arXiv subjects

Robert Berger

Publications and source records attributed to Robert Berger.

At least 19 recordsLinked to original sources

arXiv2025

A pathway towards decentralized studies of radioactive post-lead elements and their applications in beyond standard model physics

Molecules have proven to be sensitive tools for studying physics beyond the standard model, with heavy and deformed nuclei offering decisive sensitivity to parity- and time-reversal-violating effects. However, almost all elements beyond lead, occupying the 6p~to~5f atomic orbitals, lack stable isotopes, hence molecules containing them are referred to as radioactive molecules. Among those, radium monofluoride has seen particular interest, but to date, research on radioactive molecules has mainly been limited to large-scale nuclear facilities. Here, we present a scheme that allows efficient and fast harvest of radioactive ions (including short-lived Ra), and show ion gas-phase reaction studies of singly and doubly charged Ra, Po, and Pb ions with SF$_6$ gas inside an ion trap. Our results show that the chemical reaction rate of Ra$^+$ is in line with trends of other alkaline earth elements, further support by quantum chemical computations. The reaction Ra$^{2+}$ + SF$_6$ $\rightarrow$ RaF${^+}$ + SF$_5^{+}$ achieves an almost unity conversion efficiency, making it particularly suitable for the application for studies in physics beyond the standard model. The scheme enables future decentralized research avenues with short-lived radioactive molecules for fundamental physics research at laboratories without the need for local nuclear reactors or accelerators.

nucl-ex↗

arXiv2025

Enhanced sensitivity to variations of fundamental constants in highly charged molecules from analytic perturbation theory

Quasi-forbidden electronic transitions in atoms and vibronic transitions between quasi-degenerate states in molecules serve as powerful probes of hypothetical temporal variations of fundamental constants. Computation of the sensitivity of a transition to a variation of the fine-structure constant is conventionally performed by numerical variation of the speed of light in sophisticated electronic structure calculations, and therewith several individual calculations have to be performed. An approach is presented herein that obtains sensitivity coefficients as perturbation to the Dirac-Coulomb Hamiltonian and allows, for instance, computation of their leading-order contributions as expectation values of the relativistic kinetic energy and rest-mass operators. These are available in essentially all \emph{ab initio} relativistic electronic structure codes. Additionally, the corresponding operators for two-component Hamiltonians are derived, explicitly for the zeroth order regular approximation Hamiltonian. The approach is applied to demonstrate great sensitivity of highly charged polar molecules that were recently proposed for high-precision spectroscopy in [Z\"ulch \emph{et al.}, arXiv:2203.10333[physics.chem-ph]]. In particular, a high sensitivity of a wealth of quasi-degenerate vibronic transitions in \ce{PaF^3+} and \ce{CeF^2+} to temporal variations of the fine-structure constant and the electron-proton mass ratio is shown.

physics.chem-ph↗

arXiv2025

Heavier chalcogenofenchones for fundamental gas-phase studies of molecular chirality

Monoterpene ketones are frequently studied compounds that enjoy great popularity both in chemistry and in physics due to comparatively high volatility, stability, conformational rigidity and commercial availability. Herein, we explore the heavier chalcogenoketone derivatives of fenchone as promising benchmark systems -- synthetically accessible in enantiomerically pure form -- for systematic studies of nuclear charge ($Z$) dependent properties in chiral compounds. Synthesis, structural characterization, thorough gas-phase rotational and vibrational spectroscopy as well as accompanying quantum chemical studies on the density-functional-theory level reported in this work foreshadow subsequent applications of this compound class for fundamental investigations of molecular chirality under well-defined conditions.

physics.chem-ph↗

arXiv2025

Excited state assignment and state-resolved photoelectron circular dichroism in chalcogen-substituted fenchones

Excited electronic states of fenchone, thiofenchone, and selenofenchone are characterized and assigned with different gas-phase spectroscopic methods and \textit{ab initio} quantum chemical calculations. With an increasing atomic number of the chalcogen, we observe increasing bathochromic (red) shifts, which vary in strength for Rydberg states, valence-excited states, and ionization energies. The spectroscopic insight is used to state-resolve the contributions in multi-photon photoelectron circular dichroism with femtosecond laser pulses. This is shown to be a sensitive observable of molecular chirality in all studied chalcogenofenchones. Our work contributes new spectroscopic information, particularly on thiofenchone and selenofenchone. It may open a perspective for future coherent control experiments exploiting resonances in the visible and or near-ultraviolet spectral regions.

physics.chem-ph↗

arXiv2024

Towards detection of molecular parity violation via chiral co-sensing: the $^1$H/$^{31}$P model system

