arXiv subjects

Dibyendu Sardar

Publications and source records attributed to Dibyendu Sardar.

12 recordsLinked to original sources

arXiv2026

Multichannel Quantum Defect Theory with Numerical Reference Functions: Applications to Cold Atomic Collisions

We develop a method for calculating multichannel wavefunctions in the spirit of quantum defect theory, based on numerically calculated reference functions. We benchmark the method by calculating cold collisional properties of $^{85}$Rb and $^6$Li in the presence of external magnetic fields tuned across specific $s$-wave Feshbach resonances and thereby reproducing known results. We then apply the method to calculate experimentally observed $d$-wave Feshbach resonance {[\textit{Phys. Rev. Lett.} {\bf 119}, 203402 (2017)]} %MDPI: Is this a reference citation? If so, please note that ref. citations are not allowed in Abstract and should be removed. in $^{87}$Rb-$^{85}$Rb cold collisions. Our numerical results for this $d$-wave resonance show good agreement with the experimental observations. The method is applicable to arbitrary interaction potentials and to any energy range near the scattering threshold. The implementation of our method to any multichannel two-body scattering problem is straightforward.

physics.chem-ph↗

arXiv2026

Ground and excited potential energy surfaces for CaF+Ca interactions and isotope exchange reactions

We investigate the intermolecular interactions between laser-cooled CaF and Ca, in their ground and excited electronic states, aiming to understand atom-exchange reaction pathways. Using state-of-the-art \textit{ab initio} quantum chemistry methods, we compute potential energy surfaces for nine electronic states arising from the lowest three asymptotes of Ca$_2$F trimer, within the rigid rotor approximation applied to CaF. Two-dimensional potential energy surfaces are computed for the ground state and one of the excited states. We use a combination of the coupled cluster method restricted to single, double, and perturbative triple excitations, and the multireference configuration interaction method with single and double excitations. The ground (X)~$^2\mathrm{A}'$ electronic state of the trimer is significantly deep and highly anisotropic. The excited electronic states are also strongly bound. Notably, the potential energy surface of one of the excited states, (2)~$^2\mathrm{A}'$, lies below the ground-state asymptote of the trimer. By analyzing the potential energy surfaces, we discuss atom-exchange reaction pathways involving both the ground-state interaction between CaF and Ca and the excited metastable state of Ca.

physics.atom-ph↗

arXiv2026

Charge Exchange Dynamics in Cold Collisions of $^{40}$CaH$^+$ and $^{39}$K

We report the observation of charge-exchange collisions between trapped calcium monohydride molecular ions ($^{40}$CaH$^+$) and ultracold potassium atoms ($^{39}$K) in a hybrid ion-atom trap. The measured charge-exchange rate coefficient is significantly suppressed relative to the Langevin rate constant for the system. We use $\mathit{ab\ initio}$ quantum-chemical calculations to model the (CaH-K)$^+$ complex in the ground and excited electronic states and to identify possible charge-exchange mechanisms. Our calculations rule out a direct non-radiative charge-exchange reaction and instead point to a radiative mechanism, but do not quantitatively reproduce the measured rate, highlighting the need for a full-dimensional quantum dynamics treatment that includes vibrational motion and intermediate complex formation. Our work demonstrates that cold hybrid ion-atom platforms with molecular ions enable access to richer chemical complexity and collisional dynamics inaccessible in purely atomic systems.

physics.atom-ph↗

arXiv2025

CaF+CaF interactions in the ground and excited electronic states: implications for collisional losses

Accurate \textit{ab initio} potential energy surfaces are essential to understand and predict collisional outcomes in ultracold molecular systems. In this study, we explore the intermolecular interactions between two laser-cooled CaF molecules, both in their ground and excited electronic states, aiming to understand the mechanisms behind the observed collisional losses on the non-reactive, spin-polarized surface of the CaF+CaF system. Using state-of-the-art \textit{ab initio} methods, we compute twelve electronic states of the Ca$_2$F$_2$ complex within the rigid rotor approximation applied to CaF. Calculating the potential energy surfaces for the excited electronic states of Ca$_2$F$_2$ is challenging and computationally expensive. Our approach employs the multireference configuration interaction method, restricted to single and double excitations, along with a reasonably large active space to ensure the convergence in the excited states. We also compute the spin-orbit coupling between the ground state and the lowest spin-polarized triplet state, as well as the spin-spin coupling within the lowest triplet state (1) $^3\mathrm{A}'$. Additionally, we determine the electric transition dipole moments for the (1) $^3\mathrm{A}'$-(2) $^3\mathrm{A}'$ and (1) $^3\mathrm{A}'$-(1) $^3\mathrm{A}''$ transitions. Notably, we find that the lowest spin-polarized state (1) $^3\mathrm{A}'$, shifted by 1064 nm of laser light from the optical dipole trap, intersects several electronically excited states. Finally, by analyzing the potential energy surfaces, we discuss two plausible pathways that may account for the observed collisional losses on the spin-polarized surface of the CaF+CaF system.

