behavior demonstrates that rainbow RABBITT probes the dynamical phase accumulated by a Rabi-dressed wave packet rather than the instantaneous populations of the participating states. A simple analytical model captures the principal features of the numerical calculations and provides physical insight into the emergence of the intra-sideband phase structure. These results establish intra-sideband phase dispersion as a new interferometric observable for mapping coherent Rabi dynamics.

Nitric oxide (NO) is important in biological, atmospheric, plasma, industrial, and astrophysical environments, where reliable electron-collision data support modelling charged-particle interactions with matter. Its well-known experimental properties make it suitable for assessing how the target electronic-structure description affects low-energy electron scattering calculations. In this work, NO properties were evaluated using B3LYP, M06-2X, PBE0, and $\omega$B97X-D3, with basis sets ranging from minimal to quadruple-zeta quality. Bond length, dipole moment, ionisation potential, and polarisability were compared with experiment to assess the sensitivity of the target description to the functional and basis set. The aug-cc-pVQZ basis set was then used to generate target models for ab initio R-matrix calculations over 0.1--20 eV. The total cross sections show low-energy resonance features, with the strongest functional dependence around the broad peak near 0.8--1.0 eV. A sharper, higher-energy structure is also observed below 2 eV, shifting from 1.74 to 1.82 eV depending on the functional. Differential cross sections show modest functional sensitivity, with more noticeable angular differences at 7.5 and 10 eV. These results show that the DFT functional and basis set affect the target properties, with the resulting target description influencing low-energy electron-scattering observables of NO. The comparison supports $\omega$B97X-D3/aug-cc-pVTZ geometry optimisation followed by aug-cc-pVQZ target-property calculations as a practical protocol for R-matrix modelling of NO.

New particle formation (NPF) contributes to more than half of global aerosol number concentrations, with profound implications for human health and climate change. Observational studies have shown that the frequency of NPF events in urban Beijing during summer exceeds the global average. The prevailing paradigm attributes urban NPF primarily to sulfuric acid-base nucleation involving dimethylamine (DMA). However, recent field measurements in summer urban Beijing have identified several emerging amines emitted from carbon capture processes, including monoethanolamine (MEA), piperazine (PZ), diethanolamine (DEA) and N-methyldiethanolamine (MDEA), in addition to DMA. Here, we systematically evaluate the contributions of sulfuric acid-amine nucleation pathways to urban NPF. We found that emerging amines particularly DEA and PZ, can dominate nucleation pathways under polluted urban conditions, surpassing the contribution of DMA. These findings suggest that the current universal paradigm of urban nucleation should be revisited to explicitly account for the role of emerging amines. Moreover, emerging amine-mediated NPF will become increasingly important in the context of future co-control policies for air pollution and carbon reduction.

Machine learning has transformed the field of atomistic simulations by enabling the development of interatomic potentials that are computationally efficient and highly accurate. These advances have opened the door to modeling molecular vibrations and predicting infrared spectra with near ab-initio accuracy at a fraction of the computational cost. Among these approaches, message-passing neural networks (MPNNs) have emerged as a particularly powerful class of models for representing complex atomic interactions. In this study, we benchmark five MPNN architectures, SchNet, FieldSchNet, SO3Net, PaiNN, and MACE, for predicting infrared spectra of small organic molecules. SchNet and FieldSchNet are invariant models, while SO3Net, PaiNN, and MACE are equivariant, explicitly accounting for rotational symmetries in molecular representations. We evaluate their performance in terms of computational efficiency, accuracy, and robustness. All models accurately predict properties, such as energies, forces, and dipole moments, required for infrared spectra calculations. They also capture harmonic frequencies and infrared spectra derived from molecular dynamics with high fidelity for molecules in the training set. However, SchNet and FieldSchNet show limited transferability to unseen systems, while SO3Net, PaiNN, and MACE generalize more effectively. In terms of computational efficiency, SchNet is the most efficient and FieldSchNet enables field-dependent response modeling but with higher cost. PaiNN achieves the best balance between accuracy and efficiency, MACE provides the highest spectral accuracy and transferability, and SO3Net performs between PaiNN and MACE.