Fundamental weak interactions have been shown to violate parity in both nuclear and atomic systems. However, observation of parity violation in a molecular system has proven an elusive target. Nuclear spin dependent contributions of the weak interaction are expected to result in energetic differences between enantiomers manifesting in nuclear magnetic resonance (NMR) spectra as chemical shift differences on the order of $10^{-6}$ Hz to $10^{-3}$ Hz for high-$Z$ nuclei. By employing simultaneous measurements of the diastereomeric splittings for a light and a heavy nucleus in solution-state NMR, residual chemical shift differences persisting in non-chiral environment between enantiomers of chiral compounds smaller than the typical linewidth of high-field NMR may be resolved. Sources of error must be identified and minimized to verify that the observed effect is, in fact, due to parity violation and not systematic effects. This paper presents a detailed analysis of a system incorporating \textsuperscript{31}P and \textsuperscript{1}H NMR to elucidate the systematic effects and to guide experiments with higher-$Z$ nuclei where molecular parity violation may be resolved.

physics.chem-ph↗

arXiv2024

Spectroscopic footprints of quantum friction in nonreciprocal and chiral media

We investigate how the quantum friction experienced by a polarizable atom moving with constant velocity parallel to a planar interface is modified when the latter consists of chiral or nonreciprocal media, with special focus on topological insulators. We use macroscopic quantum electrodynamics to obtain the velocity-dependent Casimir-Polder frequency shift and decay rate. These results are a generalization to matter with time-reversal symmetry breaking. We illustrate our findings by examining the nonretarded and retarded limits for five examples: a perfectly conducting mirror, a perfectly reflecting nonreciprocal mirror, a three-dimensional topological insulator, a perfectly reflecting chiral mirror and an isotropic chiral medium. We find different asymptotic power laws for all these materials. Interestingly, we find two bridges between chirality and nonreciprocity through the frequency shift that arise as a consequence of the magnetoelectric coupling. Namely, the position-dependent Casimir-Polder frequency shift for the nonreciprocal case depend on a geometric magnetic field associated with photoionization of chiral molecules, the Casimir-Polder depending on the velocities for the chiral case have the optical rotatory strength as the atomic response while those for the nonreciprocal case depend on an analog of the optical rotatory strength.

quant-ph↗

arXiv2024

Quantum electrodynamic corrections for molecules: Vacuum polarisation and electron self energy in a two-component relativistic framework

Vacuum polarisation (VP) and electron self energy (SE) are implemented and evaluated as quantum electrodynamic (QED) corrections in a (quasi-relativistic) two-component zeroth order regular approximation (ZORA) framework. For VP, the Uehling potential is considered, and for SE, the effective potentials proposed by Flambaum and Ginges as well as the one proposed by Pyykk\"o and Zhao. QED contributions to ionisation energies of various atoms and group 2 monofluorides, group 1 and 11 valence orbital energies, $^2\mathrm{P}_{1/2} \leftarrow {}^{2}\mathrm{S}_{1/2}$ and $^{2}\mathrm{P}_{3/2} \leftarrow {}^{2}\mathrm{S}_{1/2}$ transition energies of Li-, Na-, and Cu-like ions of nuclear charge $Z$ = 10, 20, ..., 90 as well as $\Pi_{1/2}\leftarrow \Sigma_{1/2}$ and $\Pi_{3/2}\leftarrow\Sigma_{1/2}$ transition energies of BaF and RaF are presented. Furthermore, perturbative and self-consistent treatments of QED corrections are compared for Kohn--Sham orbital energies of gold. It is demonstrated, that QED corrections can be obtained in a two-component ZORA framework efficiently and in excellent agreement with corresponding four-component results.

physics.chem-ph↗

arXiv2024

Anisotropy factor spectra for weakly allowed electronic transitions in chiral ketones

Quantum chemical calculations of one-photon absorption, electronic circular dichroism and anisotropy factor spectra for the A-band transition of fenchone, camphor and 3-methylcyclopentanone (3MCP) are reported. While the only weakly allowed nature of the transition leads to comparatively large anisotropies, a proper theoretical description of the absorption for such a transition requires to account for non-Condon effects. We present experimental data for the anisotropy of 3MCP in the liquid phase and show that corresponding Herzberg-Teller corrections are critical to reproduce the main experimental features. The results obtained with our comprehensive theoretical model highlight the importance of the vibrational degree of freedom, paving the way for a deeper understanding of the dynamics in electronic circular dichroism.

physics.chem-ph↗

arXiv2024

Relativistic and quantum electrodynamics effects on NMR shielding tensors of Tl$X$ ($X$ = H, F, Cl, Br, I, At) molecules

