arXiv subjects

Ayaki Sunaga

Publications and source records attributed to Ayaki Sunaga.

17 recordsLinked to original sources

arXiv2026

Vibrational infrared and Raman spectra of the methanol molecule with equivariant neural-network property surfaces

Electric dipole and polarizability surfaces are developed for the methanol (CH$_3$OH) molecule using ab initio electronic structure data, computed at the CCSD/aug-cc-pVTZ level of theory, and equivariant neural networks. These property surfaces are used to compute vibrational infrared and Raman intensities with variational vibrational energies and wave functions. The energies and wave functions, fully accounting for the large-amplitude motion and tunneling splitting states, are from continued variational vibrational computations, based on earlier work [Sunaga et al., J. Chem. Phys., 2025, 163, 064101], up to 3700 cm$^{-1}$ beyond the zero-point vibration, now reaching the O-H stretching fundamental. All vibrational fundamentals, combination and overtone bands are in excellent agreement with available (gas-phase) experimental data, with a 2.2 cm$^{-1}$ root-mean-squared deviation of the fundamentals from experiment. These developments constitute an important step towards a quantitative and comprehensive exact quantum dynamics model of the methanol molecule, and a linelist for astrophysical applications.

physics.chem-ph↗

arXiv2026

Orders-of-magnitude improvement in precision spectroscopy of an inner-shell orbital clock transition in neutral ytterbium

An inner-shell orbital clock transition $^1S_0 \leftrightarrow 4f^{13}5d6s^2 \: (J=2)$ in neutral ytterbium atoms has attracted much attention as a new optical frequency standard as well as a highly sensitive probe for several new physics phenomena, such as ultralight dark matter, violation of local Lorentz invariance, and a new Yukawa potential between electrons and neutrons. Here, we demonstrate almost two-orders-of-magnitude improvement in precision spectroscopy over the previous reports on this transition, achieved by trapping atoms in a three-dimensional magic-wavelength optical lattice. In particular, we successfully observe the coherent Rabi oscillation, the relaxation dynamics of the excited state and the interorbital Feshbach resonance. To highlight the high precision of our spectroscopy, we carry out precise isotope shift measurements between five stable bosonic isotopes well below 10 Hz uncertainties, successfully setting bounds for a hypothetical boson mediating a force between electrons and neutrons. These results open up the way for various new physics search experiments and a wide range of applications to quantum science with this clock transition.

physics.atom-ph↗

arXiv2026

Rotational Splittings in Diatomic Molecules of Interest to Searches for New Physics

Diatomic molecules with an energetically low-lying $^3 Δ_1$ state are attractive platforms to detect new physics beyond the Standard Model, such as parity- and time-reversal violating phenomena. One of the advantages of using a $^3 Δ_1$ state is its tiny $Λ$-splitting due to the coupling between the electronic and rotational angular momenta, which facilitates polarizing the molecules in small external electric fields. Theoretical estimation of the magnitude of the $Λ$-splitting is helpful for planning new experiments. In this study, we present a theoretical model to calculate the $Λ$-splitting. Our model integrates the relativistic four-component wavefunction and the traditional rotational Hamiltonian based on Hund's case (a). The multireference character of the wavefunction is taken into account. Our calculations for PtH and ThF$^+$ molecules qualitatively agree with experiment. The $Λ$-splitting of TaO$^+$ for the rotational ground state is predicted to be around 9 kHz. This tiny splitting can reduce the systematic uncertainty, but in a practical experiment, it may cause depolarization during rotation ramp-up.

physics.atom-ph↗

arXiv2026

Excluding Hypothetical Light Boson Interpretation of Yb King Plot Nonlinearity with the ${}^1S_0 \leftrightarrow {}^3P_2$ Isotope Shift Measurement

We present precision spectroscopy and isotope shift measurement of the ${}^1S_0 \leftrightarrow {}^3P_2$ clock transition in neutral ytterbium ($\mathrm{Yb}$) atoms. By revealing a magic wavelength at $905.4(2)$ nm, we successfully achieve the atomic spectrum narrower than $100$ Hz. The interleaved clock operation between isotopes allows us to determine isotope shifts of four bosonic isotope pairs at Hz-level uncertainties, which is combined with those of other four ultra-narrow transitions in $\mathrm{Yb}$ and $\mathrm{Yb}^+$ to construct the King plot. Importantly, the new isotope shift data reported in this work is a key to exclude the possibility of attributing the observed nonlinearity of the three-dimensional King plot solely to the new physics, while the previous works rely on the other terrestrial bound set by the neutron scattering and $(g-2)_e$ measurements. This work paves the way for the effective use of precision isotope shift data in the King plot analysis and stimulates further measurements in $\mathrm{Yb}$ and other elements.

