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R. Berger

Publications and source records attributed to R. Berger.

At least 19 recordsLinked to original sources
arXiv2026

Formation of gaseous, doubly charged cerium monofluoride CeF$^{2+}$ and its sensitivity to new physics

Tricationic protactinium monofluoride ($^{229}$PaF$^{3+}$) has been proposed as a candidate for probing physics beyond the Standard Model of particle physics. Since studies with $^{229}$PaF$^{3+}$ require significant experimental advances, we exploit the stable, valence-isoelectronic dicationic cerium monofluoride (CeF$^{2+}$) as a surrogate. Gas-phase fluorinated-cerium molecular ions are formed and identified using the Off-Line Ion Source and TITAN mass measurement facilities at TRIUMF. Quantum chemical calculations are performed on the electronic structure of CeF$^{2+}$, revealing a parallel to that of $^{229}$PaF$^{3+}$. Moreover, these calculations provide estimates on the sensitivity of CeF$^{2+}$ itself to various $\mathcal{P,T}$-odd properties. A brief discourse on the specifics of the quantum control of CeF$^{2+}$ is presented which anticipates future searches for symmetry violations.

physics.atom-ph↗
arXiv2025

Laser spectroscopy and CP-violation sensitivity of actinium monofluoride

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons, and nuclei. Radioactive molecules have been proposed as highly sensitive probes for such searches, but experiments with most such molecules have so far been beyond technical reach. Here we report the first production and spectroscopic study of a gas-phase actinium molecule, $^{227}$AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for $^{227}$AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by three orders of magnitude. Our results thus highlight the potential of $^{227}$AcF for exceptionally sensitive searches of CP violation.

physics.atom-ph↗
arXiv2025

Community input to the European Strategy on particle physics: Searches for Permanent Electric Dipole Moments

Searches for electric dipole moments (EDMs) in fundamental particles and quantum systems with spin are pivotal experiments at the intersection of low-energy and high-precision particle physics. These investigations offer a complementary pathway to uncovering new physics beyond the Standard Model, parallel to high-energy collider searches. EDM experiments are among the most sensitive probes for detecting non-standard time-reversal (T) symmetry violations and, via the CPT theorem, CP-violation (CPV). Current EDM measurements test new physics at mass scales in or above the 10TeV to 100TeV range. This community input to the European Particle Physics Strategy Update highlights the status of the field, and describes challenges and opportunities in Europe.

hep-ex↗
arXiv2025

Probing Instantaneous Single-Molecule Chirality in the Planar Ground State of Formic Acid

We experimentally demonstrate that individual molecules of formic acid are chiral even when they are in the vibronic ground state, which has a planar equilibrium structure. We ionize the C 1s shell of the molecule and record the photoelectron in coincidence with positively charged fragments. This provides two consecutive measurements of the structure of one molecule, the first by photoelectron diffraction imaging and the second by Coulomb explosion imaging. We find that both measurements show the same handedness of the specific molecule. The phenomenon of being achiral on average but chiral at the level of individual molecules is general to most prochiral molecules and is a consequence of the three-dimensional zero-point delocalization of the nuclei in the vibrational ground state.

physics.atom-ph↗
arXiv2024

Ionization potential of radium monofluoride

The ionization potential (IP) of radium monofluoride (RaF) was measured to be 4.969(2)[10] eV, revealing a relativistic enhancement in the series of alkaline earth monofluorides. The results are in agreement with a relativistic coupled-cluster prediction of 4.969[7] eV, incorporating up to quantum electrodynamics corrections. Using the same computational methodology, an improved calculation for the dissociation energy ($D_{0}$) of 5.54[5] eV is presented. This confirms that radium monofluoride joins the small group of diatomic molecules for which $D_{0}>\mathrm{IP}$, paving the way for precision control and interrogation of its Rydberg states.

physics.atom-ph↗
arXiv2023

Observation of the distribution of nuclear magnetization in a molecule

Rapid progress in the experimental control and interrogation of molecules, combined with developments in precise calculations of their structure, are enabling new opportunities in the investigation of nuclear and particle physics phenomena. Molecules containing heavy, octupole-deformed nuclei such as radium are of particular interest for such studies, offering an enhanced sensitivity to the properties of fundamental particles and interactions. Here, we report precision laser spectroscopy measurements and theoretical calculations of the structure of the radioactive radium monofluoride molecule, $^{225}$Ra$^{19}$F. Our results allow fine details of the short-range electron-nucleus interaction to be revealed, indicating the high sensitivity of this molecule to the distribution of magnetization, currently a poorly constrained nuclear property, within the radium nucleus. These results provide a direct and stringent test of the description of the electronic wavefunction inside the nuclear volume, highlighting the suitability of these molecules to investigate subatomic phenomena.

nucl-ex↗
arXiv2023

Pinning down electron correlations in RaF via spectroscopy of excited states and high-accuracy relativistic quantum chemistry