Results of relativistic calculations of nuclear magnetic resonance shielding tensors ($\sigma$) for the thallium monocation (Tl$^+$), thallium hydride (TlH) and thallium halides (TlF, TlCl, TlBr, TlI, and TlAt) are presented as obtained within a four-component polarization propagator formalism and a two-component linear response approach within the zeroth-order regular approximation. Additionally, some quantum electrodynamical (QED) effects on those NMR shieldings are estimated. A strong dependence of $\sigma$(Tl) on the bonding partner is found, together with a very weak dependence of QED effects with them. In order to explain the trends observed, the excitation patterns associated with relativistic $ee$ (or paramagnetic-like) and $pp$ (or diamagnetic-like) contributions to $\sigma$ are analyzed. For this purpose, also the electronic spin-free and spin-dependent contributions are separated within the two-component zeroth-order regular approximation, and the influence of spin-orbit coupling on involved molecular orbitals is studied, which allows for a thorough understanding of the underlying mechanisms.

physics.chem-ph↗

arXiv2024

Stopping mass-selected alkaline-earth metal monofluoride beams of high energy via formation of unusually stable anions

Direct laser-coolability and a comparatively simple electronic structure render alkaline-earth metal monofluoride molecules versatile laboratories for precision tests of fundamental physics. In this theoretical work, a route for efficient stopping and cooling of high-energy hot beams of mass-selected alkaline-earth metal monofluorides via their anions is explored to facilitate subsequent precision experiments with trapped molecules. It is shown that these molecular anions possess an unusually strong chemical bond and that RaF$^-$ features properties favourable for efficient pre-cooling, indicating the applicability of direct laser-cooling of the anion.

physics.chem-ph↗

arXiv2023

Global analysis of CP-violation in atoms, molecules and role of medium-heavy systems

Detection of parity (P) and time-reversal (T) symmetry-odd electric dipole moments (EDMs) within currently achievable resolution would evidence physics beyond the Standard Model of particle physics. Via the CPT-theorem, which includes charge conjugation (C), such low-energy searches complement high-energy physics experiments that probe CP-violation up to the TeV scale. Heavy-elemental atoms and molecules are considered to be among the most promising candidates for a first direct detection of P,T-violation due to enhancement effects that increase steeply with increasing nuclear charge number $Z$. However, different P,T-odd sources on the subatomic level can contribute to molecular or atomic EDMs, which are target of measurements, and this complicates obtaining rigorous bounds on P,T-violation on a fundamental level. Consequently, several experiments of complementary sensitivity to these individual P,T-odd sources are required for this purpose. Herein, a simply-applicable qualitative model is developed for global analysis of the P,T-odd parameter space from an electronic-structure theory perspective. Rules of thumb are derived for the choice of atoms and molecules in terms of their angular momenta and nuclear charge number. Contrary to naive expectations from $Z$-scaling laws, it is demonstrated that medium-heavy molecules with $Z\leq54$ can be of great value to tighten global bounds on P,T-violating parameters, in particular, if the number of complementary experiments increases. The model is confirmed by explicit density functional theory calculations of all relevant P,T-odd electronic structure parameters in systems that were used in past experiments or are of current interest for future experiments, respectively: the atoms Xe, Cs, Yb, Hg, Tl, Ra, Fr and the molecules CaOH, SrOH, YO, CdH, BaF, YbF, YbOH, HfF$^+$, WC, TlF, PbO, RaF, ThO, ThF$^+$ and PaF$^{3+}$.

hep-ph↗

arXiv2023

Towards detection of molecular parity violation by microwave spectroscopy of CpRe(CH$_{3}$)(CO)(NO)

Parity-violating differences in rotational constants of a chiral 5d transition metal complex, that was previously experimentally well-characterised by broad-band microwave spectroscopy, are predicted with a recently established efficient analytical derivative technique. Relative differences $ΔX/X$ between rotational constants $X=A,B,C$ of enantiomers of the title compound are found to be on the order of $10^{-14}$, which is a favourably large effect. The quality of the theoretical estimates is carefully assessed by computing nuclear electric quadrupole coupling constants that agree well with experiment.

physics.chem-ph↗

arXiv2023

Opportunities for Fundamental Physics Research with Radioactive Molecules

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

nucl-ex↗

arXiv2022

Electric dipole moments and the search for new physics

Static electric dipole moments of nondegenerate systems probe mass scales for physics beyond the Standard Model well beyond those reached directly at high energy colliders. Discrimination between different physics models, however, requires complementary searches in atomic-molecular-and-optical, nuclear and particle physics. In this report, we discuss the current status and prospects in the near future for a compelling suite of such experiments, along with developments needed in the encompassing theoretical framework.

hep-ph↗

arXiv2022

Relativistic and QED corrections to one-bond indirect nulcear spin-spin couplings in X$_2^{2+}$ and X$_3^{2+}$ ions (X = Zn, Cd, Hg)

