arXiv subjects

M. Beyer

Publications and source records attributed to M. Beyer.

At least 19 recordsLinked to original sources

arXiv2026

Demonstrating CBM Capabilities by $\Lambda$ Baryon Reconstruction in Ni+Ni Collisions with the mCBM Experiment at SIS18 of GSI/FAIR

The Compressed Baryonic Matter (CBM) experiment at the upcoming Facility for Antiproton and Ion Research (FAIR) is a high-rate fixed-target experiment designed to investigate nuclear matter at extreme baryon densities in relativistic nucleus-nucleus collisions. To enable high-statistics measurements of rare probes, CBM is designed to operate at event rates up to 10 MHz. This necessitates the development of fast and radiation-tolerant detectors, self-triggered front-end electronics, a free-streaming data acquisition architecture, and real-time event reconstruction capabilities. Prototype versions and pre-series productions of the CBM detector systems have been deployed in the mini-CBM demonstrator setup mCBM - an experimental precursor comprising sub-components of all major CBM systems, installed at the SIS18 facility of GSI/FAIR within the FAIR Phase-0 program. In 2024, Ni+Ni collisions at a kinetic beam energy of 1.93 AGeV and an average interaction rate of about 250 kHz were successfully recorded. This dataset enables a detailed evaluation of the operational performance of the detector systems as well as the complete CBM data chain, while the reconstruction of rare $\Lambda$ baryons serves as a natural benchmark. This paper presents the first results on $\Lambda$ signal reconstruction with the mCBM experiment, demonstrating the readiness of the detector technologies and the data chain for the upcoming full-scale CBM experiment.

physics.ins-det↗

arXiv2025

Characterization of the electronic ground state of He$_2^+$ by high-resolution photoelectron spectroscopy

Excluding the very shallow potential minimum of the electronic ground state, all bound electronic states of He$_2$ have Rydberg character. Their potential-energy functions are similar to those of the He$_2^+$ states to which the Rydberg series converge. Photoionization and electron-impact ionization of the metastable $a~^3\Sigma_u^+$ state of He$_2$ are thus characterized by diagonal Franck-Condon factors and only provide access to low vibrational levels of the He$_2^+$ $X^+$ $^2\Sigma_u^+$ electronic ground state. For this reason, little experimental information is available on the excited vibrational levels of He$_2^+$. We report a measurement, by high-resolution photoelectron spectroscopy, of 17 vibrational levels of the $X^+$ $^2\Sigma_u^+$ state of $^4$He$_2^+$, with vibrational quantum number $v^+$ from 3 to 19 and covering more than 95% of the potential well. To access these states, we exploit a hump in the potential-energy function of the $c~^3\Sigma_g^+$ state, whose vibrational wavefunctions extend to large internuclear distance by quantum-mechanical tunneling through the potential barrier. Combining these results with data on the lowest ($v^+=0-2$) and highest ($v^+=22, 23$) vibrational levels of $^4$He$_2^+$, we derive a full map of the rovibrational structure of He$_2^+$ and, in a least-squares fit, an empirical effective potential-energy function that describes all experimental data within uncertainties. This function yields the positions of the 409 bound rovibrational levels and the positions and widths of the 74 shape resonances of the $X^+$ $^2\Sigma_u^+$ state of $^4$He$_2^+$. The dissociation energy of He$_2^+$ is $D{\rm e}=19,956.10(10)$ cm$^{-1}$ [$D_0(^4{\rm He}_2^+)=19,101.29(10)$ cm$^{-1}$].

physics.atom-ph↗

arXiv2025

Precision measurement of the last bound states in H$_2$ and determination of the H + H scattering length

The binding energies of the five bound rotational levels $J=0-4$ in the highest vibrational level $v=14$ in the X$^1\Sigma_g^+$ ground electronic state of H$_2$ were measured in a three-step ultraviolet-laser experiment. Two-photon UV-photolysis of H$_2$S produced population in these high-lying bound states, that were subsequently interrogated at high precision via Doppler-free spectroscopy of the F$^1\Sigma_g^+$ - X$^1\Sigma_g^+$ system. A third UV-laser was used for detection through auto-ionizing resonances. The experimentally determined binding energies were found to be in excellent agreement with calculations based on non-adiabatic perturbation theory, also including relativistic and quantum electrodynamical contributions. The $s$-wave scattering length of the H + H system is derived from the binding energy of the last bound $J=0$ level via a direct semi-empirical approach, yielding a value of $a_s$ = 0.2724(5) $a_0$, in good agreement with a result from a previously followed theoretical approach. The subtle effect of the $m\alpha^4$ relativity contribution to $a_s$ was found to be significant. In a similar manner a value for the $p$-wave scattering volume is determined via the $J=1$ binding energy yielding $a_p$ = -134.0000(6) $a_0^3$. The binding energy of the last bound state in H$_2$, the ($v=14$, $J=4$) level, is determined at 0.023(4) cm$^{-1}$, in good agreement with calculation. The effect of the hyperfine substructure caused by the two hydrogen atoms at large internuclear separation, giving rise to three distinct dissociation limits, is discussed.

