arXiv subjects

Piotr S. Żuchowski

Publications and source records attributed to Piotr S. Żuchowski.

At least 19 recordsLinked to original sources

arXiv2026

First-order Degenerate Symmetry-Adapted Perturbation Theory

Degenerate electronic states govern the photophysics of excimers and the molecular dynamics near conical intersections. In regions where potential energy surfaces are degenerate or nearly degenerate, intermolecular interactions mix the corresponding electronic configurations. We introduce a first-order degenerate formulation of symmetry-adapted perturbation theory (dSAPT) with weak symmetry forcing that quantifies the mixing in terms of electrostatic and exchange (Pauli repulsion) contributions. We compare two alternative ways of resolving the degeneracy, in which antisymmetry is enforced either after or before diagonalization of the first-order eigenproblem. Using CASSCF monomer wave functions, we apply dSAPT to two model spatially degenerate systems, H$_2$--F(${}^2P$) and H$_2$--NO(${}^2Π$), and show that exchange effects contribute significantly to configuration mixing already at intermediate intermolecular separations. For the H$_2$--NO(${}^2Π$) complex, we further demonstrate that second-order dispersion energy is essential for qualitatively reproducing the angular dependence of the diabatic mixing angle. Finally, using water and benzene excimers with monomers described either by CASSCF or CIS wave functions, we show that Pauli repulsion controls delocalization of the excitation at short intermolecular separations.

physics.chem-ph↗

arXiv2026

Isotopic effect on collisional widths and shifts of Hg clock transition induced by cold Rb atoms

We study the isotopic dependence of collisional widths and shifts of the Hg clock transition $^1$S$_0$-$^3$P$_0$ perturbed by the Rb atoms in the temperature range from 1 nK to 1 K. For this purpose, we model the Born-Oppenheimer effective interaction potential by including the leading long-range van der Waals coefficients. For elastic collisions, we show the connection between the dependence of collision line shape parameters on the reduced mass of colliding partners as well as the variation of the scattering length in the excited and ground states of the Hg-Rb system in the $\upmu$K temperature range. We confront the full quantum scattering calculations with a semi-classical approximation for collisional widths and shifts. We show that the shape resonances in excited and ground scattering states lead to significant variations of collisional line shape parameters with the change of the reduced mass of colliding atoms. We also indicate the possible influence of inelastic collisions, which could lead to universal behavior and significantly affect the dependence of collisional broadening and shifting on the isotopic combination of colliding atoms.

physics.atom-ph↗

arXiv2025

Production of ultracold asymmetric tops from Sr atoms and SrOH molecules

We report the comprehensive theoretical investigation of the Sr-SrOH system identifying it as a promising route to production of ultracold asymmetric top molecules. Combining high-level ab initio electronic structure calculations with rigorous quantum scattering simulations, we determine strongly anisotropic, non-reactive interaction potential and an exceptionally dense spectrum of near-threshold resonances. Presented excited states, transition dipole moments, and results of the one-dimensional STIRAP model suggest the possibility of coherently transferring weakly bound complexes to the rovibrational ground state.

physics.atom-ph↗

arXiv2024

Spin--spin interaction and magnetic Feshbach resonances in collisions of high-spin atoms with closed-shell atoms

We investigate interactions and ultracold collisions of high-spin but spherical atoms with closed-shell atoms, focussing on the example of Cr with Yb. Such a combination has only one potential energy curve but gives rise to a substantial intra-atomic spin--spin interaction, which causes Feshbach resonances due to rotationally excited states. We find such resonances are guaranteed below 250 G and can reach widths of 10s of G in favorable circumstances. We study the effect of hyperfine structure, which can create additional wide resonances at low fields. Finally, we consider isotopic substitution and show that for any realistic potential it is very likely at least one combination of commonly used isotopes will have resonances with experimentally favorable properties. Thus, this is a promising system for both molecule formation and studying quantum mixtures including dipolar species.