physics.atom-ph↗

arXiv2025

Exact quantum dynamics of methanol: full-dimensional ab initio potential energy surface of spectroscopic quality and variational vibrational states

The methanol molecule is a sensitive probe of astrochemistry, astrophysics, and fundamental physics. The first-principles elucidation and prediction of its rotation-torsional-vibrational motions are enabled in this work by the computation of a full-dimensional, \emph{ab initio} potential energy surface (PES) and numerically exact quantum dynamics. An active-learning approach is used to sample explicitly correlated coupled-cluster electronic energies, and the datapoints are fitted with permutationally invariant polynomials to obtain a spectroscopic-quality PES representation. Variational vibrational energies and corresponding tunnelling splittings are computed up to the first overtone of the C-O stretching mode by direct numerical solution of the vibrational Schrödinger equation with optimal internal coordinates and efficient basis and grid truncation techniques. As a result, the computed vibrational band origins finally agree with experiment within 5 cm$^{-1}$, allowing for the exploration of the large-amplitude quantum mechanical motion and tunnelling splittings coupled with the small-amplitude vibrational dynamics. These developments open the route towards simulating rovibrational spectra used to probe methanol in outer space and in precision science laboratories, as well as for probing interactions with external magnetic fields.

physics.chem-ph↗

arXiv2025

Theoretical determination of the ionization potentials of ScF, YF, LaF and AcF

We present a comprehensive theoretical study of the ionization potentials of the MF (M = Sc, Y, La, Ac) molecules using the state-of-the-art relativistic coupled cluster approach with single, double, and perturbative triple excitations (CCSD(T)). We have further corrected our results for higherorder excitations (up to full triples), the QED self-energy and vacuum-polarization contributions. We have extensively investigated the effect of the various computational parameters on the calculated ionization potentials, allowing us to assign realistic uncertainties to our predictions.

physics.atom-ph↗

arXiv2025

Strong parity-violation effects induced by large-amplitude motions: A quantum-dynamics study of substituted chiral methanols

An enhanced mechanism is proposed for the large-amplitude-motion-induced parity-violating frequency by integrating the exact quantum dynamics method with the relativistic electronic structure theory. The torsional wavefunctions and PV frequency shifts are obtained by the exact quantum dynamics method. The potential energy curve and PV energy along the torsional coordinates are calculated using the extended atomic mean-field two-component Hamiltonian. The predicted PV frequency shift for the torsional transition of CFClBrOH is approximately 100 times larger than that of the conventional C-F stretching mode of CHFClBr. The maximum PV frequency shift (3.2 Hz) is obtained in the CHBrIOH molecule.

physics.atom-ph↗

arXiv2024

Hyperfine rovibrational states of H$_3^+$ in a weak external magnetic field

Rovibrational energies, wave functions, and Raman transition moments are reported for the lowest-energy states of the H$_3^+$ molecular ion including the magnetic couplings of the proton spins and molecular rotation in the presence of a weak external magnetic field. The rovibrational-hyperfine-Zeeman Hamiltonian matrix is constructed and diagonalized using the rovibrational eigenstates and the proton spin functions. The developed methodology can be used to compute hyperfine-Zeeman effects also for higher-energy rovibrational excitations of H$_3^+$ and other polyatomic molecules. These developments will guide future experiments extending quantum logic spectroscopy to polyatomic systems.

physics.chem-ph↗

arXiv2024

Variational Vibrational States of Methanol (12D)