physics.atom-ph↗

arXiv2024

The Rotational and Spin-Rotational Level Structure of para-H$_{2}^+$ from High-Resolution MQDT-Assisted Rydberg-State Spectroscopy

The structure of the low-lying rotational levels of the X$^+$ $ ^2 \Sigma_g ^+$ ($v^+=0$) vibronic ground state of para-H$_2^+$ has been determined by combining frequency-comb calibrated continuous-wave high-resolution laser spectroscopy of $n$f Rydberg series in the range of principal quantum number $n$ between 28 and 115 and Rydberg-series extrapolation using multichannel-quantum-defect theory (MQDT). The use of accurate quantum-defect parameters obtained from new ab initio calculations enabled the experimental determination of the pure rotational term values of the $N^+= 2$, 4 and 6 rotational levels of H$_2^+$ with sub-MHz accuracy (174.236\,744\,6(77), 575.455\,632\,5(86) and 1191.385\,571(240) cm$^{-1}$, respectively), and of the corresponding spin-rotational coupling constants with an accuracy of better than 100 kHz (42.21(4), 41.26(8) and 40.04(8) MHz, respectively). These values are in agreement with the results of first-principles calculations that include high-order relativistic and quantum-electrodynamics corrections to the level energies. To reach the reported accuracy in the Rydberg series extrapolation, it was necessary to correct for artificial level shifts arising in the MQDT calculations in the vicinity of local perturbations of high-$n$ Rydberg states with a $v^+=0$ H$_2^+$ ion core caused by low-$n$ core-excited Rydberg states, and resulting from approximations in the treatment of the Rydberg-electron energy in the interacting channels.

physics.atom-ph↗

arXiv2023

Spectroscopic study of the F$^1\Sigma_g^+$ outer well state in H$_2$, HD and D$_2$

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the \X\ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H$_2$S, HDS and D$_2$S are used to produce vibrationally hot H$_2$, HD and D$_2$. The wave function density at large internuclear separation is excited via two-photon transitions in the \F\ - \X\ system to probe ro-vibrational levels in the first excited \F\ outer well state of \emph{gerade} symmetry. Combining with accurate knowledge of the \X($v,J$) levels from advanced ab initio calculations, energies of rovibrational levels in the \F\ state are determined. For the H$_2$ isotopologue a three-laser scheme is employed yielding level energies at accuracies of $4 \times 10^{-3}$ \wn\ for F($v=0,J$) up to $J=21$ and for some low $J$ values of F($v=1$). A two-laser scheme was applied to determine level energies in H$_2$ for F($v=0-4$) levels as well as for various F levels in HD and D$_2$, also up to large rotational quantum numbers. The latter measurements in the two-laser scheme are performed at lower resolution and the accuracy is strongly limited to 0.5 \wn\ by ac-Stark effects. For H$_2$ a new quasibound resonance ($v=6$, $J=23$) is detected through the Q(23) and O(23) transitions in the F0-X6 band. The experimental results on F($v,J$) level energies are compared with previously reported theoretical results from multi-channel quantum-defect calculations as well as with results from newly performed nonadiabatic quantum calculations.

physics.atom-ph↗

arXiv2022

Black-body radiation induced photodissociation and population redistribution of weakly bound states in H$_2^+$

Molecular hydrogen ions in weakly bound states close to the first dissociation threshold are attractive quantum sensors for measuring the proton-to-electron mass ratio and hyperfine-induced ortho-para mixing. The experimental accuracy of previous spectroscopic studies relying on fast ion beams could be improved by using state-of-the-art ion trap setups. With the electric dipole moment vanishing in H$_2^+$ and preventing fast spontaneous emission, radiative lifetimes of the order of weeks are found. We include the effect of black-body radiation that can lead to photodissociation and rovibronic state redistribution to obtain effective lifetimes for trapped ion experiments. Rate coefficients for bound-bound and bound-continuum processes were calculated using adiabatic nuclear wave functions and nonadiabatic energies, including relativistic and radiative corrections. Effective lifetimes for the weakly bound states were obtained by solving a rate equation model and lifetimes in the range of 4 to 523~ms and $>$215~ms were found at room temperature and liquid nitrogen temperature, respectively. Black-body induced photodissociation was identified as the lifetime-limiting effect, which guarantees the purity of state-selectively generated molecular ion ensembles. The role of hyperfine-induced $g/u$-mixing, which allows pure rovibrational transitions, was found to be negligible.