We report the spectroscopy of the 14 lowest excited electronic states in the radioactive molecule radium monofluoride (RaF). The observed excitation energies are compared with fully relativistic state-of-the-art Fock-space coupled cluster (FS-RCC) calculations, which achieve an agreement of >=99.64% (within ~12 meV) with experiment for all states. Guided by theory, a firm assignment of the angular momentum and term symbol is made for 10 states and a tentative assignment for 4 states. The role of high-order electron correlation and quantum electrodynamics effects in the excitation energy of excited states is studied, found to be important for all states. Establishing the simultaneous accuracy and precision of calculations is an important step for research at the intersection of particle, nuclear, and chemical physics, including searches of physics beyond the Standard Model, for which RaF is a promising probe.

physics.atom-ph↗
arXiv2023

In-source and in-trap formation of molecular ions in the actinide mass range at CERN-ISOLDE

The use of radioactive molecules for fundamental physics research is a developing interdisciplinary field limited dominantly by their scarce availability. In this work, radioactive molecular ion beams containing actinide nuclei extracted from uranium carbide targets are produced via the Isotope Separation On-Line technique at the CERN-ISOLDE facility. Two methods of molecular beam production are studied: extraction of molecular ion beams from the ion source, and formation of molecular ions from the mass-separated ion beam in a gas-filled radio-frequency quadrupole ion trap. Ion currents of U$^+$, UO$_{1-3}^+$, UC$_{1-3}^+$, UF$_{1-4}^+$, UF$_{1,2}$O$_{1,2}^+$ are reported. Metastable tantalum and uranium fluoride molecular ions are identified. Formation of UO$_{1-3}^+$, U(OH)$_{1-3}^+$, UC$_{1-3}^+$, UF$_{1,2}$O$_{1,2}^+$ from mass-separated beams of U$^+$, UF$_{1,2}^+$ with residual gas is observed in the ion trap. The effect of trapping time on molecular formation is presented.

physics.ins-det↗
arXiv2023

A new route for enantio-sensitive structure determination by photoelectron scattering on molecules in the gas phase

X-ray as well as electron diffraction are powerful tools for structure determination of molecules. Studies on randomly oriented molecules in the gas-phase address cases in which molecular crystals cannot be generated or the interaction-free molecular structure is to be addressed. Such studies usually yield partial geometrical information, such as interatomic distances. Here, we present a complementary approach, which allows obtaining insight to the structure, handedness and even detailed geometrical features of molecules in the gas phase. Our approach combines Coulomb explosion imaging, the information that is encoded in the molecular frame diffraction pattern of core-shell photoelectrons and ab initio computations. Using a loop-like analysis scheme we are able to deduce specific molecular coordinates with sensitivity even to the handedness of chiral molecules and the positions of individual atoms, as, e.g., protons.

physics.chem-ph↗
arXiv2021

Fourfold Differential Photoelectron Circular Dichroism

We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s-photoelectrons in coincidence with fragment ions, we deduce the molecule's orientation and photoelectron emission direction in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase.

physics.atm-clus↗
arXiv2021

Isotope Shifts of Radium Monofluoride Molecules

Isotope shifts of $^{223-226,228}$Ra$^{19}$F were measured for different vibrational levels in the electronic transition $A^{2}{}Π_{1/2}\leftarrow X^{2}{}Σ^{+}$. The observed isotope shifts demonstrate the particularly high sensitivity of radium monofluoride to nuclear size effects, offering a stringent test of models describing the electronic density within the radium nucleus. Ab initio quantum chemical calculations are in excellent agreement with experimental observations. These results highlight some of the unique opportunities that short-lived molecules could offer in nuclear structure and in fundamental symmetry studies.

nucl-ex↗
arXiv2021

Closed-loop recycling of rare liquid samples for gas-phase experiments

Many samples of current interest in molecular physics and physical chemistry exist in the liquid phase and are vaporized for the use in gas cells, diffuse gas targets or molecular gas jets. For some of these techniques the large sample consumption is a limiting factor. When rare, expensive molecules, such as chiral molecules or species with isotopic labels are used, wasting them in the exhaust line of the pumps is a quite expensive and inefficient approach. Therefore, we developed a closed-loop recycling system for molecules with vapor pressures below atmospheric pressure. Once filled, only a few valves have to be opened or closed and a cold trap must be moved. The recycling efficiency per turn exceeds 95 %.

physics.ins-det↗
arXiv2020

Ab initio study and assignment of electronic states in molecular RaCl

Radium compounds have attracted recently considerable attention due to both development of experimental techniques for high-precision laser spectroscopy of molecules with short-lived nuclei and amenability of certain radium compounds for direct cooling with lasers. Currently, radium monofluoride (RaF) is one of the most studied molecules among the radium compounds, both theoretically and recently also experimentally. Complementary studies of further diatomic radium derivatives are highly desired to assess the influence of chemical substitution on diverse molecular parameters, especially on those connected with laser cooling, such as vibronic transition probabilities, and those related to violations of fundamental symmetries. In this article high-precision \emph{ab initio} studies of electronic and vibronic levels of diatomic radium monochloride (RaCl) are presented. Recently developed approaches for treating electronic correlation with Fock-space coupled cluster methods are applied for this purpose. Theoretical results are compared to an early experimental investigation by Lagerqvist and used to partially reassign the experimentally observed transitions and molecular electronic levels of RaCl. Effective constants of $\mathcal{P}$-odd hyperfine interaction $W_{\rm{a}}$ and $\mathcal{P,T}$-odd scalar-pseudoscalar nucleus-electron interaction $W_{\rm{s}}$ in the ground electronic state of RaCl are estimated within the framework of a quasirelativistic Zeroth-Order Regular Approximation approach and compared to parameters in RaF and RaOH.