We present an accelerated ``on-the-fly'' coupled-cluster path-integral molecular dynamics (PIMD) method for finite-temperature simulations in which electron correlation and nuclear quantum effects are treated simultaneously. The approach is based on our quantum ring-polymer contraction (qRPC) technique, in which the inexpensive Hartree-Fock potential is evaluated on the full ring-polymer, while the expensive coupled-cluster correction is evaluated on the centroid only. This qRPC decomposition is combined with a second-generation Car-Parrinello-like dynamics of the Hartree-Fock reference and a basis-consistent extrapolation of the coupled-cluster and de-excitation amplitudes. The combination of all three acceleration layers is essential for making correlated PIMD calculations feasible. We apply the method to a proton shared by water and formaldehyde. Relative to classical nuclei, nuclear quantum effects broaden covalent X--H bond-length distributions, reduce the bias of the shared proton toward formaldehyde, and shift the mean proton-transfer coordinate from 0.206 to 0.135A. The probability of finding the proton closer to formaldehyde decreases from 81.7$\%$ to 61.1$\%$. The corresponding nuclear magnetic shielding tensors show that electron correlation and nuclear quantum effects are of comparable magnitude and can act in opposite directions. These results demonstrate that predictive simulations of asymmetric hydrogen bonds require a simultaneous treatment of correlated electronic structure and nuclear quantum fluctuations.

We investigate the fluorescence and relaxation dynamics of Cs atoms embedded in a cryogenic argon matrix using spectroscopy measurements combined with diatomic-in-molecule (DIM) simulations. The data reveal complex emission spectra, large Stokes shifts, and slow relaxation effects, indicating strong host-guest interactions and substantial lattice reorganization. Although the spectra are superimposed on a broad background, possibly due to low-symmetry, defect-related, or grain-boundary trapping sites, the main spectral structure is consistent with two dominant trapping environments that give rise to two triplet absorption features with distinct fluorescence behavior of the doublet and singlet components. Polarization measurements further suggest that these sites may differ in symmetry, although a unique structural assignment remains difficult

The natural and Dyson orbitals are studied for small helium drops comprising 5 to 20 helium atoms interacting via a soft two-body gaussian potential. The wave functions of these drops have been obtained in the hyperspherical cluster model (HCM) which provides a correct description of the single-particle behaviour at large separations from the system. The natural orbitals are obtained from diagonalization of the nonlocal one-body density matrix, while Dyson orbitals are constructed by direct overlap of the wave functions of two drops differing by one boson. This overlap converges with increasing basis of the HCM. The shapes and occupancies of the natural orbitals as well as their link to Dyson overlaps and evolution with increasing number of atoms are discussed. Both natural and Dyson orbitals can be used to represent the density of the system. However, the natural orbitals representation is demonstrated to be superior. With increasing boson numbers the difference between Dyson and natural orbitals becomes less prominent and it is expected to disappear in infinitely large systems of identical bosons.

We present an analytical emission model that accurately predicts the properties of effusive sources formed by long collimation tubes. By construction, it captures the full range of molecular flow, from the transparent flux regime, which occurs in highly rarefied gases, to the opaque regime, which arises as the flux increases and interparticle collisions become non-negligible. The model is based on a previously developed secondary-emission-surface approach, improved here to overcome its internal limitations and recover the well-established axial flux intensity. It provides accurate analytical predictions of the angular intensity distribution in the molecular flow regime, offering valuable guidance for the design of efficient primary sources across a broad range of experiments in atomic and molecular physics

The OH$^+$ ($X ^3\Sigma^-$) radical cation has been investigated by combining a 4 K 22-pole ion trap apparatus with high-resolution IR and THz radiation sources. Applying different types of action spectroscopic methods, the fundamental vibrational band in the 3 $\mu$m range and the spin manifold of the $N=1 \leftarrow 0$ rotational transition around 1 THz have been extended and refined. Additionally, the spin manifold of the $N=2 \leftarrow 1$ rotational transition, scattered around 2 THz, has been measured for the first time with microwave accuracy. Although all hyperfine components of the pure rotational transitions are affected by considerable Zeeman splittings, a simulation of their contours allowed us to extract the field-free center frequencies with high accuracy. A global fit combining rovibrational and pure rotational transitions from the literature with those newly obtained in this work was performed, leading to improvements in the spectroscopic constants of OH$^+$, particularly those in the ground vibrational state.