physics.atom-ph↗

arXiv2022

Improved Ionization and Dissociation Energies of the Deuterium Molecule

The ionization energy of D$_2$ has been determined experimentally from measurements involving two-photon Doppler-free vacuum-ultraviolet pulsed laser excitation and near-infrared continuous-wave laser excitation to yield $E_\mathrm{I}(\mathrm{D}_2)=124\,745.393\,739(26)$ \wn. From this value, the dissociation energy of D$_2$ is deduced to be $D_0$(D$_2$) = 36\,748.362\,282(26) \wn, representing a 25-fold improvement over previous values, and found in good agreement (at $1.6\sigma$) with recent ab initio calculations of the 4-particle nonadiabatic relativistic energy and of quantum-electrodynamic corrections up to order $m\alpha^6$. This result constitutes a test of quantum electrodynamics in the molecular domain, while a perspective is opened to determine nuclear charge radii from molecules.

physics.atom-ph↗

arXiv2021

Precision measurement of quasi-bound resonances in H$_2$ and the H + H scattering length

Quasi-bound resonances of H$_2$ are produced via two-photon photolysis of H$_2$S molecules as reactive intermediates or transition states, and detected before decay of the parent molecule into three separate atoms. As was previously reported [K.F. Lai et al., Phys. Rev. Lett. 127, 183001 (2021)] four centrifugally bound quantum resonances with lifetimes of multiple $\mu$s, lying energetically above the dissociation limit of the electronic ground state X$^1\Sigma_g^+$ of H$_2$, were observed as X($v,J$) = (7,21)$^*$, (8,19)$^*$, (9,17)$^*$, and (10,15)$^*$, while also the short-lived ($\sim 1.5$ ns) quasi-bound resonance X(11,13)$^*$ was probed. The present paper gives a detailed account on the identification of the quasi-bound or shape resonances, based on laser detection via F-X two-photon transitions, and their strongly enhanced Franck-Condon factors due to the shifting of the wave function density to large internuclear separation. In addition, the assignment of the rotational quantum number is verified by subsequent multi-step laser excitation into autoionization continuum resonances. Existing frameworks of full-fledged ab initio computations for the bound region in H$_2$, including Born-Oppenheimer, adiabatic, non-adiabatic, relativistic and quantum-electrodynamic contributions, are extended into the energetic range above the dissociation energy. These comprehensive calculations are compared to the accurate measurements of energies of quasi-bound resonances, finding excellent agreement. Etc.

physics.atom-ph↗

arXiv2021

Shape resonances in H$_2$ as photolysis reaction intermediates

Shape resonances in H$_2$, produced as reaction intermediates in the photolysis of H$_2$S precursor molecules, are measured in a half-collision approach. Before desintegrating into two ground state H atoms, the reaction is quenched by two-photon Doppler-free excitation to the F electronically excited state of H$_2$. For $J=13,15,17,19$ and 21, resonances with lifetimes in the range of nano to milliseconds were observed with an accuracy of 30~MHz (1.4~mK). The experimental resonance positions are found to be in excellent agreement with theoretical predictions when nonadiabatic and quantum electrodynamical corrections are included. This is the first time such effects are observed in collisions between neutral atoms. From the potential energy curve of the H$_2$ molecule, now tested at high accuracy over a wide range of internuclear separations, the s-wave scattering length for singlet H(1s)+H(1s) scattering is determined at $a = 0.2735^{39}_{31}~a_0$.

physics.atom-ph↗

arXiv2021

Photolysis production and spectroscopic investigation of the highest vibrational states in H$_2$ (X$^1\Sigma_g^+$ $v=13,14$)

Rovibrational quantum states in the $X^1\Sigma_g^+$ electronic ground state of H$_2$ are prepared in the $v=13$ vibrational level up to its highest bound rotational level $J=7$, and in the highest bound vibrational level $v=14$ (for $J=1$) by two-photon photolysis of H$_2$S. These states are laser-excited in a subsequent two-photon scheme into $F^1\Sigma_g^+$ outer well states, where the assignment of the highest ($v,J$) states is derived from a comparison of experimentally known levels in \F, combined with \emph{ab initio} calculations of \X\ levels. The assignments are further verified by excitation of $F^1\Sigma_g^+$ population into autoionizing continuum resonances which are compared with multi-channel quantum defect calculations. Precision spectroscopic measurements of the $F-X$ intervals form a test for the \emph{ab initio} calculations of ground state levels at high vibrational quantum numbers and large internuclear separations, for which agreement is found.