physics.chem-ph↗
arXiv2020

Lasercooled radium monofluoride: A molecular all-in-one probe for new physics

The particular advantages of using the diatomic molecule radium monofluoride (RaF) as a versatile molecular probe for physics beyond the Standard Model are highlighted. i) RaF was previously suggested as being potentially amenable to direct cooling with lasers. As shown in the present work, RaF's energetically lowest electronically excited state is of ${}^{2}Π$ symmetry (in contrast to BaF), such that no low-lying ${}^{2}Δ$ state prevents efficient optical cooling cycles. ii) The effective electric field acting on the unpaired electron in the electronic ground state of RaF is estimated larger than in YbF, from which the best restrictions on the electron electric dipole moment (eEDM) were obtained experimentally. iii) Favourable crossings of spin-rotational levels of opposite parity in external magnetic fields exist, which are important for the measurement of the nuclear anapole moment of nuclei with a valence neutron. Thus, RaF appears currently as one of the most attractive candidates for investigation of parity-odd as well as simultaneously parity- and time-reversal-odd interactions in the realms of molecular physics.

physics.chem-ph↗
arXiv2019

Spectroscopy of short-lived radioactive molecules: A sensitive laboratory for new physics

The study of molecular systems provides exceptional opportunities for the exploration of the fundamental laws of nature and for the search for physics beyond the Standard Model of particle physics. Measurements of molecules composed of naturally occurring nuclei have provided the most stringent upper bounds to the electron electric dipole moment to date, and offer a route to investigate the violation of fundamental symmetries with unprecedented sensitivity. Radioactive molecules - where one or more of their atoms possesses a radioactive nucleus - can contain heavy and deformed nuclei, offering superior sensitivity for EDM measurements as well as for other symmetry-violating effects. Radium monofluoride, RaF, is of particular interest as it is predicted to have an appropriate electronic structure for direct laser cooling. Furthermore, some Ra isotopes are known to be octupole deformed, thereby resulting in a large enhancement of their symmetry-violating nuclear moments. Until now,however, no experimental measurements of RaF have been performed, and their study is impeded by major experimental challenges, as no stable isotopes of radium exist. Here, we present a novel experimental approach to study short-lived radioactive molecules using the highly sensitive collinear resonance ionisation method. With this technique we have measured, for the first time, the energetically low-lying electronic states for each of the isotopically pure RaF molecules at the ISOLDE-CERN. Our results provide strong evidence of the existence of a suitable laser-cooling scheme for these molecules and constitute a pivotal step towards high-precision studies in these systems. Our findings open up new opportunities in the synthesis, manipulation and study of short-lived radioactive molecules, which will have a direct impact in many-body physics, astrophysics, nuclear structure, and fundamental physics research.

nucl-th↗
arXiv2018

Absolute ion detection efficiencies of microchannel plates and funnel microchannel plates for multi-coincidence detection

Modern momentum imaging techniques allow for the investigation of complex molecules in the gas phase by detection of several fragment ions in coincidence. For these studies, it is of great importance that the single-particle detection efficiency e is as high as possible, as the overall efficiency scales with e over n, i.e. the power of the number of detected particles. Here we present measured absolute detection efficiencies for protons of several micro-channel plates (MCPs), including efficiency enhanced "funnel MCPs". Furthermore, the relative detection efficiency for two-, three-, four-, and five-body fragmentation of CHBrClF has been examined. The "funnel" MCPs exhibit an efficiency of approx. 90 percent, gaining a factor of 24 (as compared to "normal" MCPs) in case of a five-fold ion coincidence detection.

physics.ins-det↗
arXiv2016

Perturbation of Exciton Aggregate Coupling by Optical Excitation in Crystalline Perfluoropentacene Films

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states promises to overcome the Shockley-Queisser limit as up to two electrons may be harvested per absorbed photon. Intermolecular coupling is deemed mandatory for both, singlet exciton fission and a band-like transport. Such a coupling is manifested, $e.g.$, by the Davydov-splitting of the lowest-energy exciton transition in crystalline organic solids. For the model system perfluoropentacene, the corresponding transitions in the experimental, polarisation-resolved absorption spectra are identified by theoretical calculations based on the concept of H- and J- aggregation. Optical injection into the first vibronic progression of the fundamental exciton transitions significantly perturbs the higher-energy transitions that are associated to H-type aggregates of the $S_0 \rightarrow S_3$ transition during and following efficient singlet exciton fission. These findings underline the necessity for efficient carrier extraction as triplet accumulation may be detrimental to both, singlet exciton fission and any potentially band like transport. More generally, our observations indicate that electronic excitations can perturb the electronic band structure in organic crystals and highlight their correlated nature by potentially distorting the lattice.

cond-mat.mtrl-sci↗