Active control of dark long-lived excitonic states in molecular aggregates using local electric fields is a pivotal challenge for advancing nanoscale optoelectronics and quantum device engineering. This experimental study investigates the collective excitonic states in aggregates composed of radical chromophores. With the strong optical enhancement provided by tip-enhanced photoluminescence (TEPL) spectroscopy, bright and dark excitonic modes are observed emerging due to interexciton coupling and induce changes in their spectra with the electric field locally applied within the nanocavity gap. Proportionally scaling Stark shifts are revealed as well as the emission peak sharpening of the dark states and a divergent behavior of the bright states in asymmetric measurement positions of the nanocavity above the aggregates. The observed complex behavior is discussed in terms of influence of the field, molecule arrangement, nanocavity coupling, dark mode lifetimes and electrostatic charge inhomogeneities in the clusters. This sensitivity to the external parameters demonstrates an effective means of control over radical excitonic aggregates.

This article is a collection of contributions from speakers at the 2025 DEAMN [Dynamics of Electrons in Atomic and Molecular Nanoclusters] workshop at the Majorana Centre in Erice. Not ordinary contributions to a conference proceeding, this gives a new and different perspective on the work done by the workshop participants.

Classical optical frameworks such as the discrete dipole approximation (DDA) assume that the linear spectrum of coupled quantum emitters can be computed solely from the linear susceptibilities of individual constituents. However, recent polariton studies show that cavity linear response can encode nonlinear optical susceptibilities. Here, we demonstrate that this phenomenon is more general: emitter-emitter interactions allow nonlinearities of individual emitters to emerge in the linear response of arrays, without cavities or permutational symmetry. To illustrate this phenomenon, we show linear spectra for coupled heterodimers and linear chains, and demonstrate that Raman features of individual monomers show up as vibrational sidebands of collective resonances. Moreover, tuning Raman-type anharmonicities enables systematic control of spectral features, establishing a genuine quantum optical effect in molecular aggregates and quantum emitter arrays, which goes beyond mean-field descriptions in light-matter interactions.

We present velocity map imaging data on intramolecular ion-pair dissociation (IPD) of carbonyl sulfide (OCS) induced by electron impact over the 20 eV to 45 eV energy range. Two distinct IPD pathways were resolved: CO+ + S- (threshold 14.8 +- 0.7 eV) and CS+ + O- (threshold 16.8 +- 0.7 eV). The kinetic energy release spectra display a single peak for S- but split into two components for O-; in both channels the maximum kinetic energies level off once the beam energy exceeds roughly 30 eV, pointing to excitation through discrete superexcited states of quasi-resonant character. Partial wave decomposition of the fragment angular distributions reveals that the momentum-transfer parameter (beta) surpasses unity at every energy studied, invalidating the dipole-Born approximation, and that the dominant partial wave character shifts systematically with beam energy. These patterns are consistent with a mechanism in which the incident electron deposits energy through inelastic scattering, populating hybrid Rydberg-ion-pair superexcited configurations that subsequently undergo state-specific unimolecular dissociation along nonadiabatic pathways. From an applied standpoint, intramolecular ion-pair dissociation matters for astrochemistry and radiation biophysics because it generates reactive anions and cations without photon emission, redistributing excess molecular energy nonadiabatically in environments ranging from interstellar clouds to biological systems.

Recently, it has been demonstrated theoretically that the interaction of two PsH atoms, each being a stable bound state of a hydrogen atom and a positronium atom, is attractive, leading to the formation of a molecular complex denoted as (PsH)2. However, the physical nature of this interaction has remained elusive. In the present study, we show that the stabilizing mechanism is entirely encoded in the quantum correlations between the two positrons and, to a lesser extent, in the electron-positron correlations. Notably, the interaction cannot be recovered at the mean-field (Hartree-Fock) level, nor by computational models that include only electron-electron correlation effects. Accordingly, the bond formed between PsH units, termed here a two-positron gluic bond to emphasize its fundamentally distinct character from the two-positron covalent bonds present in pure antimatter molecules, emerges only when matter and antimatter particles form a common bound state. When classified within the framework of known bonding mechanisms, this gluic bond falls into the category of stabilizing dispersion interactions, giving rise to a van der Waals complex. However, its remarkably large bond dissociation energy, compared with those of strongly bonded van der Waals complexes of similar size, reveals an anomalously strong interaction. For this reason, we propose that (PsH)2 is most appropriately described as a "super" van der Waals complex stabilized by a "super" van der Waals bond.