Full-dimensional (12D) vibrational states of the methanol molecule (CH$_3$OH) have been computed using the GENIUSH-Smolyak approach and the potential energy surface from Qu and Bowman (2013). All vibrational energies are converged better than 0.5 cm$^{-1}$ with respect to the basis and grid size up to the first overtone of the CO stretch, ca. 2000 cm$^{-1}$ beyond the zero-point vibrational energy. About seventy torsion-vibration states are reported and assigned. The computed vibrational energies agree with the available experimental data within less than a few cm$^{-1}$ in most cases, which confirms the good accuracy of the potential energy surface. The computations are carried out using curvilinear normal coordinates with the option of path-following coefficients which minimize the coupling of the small- and large-amplitude motions. It is important to ensure tight numerical fulfilment of the $C_{3\mathrm{v}}$(M) molecular symmetry for every geometry and coefficient set used to define the curvilinear normal coordinates along the torsional coordinate to obtain a faithful description of degeneracy in this floppy system. The reported values may provide a computational reference for fundamental spectroscopy, astrochemistry, and for the search of the proton-to-electron mass ratio variation using the methanol molecule.

physics.chem-ph↗

arXiv2023

Measuring the nuclear magnetic quadrupole moment of optically trapped ytterbium atoms in the metastable state

We propose a scheme to measure a nuclear magnetic quadrupole moment (MQM), a CP-violating electromagnetic moment that appears in the nuclear sector, using the long-lived $^3P_2$ metastable state in neutral $^{173}$Yb atoms. Laser-cooling and trapping techniques enable us to prepare ultracold $^{173}$Yb atoms in the $^3P_2$ state trapped in an optical lattice or an optical tweezer array, providing an ideal experimental platform with long spin coherence time. In addition, our relativistic configuration interaction calculation for the $^3P_2$ electronic wavefunction reveals a large magnetic field gradient generated by the atomic electrons in this state, which amplifies the measurable effect of an MQM. Our scheme could lead to an improvement of more than one order of magnitude in MQM sensitivity compared to the best previous measurement [S. A. Murthy et al., Phys. Rev. Lett. 63, 965 (1989)]

physics.atom-ph↗

arXiv2023

Observation of an Inner-Shell Orbital Clock Transition in Neutral Ytterbium Atoms

We observe a weakly allowed optical transition of atomic ytterbium from the ground state to the metastable state $4f^{13}5d6s^2 \: (J=2)$ for all five bosonic and two fermionic isotopes with resolved Zeeman and hyperfine structures. This inner-shell orbital transition has been proposed as a new frequency standard as well as a quantum sensor for new physics. We find magic wavelengths through the measurement of the scalar and tensor polarizabilities and reveal that the measured trap lifetime in a three-dimensional optical lattice is 1.9(1) s, which is crucial for precision measurements. We also determine the $g$ factor by an interleaved measurement, consistent with our relativistic atomic calculation. This work opens the possibility of an optical lattice clock with improved stability and accuracy as well as novel approaches for physics beyond the standard model.

physics.atom-ph↗

arXiv2022

Enhancement of parity-violating energy difference of CHFClBr, CHFClI, and CHFBrI by breaking the cancellation among valence orbital contributions

The enhancement of the parity-violating energy difference (PVED) by electronic excitation is studied for H$_2X_2$ ($X =$ O, S, Se, Te), CHFClBr, CHFClI, and CHFBrI. To clarify the enhancement mechanism, the dihedral angle dependence of the PVED of H$_2X_2$ in excited states is studied. If the contribution from the highest occupied molecular orbital (HOMO) to the PVED in the ground state is larger than the sum of those from all occupied orbitals, the PVED in the first excited state has a much larger value compared to the ground state due to cancellation breaking among valence orbital contributions. This enhancement is named cancellation breaking enhancement. The PVED enhancement is also studied for CHFClBr, CHFClI, and CHFBrI in excited states, and the cancellation breaking enhancement is confirmed. When the PVED contribution from the HOMO is larger than any other contribution, the cancellation breaking enhancement hypothesis provides the estimate of PVED in the first excited state from the HOMO contribution.

physics.chem-ph↗

arXiv2022

4-component relativistic Hamiltonian with effective QED potentials for molecular calculations