physics.atom-ph↗

arXiv2022

Observation of the P-wave Shape Resonance

Partial wave resonances are quasi-bound states that are formed by tunneling through the centrifugal barrier. Such states are important to collisions that deviate from the Langevin limit where direct collision path is suppressed due to quantum symmetry or a potential barrier. In such a case, quantum resonances play a major role in the dynamics at low temperatures where scattering process can be described mainly by the resonance contribution. Here we present the first observation of the lowest-lying partial wave resonance in the Penning ionization collisions between metastable neon and rovibrationally ground state HD molecules. We observe the resonance at an energy of $k_B\times$22mK taking advantage of the phase space correlation similar to atomic delta kick cooling. We show that the $p$-wave resonance position is particularly sensitive to the leading term of the van der Waals interaction. We also measure shape resonance at a higher angular momentum state, $l=6$, allowing us to disentangle and probe the short and the long parts of the intermolecular interaction.

physics.atom-ph↗

arXiv2020

Benchmarking the accuracy of seniority-zero wavefunction methods for non-covalent interactions

In this paper, we scrutinize the ability of seniority-zero wavefunction-based methods to model different types of non-covalent interactions, such as hydrogen bonds, dispersion, and mixed non-covalent interactions as well as prototypical model systems with various contributions of dynamic and static electron correlation effects. Specifically, we focus on the pair Coupled-Cluster Doubles (pCCD) ansatz combined with two different flavours of dynamic energy corrections, (i) based on a perturbation theory correction and (ii) on a linearized coupled-cluster ansatz on top of pCCD. We benchmark these approaches against the A24 data set [Řez{á}{č} and Hobza, J.~Chem.~Theory~Comput., 9, 2151-2155 (2013)] extrapolated to the basis set limit and some model non-covalent complexes that feature covalent bond breaking. By dissecting different types of interactions in the A24 data set within the Symmetry-Adapted Perturbation Theory (SAPT) framework, we demonstrate that pCCD can be classified as a dispersion-free method. Furthermore, we found that both flavours of post-pCCD approaches represent encouraging and computationally more efficient alternatives to standard electronic structure methods to model weakly-bound systems, resulting in small statistical errors. Finally, a linearized coupled cluster correction on top of pCCD proved to be most reliable for the majority of investigated systems, featuring smaller non-parallelity errors compared to perturbation-theory-based approaches.

physics.chem-ph↗

arXiv2019

Product-state distribution after isotopic substitution in ultracold atom-molecule collisions

We show that products of the isotopic substitution reactions in experimentally accessible molecules such as NaK, RbCs, and SrF are cold according to their translational energy below hundreds of mK. For these chemical reactions, molecular products may occupy only the lowest rotational states. We also discuss the possibility of controlling the chemical reactions by the electric field in ultracold mixtures of molecules and atoms with low kinetic energy release, where one of the constituent atoms of colliding molecule is replaced by its isotope. This letter opens new avenues in investigating the branching ratios of chemical reactions in ultracold conditions.

physics.chem-ph↗

arXiv2019

Quantum chaos in Feshbach resonances of the ErYb system

We investigate ultracold magnetic-field-assisted collisions in the so far unexplored ErYb system. The nonsphericity of the Er atom leads to weakly anisotropic interactions that provide the mechanism for Feshbach resonances to emerge. The resonances are moderately sparsely distributed with a density of $0.1\,{\rm G}^{-1}-0.3\,{\rm G}^{-1}$ and exhibit chaotic statistics characterized by a Brody parameter $η\approx 0.5-0.7$. The chaotic behaviour of Feshbach resonances is accompanied by strong mixing of magnetic and rotational quantum numbers in near-threshold bound states. We predict the existence of broad resonances at fields $<300\,{\rm G}$ that may be useful for the precise control of scattering properties and magnetoassociation of ErYb molecules. The high number of bosonic Er-Yb isotopic combinations gives many opportunities for mass scaling of interactions. Uniquely, two isotopic combinations have nearly identical reduced masses (differing by less than $10^{-5}$ relative) that we expect to have strikingly similar Feshbach resonance spectra, which would make it possible to experimentally measure their sensitivity to hypothetical variations of proton-to-electron mass ratio.