physics.chem-ph↗

arXiv2018

Dissociation energy of the hydrogen molecule at 10$^{-9}$ accuracy

The ionization energy of ortho-H$_2$ has been determined to be $E^\mathrm{o}_\mathrm{I}(\mathrm{H}_2)/(hc)=124\,357.238\,062(25)$ cm$^{-1}$ from measurements of the GK(1,1)--X(0,1) interval by Doppler-free two-photon spectroscopy using a narrow band 179-nm laser source and the ionization energy of the GK(1,1) state by continuous-wave near-infrared laser spectroscopy. $E^\mathrm{o}_\mathrm{I}$(H$_2$) was used to derive the dissociation energy of H$_2$, $D^{N=1}_{0}$(H$_2$), at $35\,999.582\,894(25)$ cm$^{-1}$ with a precision that is more than one order of magnitude better than all previous results. The new result challenges calculations of this quantity and represents a benchmark value for future relativistic and QED calculations of molecular energies.

physics.chem-ph↗

arXiv2012

Ultrafast transient generation of spin-densitywave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2 following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

cond-mat.supr-con↗

arXiv2011

Photoinduced melting of superconductivity in the high-Tc superconductor La2-xSrxCuO4 probed by time-resolved optical and THz techniques

Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4 thin films is investigated utilizing optical pump-probe and optical pump - THz probe techniques as a function of temperature and excitation fluence. The absorbed energy density required to suppress superconductivity is found to be about 8 times higher than the thermodynamically determined condensation energy density and nearly temperature independent between 4 and 25 K. These findings indicate that during the time when superconducting state suppression takes place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the phonons with energy lower than twice the maximum value of of the SC gap and only 10% is spent on Cooper pair breaking.

cond-mat.supr-con↗

arXiv2010

Disentanglement of the electronic and lattice parts of the order parameter in a 1D Charge Density Wave system probed by femtosecond spectroscopy

We report on the high resolution studies of the temperature (T) dependence of the q=0 phonon spectrum in the quasi one-dimensional charge density wave (CDW) compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below Tc show pronounced T-dependences of their amplitudes, frequencies and dampings. Utilizing the time-dependent Ginzburg-Landau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.

cond-mat.str-el↗

arXiv2010

Femtosecond Quasiparticle and Phonon Dynamics in Superconducting YBa2Cu3O7 Studied by Wideband Terahertz Spectroscopy

We measure the anisotropic mid-infrared response of electrons and phonons in bulk YBa2Cu3O7 after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

cond-mat.supr-con↗

arXiv2009

Dynamical holographic QCD with area-law confinement and linear Regge trajectories

We construct a new solution of five-dimensional gravity coupled to a dilaton which encodes essential features of holographic QCD backgrounds dynamically. In particular, it implements linear confinement, i.e. the area law behavior of the Wilson loop, by means of a dynamically deformed anti-de Sitter metric. The predicted square masses of the light-flavored natural-parity mesons and their excitations lie on linear trajectories of approximately universal slope with respect to both radial and spin quantum numbers and are in satisfactory agreement with experimental data.

hep-ph↗

arXiv2009

Light-front Nambu--Jona-Lasinio model at finite temperature and density

In recent years light-front quantisation has been extended to allow for a consistent treatment of systems at finite temperature and density. This is in particular interesting for an investigation of the processes in nuclear matter under extreme condition as occurring, e.g., during a heavy ion collision. Utilising a Dyson expansion to the N-point Green functions at finite temperature and density we focus on the occurrence of pionic and scalar diquark dynamics in quark matter and compute the masses and the Mott dissociation using a separable t-matrix approach. For the scalar quark-quark correlation we determine the critical temperature of colour superconductivity using the Thouless criterion. On the same footing the properties of the nucleon in a medium of quark matter are computed within a Faddeev approach. Critical lines for nucleon breakup are given. Presently, we use a light-front Nambu--Jona-Lasinio model that allows us to compare these results of this novel approach to the more traditional instant form approach, where applicable.

hep-ph↗

arXiv2009

Dynamics of photoinduced Charge Density Wave-metal phase transition in K0.3MoO3

We present first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic (M) state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast non-thermal CDW melting is achieved at the absorbed energy density that corresponds to the electronic energy difference between the metallic and CDW states. The results imply that on the sub-picosecond timescale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

cond-mat.str-el↗