Allostery, the intriguing phenomenon of long-range communication between distant sites in proteins, plays a central role in biomolecular regulation and signal transduction. While it is commonly attributed to conformational rearrangements, the underlying dynamical mechanisms remain poorly understood. The contact cluster model of allostery [J. Chem. Theory Comput. 2024, 20, 10731-10739] identifies localized groups of highly correlated contacts that mediate interactions between secondary structure elements. This framework proposes that allostery proceeds through a multistep process involving cooperative contact changes within clusters and communication between distant clusters, transmitted through rigid secondary structures. To demonstrate the validity and generality of the model, this Perspective employs extensive molecular dynamics simulations ($\sim1\,$ms total simulation time) of four different photoswitchable PDZ domains and studies how different domains, ligands, and perturbations influence both the contact clusters and their dynamical evolution. These analyses reveal several recurring clusters that represent shared flexible structural modules, such as loops connecting $\beta$-sheets, and show that the characteristic time scales of the nonequilibrium protein response can be directly associated with the motions of individual contact clusters. Thus, the dynamic decomposition of PDZ domains into contact clusters uncovers a modular, dynamics-based architecture that underlies and facilitates long-range allosteric communication.

We present SCULPT (Supervised Clustering and Uncovering Latent Patterns with Training), a comprehensive software platform for analyzing tabulated high-dimensional multi-particle coincidence data from Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) experiments. The software addresses critical challenges in modern momentum spectroscopy by integrating advanced machine learning techniques with physics-informed analysis in an interactive web-based environment. SCULPT implements Uniform Manifold Approximation and Projection (UMAP) for non-linear dimensionality reduction to reveal correlations in highly dimensional data. We also discuss potential extensions to deep autoencoders for feature learning, and genetic programming for automated discovery of physically meaningful observables. A novel adaptive confidence scoring system provides quantitative reliability assessments by evaluating user-selected clustering quality metrics with predefined weights that reflect each metric's robustness. The platform features configurable molecular profiles for different experimental systems, interactive visualization with selection tools, and comprehensive data filtering capabilities. Utilizing a subset of SCULPT's capabilities, we analyze photo double ionization data measured using the COLTRIMS method for 3-body dissociation of the D2O molecule, revealing distinct fragmentation channels and their correlations with physics parameters. The software's modular architecture and web-based implementation make it accessible to the broader atomic and molecular physics community, significantly reducing the time required for complex multi-dimensional analyses. This opens the door to finding and isolating rare events exhibiting non-linear correlations on the fly during experimental measurements, which can help steer exploration and improve the efficiency of experiments.

A theoretical approach to describing transport of an entire ensemble of clusters with different sizes as a single species in gas has been developed. The major assumption is an existence of local partial chemical equilibrium between the clusters. It is shown that thermal diffusion emerges in the collective description as a significant factor even if it is negligible when transport of the original molecular species is considered. Analytical expressions for the effective diffusion and thermal diffusion coefficients at temperature, pressure, and chemical composition gradients have been derived. The theory has been applied to a technology of H2S conversion in a centrifugal plasma-chemical reactor and has made it possible to account for sulfur clusters in numerical process modeling.

We present an investigation of one-photon valence-shell photoelectron spectroscopy and photoelectron circular dichroism (PECD) for the chiral molecule (1R,4R)-3-(heptafluorobutyryl)-(+)-camphor (HFC) and its europium complex Eu(III) tris[3-(heptafluorobutyryl)-(1R,4R)-camphorate] (Eu-HFC$_{3}$), the latter of which constitutes the heaviest organometallic molecule for which PECD has yet been measured. We discuss the role of keto-enol tautomerism in HFC, both as a free molecule and complexed in Eu-HFC$_{3}$. PECD is a uniquely sensitive probe of molecular chirality and structure such as absolute configuration, conformation, isomerisation, and substitution, and as such is in principle well suited to unambiguously resolving tautomers; however modeling remains challenging. For small organic molecules, theory is generally capable of accounting for experimentally measured PECD asymmetries, but significantly poorer agreement is typically achieved for the case of large open-shell systems. Here, we report PECD asymmetries ranging up to $\sim8\%$ for HFC and $\sim7\%$ for Eu-HFC$_{3}$, of similar magnitude to those reported previously for smaller isolated chiral molecules, indicating that PECD remains a practical experimental technique for the study of large, complicated chiral systems.

The effect of Diabatisation is reported in the excited argon isomers using the Diatomic-In-Molecules (DIM) method. In previous work using DIM, the lowest energy isomers of Ar$_N^*$ were shown as Ar$_3^*-$Ar$_{N-3}$, however, using the Hole-Particle-Psedopotential (HPP) method, it was shown that the excitation is localised over dimer not trimer; Ar$_2^*-$Ar$_{N-2}$. In this work we improve the DIM calculations by including previously ignored strongly avoided crossing between 3p4s and 3p4p $^{1,3}\Sigma$ states.