physics.atom-ph↗

arXiv2019

Nonrigidity effects -- a missing puzzle piece in the description of low-energy anisotropic molecular collisions

Cold collisions serve as a very sensitive probe of the interaction potential. In the recent study of Klein et al. (Nature Phys. 13, 35-38 (2017)) the one-parameter scaling of the interaction potential was necessary to obtain agreement between theoretical and observed patterns of the orbiting resonances for excited metastable helium atoms colliding with hydrogen molecules. Here we show that the effect of nonrigidity of the H$_2$ molecule on the resonant structure, absent in the previous study, is critical to predict correct positions of the resonances in that case. We have complemented the theoretical description of the interaction potential and revised reaction rate coefficients by proper inclusion of the flexibility of the molecule. The calculated reaction rate coefficients are in remarkable agreement with the experimental data without empirical adjustment of the interaction potential. We have shown that even state-of-the-art calculations of the interaction energy cannot ensure agreement with the experiment if such an important physical effect as flexibility of the interacting molecule is neglected. Our findings about the significance of the nonrigidity effects can be especially crucial in cold chemistry, where the quantum nature of molecules is pronounced.

physics.chem-ph↗

arXiv2019

Magnetic Feshbach resonances in ultracold collisions between Cs and Yb atoms

We investigate magnetically tunable Feshbach resonances in ultracold collisions between ground-state Yb and Cs atoms, using coupled-channel calculations based on an interaction potential recently determined from photoassociation spectroscopy. We predict resonance positions and widths for all stable isotopes of Yb, together with resonance decay parameters where appropriate. The resonance patterns are richer and more complicated for fermionic Yb than for spin-zero isotopes, because there are additional level splittings and couplings due to scalar and tensorial Yb hyperfine interactions. We examine collisions involving Cs atoms in a variety of hyperfine states, and identify resonances that appear most promising for experimental observation and for magnetoassociation to form ultracold CsYb molecules.

physics.atom-ph↗

arXiv2018

Magnetically tunable Feshbach resonances in ultracold gases of europium atoms and mixtures of europium and alkali-metal atoms

We investigate magnetically tunable Feshbach resonances between ultracold europium atoms and between europium and alkali-metal atoms using multichannel quantum scattering calculations. For ultracold gases of europium atoms both homonuclear $^{153}$Eu+$^{153}$Eu and heteronuclear $^{151}$Eu+$^{153}$Eu systems are studied. Calculations for mixtures of europium and alkali-metal atoms are carried out for prototype systems of $^{153}$Eu+$^{87}$Rb and $^{153}$Eu+$^7$Li. We analyze the prospects for the control of scattering properties, observation of quantum chaotic behavior, and magnetoassociation into ultracold polar and paramagnetic molecules. We show that favorable resonances can be expected at experimentally feasible magnetic field strengths below 1000$\,$G for all investigated atomic combinations. For Eu atoms, a rich spectrum of resonances is expected as a result of the competition between relatively weak short-range spin-exchange and strong long-range magnetic dipole-dipole interactions, where the dipolar interaction induces measurable resonances. A high density of resonances is expected at magnetic field strengths below 200$\,$G without pronounced quantum chaos signatures. The present results may be useful for the realization and application of dipolar atomic and molecular quantum gases based on europium atoms in many-body physics.

physics.atom-ph↗

arXiv2018

Experimental and theoretical study of the B(2)$^2 Σ^+ \rightarrow$ X(1)$^2 Σ^+$ system in the KSr molecule