We report the implementation of effective QED potentials for all-electron 4-component relativistic molecular calculations using the DIRAC code. The potentials are also available for 2-component calculations, proper picture-change being mandatory. Specificially, we have implemented the Uehling potential [E. A. Uehling, Phys. Rev. 48 , 55 (1935)] for vacuum polarization and two effective potentials [P. Pyykkö and L.-B. Zhao, J. Phys. B 36 , 1469 (2003); V. V. Flambaum and J. S. M. Ginges, Phys. Rev. A 72 , 052115 (2005)] for electron self-energy. We provide extensive theoretical background for these potentials. We report the following sample applications: i) we confirm the conjecture of Pyykkö that QED effects are observable for the AuCN molecule by directly calculating ground-state rotational constants $B_0$ of the three isotopomers studied by MW spectroscopy; QED brings the corresponding substitution Au-C bond length $r_s$ from 0.23 to 0.04 pm agreement with experiment, ii) spectroscopic constants of van der Waals dimers M$_2$ (M=Hg, Rn, Cn, Og) iii) there is a significant change of valence s population of Pb in the reaction PbH$_4$ -> PbH$_2$ + H$_2$, which is thereby a good candidate for observing QED effects in chemical reactions, as proposed in [K. G. Dyall et al., Chem. Phys. Lett. 348 , 497 (2001)]. QED contributes 0.32 kcal/mol to the reaction energy, thereby reducing its magnitude by -1.27 %. For corresponding hydrides of superheavy flerovium, the electronic structures are quite similar. Interestingly, the QED contribution to the reaction energy is of quite similar magnitude (0.35 kcal/mol), whereas the relative change is significantly smaller (-0.50 %). This curious observation can be explained by the faster increase of negative vacuum polarization over positive electron self-energy contributions as a function of nuclear charge.

physics.chem-ph↗

arXiv2022

Spectroscopic and Electric Properties of the TaO$^+$ Molecule Ion for the Search of New Physics: A Platform for Identification and State Control

The TaO$^+$ cation is an attractive molecular species to search for parity- and time-reversal-violating interactions, in particular of hadronic origin. For the spectroscopic detection and preparation of TaO$^+$ cation in a desired state detailed knowledge of spectroscopic and electric properties in excited states is essential information. In this work we present spectroscopy constants for TaO$^+$ in the electronic ground and 29 excited states calculated with relativistic configuration interaction theory. The equilibrium bond lengths ($R_\mathrm{e}$), harmonic vibrational frequencies ($ω_\mathrm{e}$), transition dipole moments (TDM), vertical excitation energies and static molecular dipole moments (PDM) are summarized. We include a detailed characterization of all electronic states in terms of their spinor occupations. This work supports the realization of experiments using TaO$^+$ ions to search for new physics beyond the standard model of elementary particles.

physics.atom-ph↗

arXiv2021

Enhancement of parity-violating energy difference of H$_2 $X$_2$ molecules by electronic excitation

The parity-violating energy difference (PVED) between two enantiomers of a chiral molecule is caused by the weak interaction. Because of the smallness of the PVED, nonzero PVED is yet to be discovered in experimental searches. To detect the PVED, the search for molecules with large PVED values is important. Previously, one of the authors proposed that the PVED may be significantly enhanced in ionized or excited states. The significant enhancement of the PVED in some electronic excited states is proven in this study using H$_2$X$_2$ (X=O, S, Se, Te) molecules as examples. The maximum enhancement was an about 360-fold increase for H$_2$Se$_2$. For the PVED calculation, we employ the finite-field perturbation theory (FFPT) within the equation-of-motion coupled-cluster theory based on the exact two-component molecular-mean field Hamiltonian. The relation between the enhancement of the PVED and the contribution to the PVED from the highest occupied molecular orbital is also examined. The effects of computational elements, such as parameters related to the electron correlation and FFPT on PVED values in excited states of H$_2$X$_2$ molecules are studied.

physics.chem-ph↗

arXiv2021

Towards highly accurate calculations of parity violation in chiral molecules: relativistic coupled-cluster theory including QED-effects

Parity-violating energies $E_{PV}$ of the $H_2X_2$ X = O, S, Se, Te, Po) molecules are reported, calculated as analytical expectation values at the relativistic coupled-cluster singles-and-doubles (CCSD) level using property-optimized basis sets. Radiative corrections to the $E_{PV}$ was investigated using effective QED-potentials and found to reach a maximal value of 2.38% for $H_2Po_2$ . However, this result depends on the choice of effective self-energy potential and may indicate limitations to their domain of validity.

physics.chem-ph↗

arXiv2020

The DIRAC code for relativistic molecular calculations

DIRAC is a freely distributed general-purpose program system for 1-, 2- and 4-component relativistic molecular calculations at the level of Hartree--Fock, Kohn--Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, coupled cluster and electron propagator theory. At the self-consistent-field level a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding, and frozen density embedding models. DIRAC was one of the earliest codes for relativistic molecular calculations and remains a reference in its field.

physics.chem-ph↗