The indirect nuclear spin-spin coupling tensor, $\mathbf J$, between mercury nuclei in Hg-containing systems can be of the order of few kHz and one of the largest measured. We conduct an analysis of the physics behind the electronic mechanisms that contribute to the one- and two-bond couplings $^n {\mathbf J}_{\mathrm{Hg}-\mathrm{Hg}}$ ($n=1, 2$). We performed calculations for $J$-couplings in X$_2^{2+}$ and $X_3^{2+}$ ions ($X$ = Zn, Cd, Hg), within polarization propagator theory, using the random phase approximation (RPA) and the pure zeroth order approximation (PZOA), with Dirac-Hartree-Fock (DHF) and Dirac-Kohn-Sham (DKS) orbitals, both at four-component and ZORA levels. We show that the "paramagnetic-like" mechanism contribute with more than 99.98\% to the total isotropic component of the coupling tensor. By means of an analysis of the molecular and atomic orbitals involved in the total value of the response function, we find that the $s$-type valence atomic orbitals have a predominant role in the description of the coupling. This fact allows us to develop an effective model from which quantum electrodynamics (QED) effects on $J$-coupling in the aforementioned ions can be estimated. The estimated QED corrections were found in the interval $(0.7; ~ 1.7)$\% of the total relativistic effect on isotropic one-bond $^1 {\mathbf J}$ coupling and from the interval $(-0.2; ~ -0.4)$\%, in Zn-containing ions, to $(-0.8; ~ -1.2)$\%, in Hg-containing ions, of the total isotropic coupling constant in the studied systems. We also show that estimated QED corrections cast a visible dependence on the nuclear charge $Z$ of each atom $X$ in the form of a power-law $\propto Z^5$.

physics.chem-ph↗

arXiv2022

Cool molecular highly charged ions for precision tests of fundamental physics

Molecules and atomic highly charged ions provide powerful low-energy probes of the fundamental laws of physics: Polar molecules possess internal fields suitable to enhance fundamental symmetry violation by several orders of magnitudes, whereas atoms in high charge states can feature large relativistic effects and compressed level structures, ideally posed for high sensitivity to variations of fundamental constants. Polar, highly charged molecules could benefit from both: large internal fields and large relativistic effects. However, a high charge dramatically weakens chemical bonding and drives systems to the edge of Coulomb explosion. Herein, we propose multiply-charged polar molecules, that contain actinides, as promising candidates for precision tests of physics beyond the standard model. Explicitly, we predict PaF$^{3+}$ to be thermodynamically stable, coolable and well-suited for precision spectroscopy. The proposed class of compounds, especially with short-lived actinide isotopes from the territory of pear-shaped nuclei, has potential to advance our understanding of molecules under extreme conditions, to provide a window into unknown properties of atomic nuclei, and to boost developments in molecular precision spectroscopy in various areas, such as optical clocks and searches for new physics.

physics.chem-ph↗

arXiv2021

Accurate \textit{ab initio} calculations of RaF electronic structure indicate the need for more laser-spectroscopical measurements

Recently a breakthrough has been achieved in laser-spectroscopic studies of short-lived radioactive compounds with the first measurements of the radium monofluoride molecule (RaF) UV/vis spectra. We report results from high accuracy \emph{ab initio} calculations of the RaF electronic structure for ground and low-lying excited electronic states. Two different methods agree excellently with experimental excitation energies from the electronic ground state to the $^2Π_{1/2}$ and $^2Π_{3/2}$ states, but lead consistently and unambiguously to deviations from experimental-based adiabatic transition energy estimates for the $^2Σ_{1/2}$ excited electronic state and show that more measurements are needed to clarify spectroscopic assignment of the $^2Δ$ states.

physics.atom-ph↗

arXiv2021

Quasi-relativistic approach to analytical gradients of parity violating potentials

An analytic gradient approach for the computation of derivatives of parity-violating (PV) potentials with respect to displacements of the nuclei in chiral molecules is described and implemented within a quasirelativistic mean-field framework. Calculated PV potential gradients are utilised for estimating PV frequency splittings between enantiomers in rotational and vibrational spectra of four chiral polyhalomethanes, i.e. CHBrClF, CHClFI, CHBrFI and CHAtFI. Values calculated within the single-mode approximation for the frequency shifts agree well with previously reported theoretical values. The influence of non-separable anharmonic effects (multi-mode effects) on the vibrational frequency shifts, which are readily accessible with the present analytic derivative approach, are estimated for the C--F stretching fundamental of all four molecules and computed for each of the fundamentals in CHBrClF and CHAtFI. Multi-mode effects are found to be significant, in particular for the C--F stretching modes, being for some modes and cases of similar size as the single-mode contribution.

physics.chem-ph↗