Spectral bands for the B(2)$^{2}Σ^{+}\rightarrow$ X(1)$^{2}Σ^{+}$ electronic transition in the doubly-polar open-shell KSr molecule are recorded at moderate resolution using the thermoluminescence technique. The spectra are simulated using three kinds of advanced electronic structure calculations, allowing for an assessment of their accuracy on one hand, and for the derivation of fundamental spectroscopic constants of the X(1)$^{2}Σ^{+}$ KSr ground state and the excited electronic state B(2)$^{2}Σ^{+}$, on the other hand. These results should facilitate further studies aiming at creating ultracold bosonic or fermionic KSr molecules.

physics.atom-ph↗

arXiv2018

Atom-molecule collisions, spin relaxation, and sympathetic cooling in an ultracold spin-polarized Rb($^2\mathrm{S}$)-SrF$(^2Σ^+)$ mixture

We explore the suitability of ultracold collisions between spin-polarized SrF($^2Σ^+$) molecules and Rb($^2$S) atoms as elementary steps for the sympathetic cooling of SrF($^2Σ^+$) molecules in a magnetic trap. To this end, we carry out quantum mechanical scattering calculations on ultracold Rb+SrF collisions in a magnetic field based on an accurate potential energy surface for the triplet electronic state of Rb-SrF developed ab initio using a spin-restricted coupler cluster method with single, double and noniterative triple excitations [RCCSD(T)]. The Rb-SrF interaction has a global minimum with a well depth of 3444 cm$^{-1}$ in a bent geometry and a shallow local minimum in the linear geometry. Despite such a strong and anisotropic interaction, we find that converged close-coupling scattering calculations on Rb+SrF collisions in a magnetic field are still possible using rotational basis sets including up to 125 closed rotational channels in the total angular momentum representation. Our calculations show that electronic spin relaxation in fully spin-polarized Rb-SrF collisions occurs much more slowly than elastic scattering over a wide range of magnetic fields (1-1000 G) and collision energies ($10^{-5}-10^{-3}$ K) suggesting good prospects of sympathetic cooling of laser-cooled SrF($^2Σ^+$) molecules with spin-polarized Rb($^2$S) atoms in a magnetic trap. We show that incoming $p$-wave scattering plays a significant role in ultracold collisions due to the large reduced mass of the Rb-SrF collision pair. The calculated magnetic field dependence of the inelastic cross sections at 1.4 $μ$K displays a rich resonance structure including a low-field $p$-wave resonance, which suggests that external magnetic fields can be used to enhance the efficiency of sympathetic cooling in heavy atom-molecule mixtures.

physics.atom-ph↗

arXiv2018

First-order symmetry-adapted perturbation theory for multiplet splittings

We present a symmetry-adapted perturbation theory (SAPT) for the interaction of two high-spin open-shell molecules (described by their restricted open-shell Hartree-Fock determinants) resulting in low-spin states of the complex. The previously available SAPT formalisms, except for some system-specific studies for few-electron complexes, were restricted to the high-spin state of the interacting system. Thus, the new approach provides, for the first time, a SAPT-based estimate of the splittings between different spin states of the complex. We have derived and implemented the lowest-order SAPT term responsible for these splittings, that is, the first-order exchange energy. We show that within the so-called S2 approximation commonly used in SAPT (neglecting effects that vanish as fourth or higher powers of intermolecular overlap integrals), the first-order exchange energies for all multiplets are linear combinations of two matrix elements: a diagonal exchange term that determines the spin-averaged effect and a {\em spin-flip term} responsible for the splittings between the states. The numerical factors in this linear combination are determined solely by the Clebsch-Gordan coefficients: accordingly, the S2 approximation implies a Heisenberg Hamiltonian picture with a single coupling strength parameter determining all the splittings. The new approach is cast into both molecular-orbital and atomic-orbital expressions: the latter enable an efficient density-fitted implementation. We test the newly developed formalism on several open-shell complexes ranging from diatomic systems (Li+H, Mn+Mn,...) to the phenalenyl dimer.

physics.chem-ph↗

arXiv2017

Optical Feshbach resonances and ground state molecule production in the RbHg system