Relativistic effects strongly influence the electronic structures of the heaviest elements, thereby shaping their chemical and physical properties. Studying ion mobility within a noble gas environment reveals how the ion-neutral interactions depend on the ion's electronic configurations, thus providing an avenue for exploring these effects. An ion mobility spectrometer with a cryogenic drift tube was developed to precisely measure the low-field reduced mobility of heavy lanthanide and actinide cations. The apparatus was characterized by optimizing the bunching operation of ions with a miniature RF coulomb buncher and evaluating the chromatography performance of the drift tube operated with helium buffer gas at a temperature of 298K. Systematic ion mobility measurements of lutetium cations (Lu$^{+}$) drifting in helium gas were carried out as a case study. The electronic state chromatography of Lu$^{+}$ has been demonstrated. The low-field reduced ion mobility for the ground and lowest meta-stable state of Lu$^{+}$ have been examined. In addition, the variation of both states' reduced mobility and the quenching of meta-stable population has been investigated under different reduced electric fields ($E/n_0$), the ratio of an electric field to neutral gas number density.

Helium nanodroplets doped with polar molecules are studied by electrostatic deflection. This broadly applicable method allows even polyatomic molecules to attain sub-Kelvin temperatures and nearly full orientation in the field. The resulting intense force from the field gradient strongly deflects even droplets with tens of thousands of atoms, the most massive neutral systems studied by beam "deflectometry." We use the deflections to extract droplet size distributions. Moreover, since each host droplet deflects according to its mass, spatial filtering of the deflected beam translates into size filtering of neutral fragile nanodroplets. As an example, we measure the dopant ionization probability as a function of droplet radius and determine the mean free path for charge hopping through the helium matrix. The technique will enable separation of doped and neat nanodroplets and size-dependent spectroscopic studies.

Dimers of tetracene molecules are formed inside helium nanodroplets and identified through covariance analysis of the emission directions of kinetic tetracene cations stemming from femtosecond laser-induced Coulomb explosion. Next, the dimers are aligned in either one or three dimensions under field-free conditions by a nonresonant, moderately intense laser pulse. The experimental angular covariance maps of the tetracene ions are compared to calculated covariance maps for seven different dimer conformations and found to be consistent with four of these. Additional measurements of the alignment-dependent strong-field ionization yield of the dimer narrows the possible conformations down to either a slipped-parallel or parallel-slightly-rotated structure. According to our quantum chemistry calculations, these are the two most stable gas-phase conformations of the dimer and one of them is favorable for singlet fission.

We use laser-induced rotation of single molecules embedded in superfluid helium nanodroplets to reveal angular momentum dynamics and transfer in a controlled setting, under far-from-equilibrium conditions. As an unexpected result, we observe pronounced oscillations of time-dependent molecular alignment that have no counterpart in gas-phase molecules. Angulon theory reveals that these oscillations originate from the unique rotational structure of molecules in He droplets and quantum-state-specific transfer of rotational angular momentum to the many-body He environment on picosecond timescales. Our results pave the way to understanding collective effects of macroscopic angular momentum exchange in solid state systems in a bottom-up fashion.

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Many strategies have been developed to reveal structural or dynamical information about chiral molecules, from the microwave to the extreme ultraviolet range. Most schemes employ circularly or elliptically polarized radiation, and more sophisticated configurations involve, for instance, light pulses with time-varying polarization states. In all these schemes, the polarization state of light is always considered as constant over one optical cycle. In this study, we zoom into the optical cycle in order to resolve and control a subcyle attosecond chiroptical process. We engineer an electric field whose instantaneous chirality can be controlled within the optical cycle, by combining two phase-locked orthogonally polarized fundamental and second harmonic fields. While the composite field has zero net ellipticity, it shows an instantaneous optical chirality which can be controlled via the two-color delay. We theoretically and experimentally investigate the photoionization of chiral molecules with this controlled chiral field. We find that electrons are preferentially ejected forward or backward relative to the laser propagation direction depending on the molecular handedness, similarly to the well-established photoelectron circular dichroism process. However, since the instantaneous chirality switches sign from one half cycle to the next, electrons ionized from two consecutive half cycles of the laser show opposite forward/backward asymmetries. This chiral signal provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. Our results demonstrate the important role of sub-cycle polarization shaping of electric fields, as a new route to study and manipulate chiroptical processes.

It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500 eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O $1s$-excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion, i.e. it changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the X-ray regime.