We present the prospects for photoassociation, optical control of interspecies scattering lengths and finally, the production of ultracold absolute ground state molecules in the Rb+Hg system. We use the "gold standard" ab initio methods for the calculations of ground (CCSD(T)) and excited state (EOM-CCSD) potential curves. The RbHg system, thanks to the wide range of stable Hg bosonic isotopes, offers possibilities for mass-tuning of ground state interactions. The optical lengths describing the strengths of optical Feshbach resonances near the Rb transitions are favorable even at large laser detunings. Ground state RbHg molecules can be produced with efficiencies ranging from about 20% for deeply bound to at least 50% for weakly bound states close to the dissociation limit. Finally, electronic transitions with favorable Franck-Condon factors can be found for the purposes of a STIRAP transfer of the weakly bound RbHg molecules to the absolute ground state using commercially available lasers.

cond-mat.quant-gas↗

arXiv2017

Observation of Feshbach resonances between alkali and closed-shell atoms

Magnetic Feshbach resonances are an invaluable tool for controlling ultracold atoms and molecules. They can be used to tune atomic interactions and have been used extensively to explore few- and many-body phenomena. They can also be used for magnetoassociation, in which pairs of atoms are converted into molecules by ramping an applied magnetic field across a resonance. Pairs of open-shell atoms, such as the alkalis, chromium, and some lanthanides, exhibit broad resonances because the corresponding molecule has multiple electronic states. However, molecules formed between alkali and closed-shell atoms have only one electronic state and no broad resonances. Narrow resonances have been predicted in such systems, but until now have eluded observation. Here we present the first observation of magnetic Feshbach resonances in a system containing a closed-shell atom, Sr, interacting with an alkali atom, Rb. These resonances pave the way to creating an ultracold gas of strongly polar, open-shell molecules, which will open up new possibilities for designing quantum many-body systems and for tests of fundamental symmetries.

physics.atom-ph↗

arXiv2016

Direct probe of anisotropy in atom-molecule collisions via quantum scattering resonances

Anisotropy is a fundamental property of particle interactions. It occupies a central role in cold and ultra-cold molecular processes, where long range forces have been found to significantly depend on orientation in ultra-cold polar molecule collisions. Recent experiments have demonstrated the emergence of quantum phenomena such as scattering resonances in the cold collisions regime due to quantization of the intermolecular degrees of freedom. Although these states have been shown to be sensitive to interaction details, the effect of anisotropy on quantum resonances has eluded experimental observation so far. Here, we directly measure the anisotropy in atom-molecule interactions via quantum resonances by changing the quantum state of the internal molecular rotor. We observe that a quantum scattering resonance at a collision energy of $k_B$ x 270 mK appears in the Penning ionization of molecular hydrogen with metastable helium only if the molecule is rotationally excited. We use state of the art ab initio and multichannel quantum molecular dynamics calculations to show that the anisotropy contributes to the effective interaction only for $H_2$ molecules in the first excited rotational state, whereas rotationally ground state $H_2$ interacts purely isotropically with metastable helium. Control over the quantum state of the internal molecular rotation allows us to switch the anisotropy on or off and thus disentangle the isotropic and anisotropic parts of the interaction. These quantum phenomena provide a challenging benchmark for even the most advanced theoretical descriptions, highlighting the advantage of using cold collisions to advance the microscopic understanding of particle interactions.

physics.chem-ph↗

arXiv2015

Ab initio interaction potentials and scattering lengths for ultracold mixtures of metastable helium and alkali-metal atoms

We have obtained accurate ab initio quartet potentials for the diatomic metastable triplet helium + alkali-metal (Li, Na, K, Rb) systems, using all-electron restricted open-shell coupled cluster singles and doubles with noniterative triples corrections [CCSD(T)] calculations and accurate calculations of the long-range $C_6$ coefficients. These potentials provide accurate ab initio quartet scattering lengths, which for these many-electron systems is possible, because of the small reduced masses and shallow potentials that results in a small amount of bound states. Our results are relevant for ultracold metastable triplet helium + alkali-metal mixture experiments.

cond-mat.quant-gas↗