physics.atom-ph↗

arXiv2023

Sticking lifetime of ultracold CaF molecules in triplet interactions

A six-dimensional potential energy surface is constructed for the spin-polarized triplet state of CaF-CaF by \textit{ab initio} calculations at the CCSD(T) level of theory, followed by Gaussian process interpolation. The potential is utilized to calculate the density of states for this bi alkaline-earth-halogen system where we find the value 0.038 $μ$K$^{-1}$, implying a mean resonance spacing of 26 $μ$K in the collision complex. This value implies an associated Rice-Ramsperger-Kassel-Marcus lifetime of 18 $μ$s, thus predicting long-lived complexes in collisions at ultracold temperatures.

physics.atom-ph↗

arXiv2022

Four-body singlet potential energy surface for reactions of calcium monofluoride

A full six-dimensional Born-Oppenheimer singlet potential energy surface is constructed for the reaction CaF + CaF $\rightarrow$ CaF$_2$ + Ca using a multireference configuration interaction (MRCI) electronic structure calculation. The {\it ab initio} data thus calculated are interpolated by Gaussian process (GP) regression. The four-body potential energy surface features one $D_{2h}$ global minimum and one $C_s$ local minimum, connected by a barrierless transition state that lends insight to the reaction mechanism. This surface is intended to be of use in understanding ultracold chemistry of CaF molecules.

physics.atom-ph↗

arXiv2022

Electronic structure, spectroscopy, cold ion-atom elastic collision properties and photoassociation formation prediction of (MgCs)$^+$ molecular ion

Studies on the interactions between an alkali atom and an alkaline earth ion at low energy are important in the field of cold chemistry. In this paper we, extensively, study the structure, interactions, and dynamics of (MgCs)$^+$ molecular ion. We use an ab initio approach based on the formalism of non-empirical pseudo-potential for Mg$^{2+}$ and Cs$^+$ cores, large Gaussian basis sets and full valence configuration interaction. In this context, the (MgCs)$^+$ cation is treated as an effective two-electron system. Potential energy curves and their spectroscopic constants for the ground and the first 41 excited states of $^{1,3}Σ^+$, $^{1,3}Π$ and $^{1,3}Δ$ symmetries are determined. Furthermore, we identify the avoided crossings between the electronic states of $^{1,3}Σ^+$ and $^{1,3}Π$ symmetries. These crossings are related to the charge transfer process between the two ionic limits Mg/Cs$^+$ and Mg$^+$/Cs. In addition, vibrational-level spacings, the transition and permanent dipole moments are presented and analysed. Using the produced potential energy data, the ground-state scattering wave functions and elastic cross sections for a wide range of energies are performed. Furthermore, we predict the formation of translationally and rotationally cold molecular ion (MgCs) + in the ground state electronic potential energy by stimulated Raman type process aided by ion-atom cold collision. In the low energy limit (< 1 mK), elastic scattering cross sections exhibit Wigner law threshold behaviour; while in the high energy limit the cross sections as a function of energy E go as E$^{-1/3}$. A qualitative discussion about the possibilities of forming the cold (MgCs)$^+$ molecular ions by photoassociative spectroscopy is presented.

physics.atom-ph↗

arXiv2020

Realizing negative index of refraction in an ensemble of ground-state polar molecules with lasers

We propose a coherent optical method for creating negative refractive index (NRI) for a gaseous ensemble of ground-state polar molecules possessing both permanent electric and magnetic moments. Exploiting the pure rotational transition between the two lowest rotational levels of the ground vibrational state one can generate two dressed states of mixed parity using a microwave laser. These dressed states are then used as the two lower states of a Λ-type three-level scheme using two infrared lasers to couple them to another ro-vibrational level in the ground-state manifold. One of the infrared lasers is used as a weak probe and the other as a control field with a fixed detuning. We take into account local-field effects on the dielectric response of the medium at the probe frequency in terms of Clausius-Mossoti relation. We extract magnetic response out of purely dielectric response and calculate the magnetic permeability of the medium in terms of dielectric susceptibility. Our results show that there is a small frequency window near the two-photon resonance where both electric permittivity and magnetic permeability are negative with vanishingly small absorption. The figure of merit for the medium is shown to exceed unity. We interpret our results in terms of the proximity of EIT and quantum interference. We discuss the possible realization of our method using cold polar molecules that are recently experimentally produced.

physics.optics↗

arXiv2019

Modeling atom-atom interactions at low energy by Jost-Kohn potentials

More than 65 years ago, Jost and Kohn [R. Jost and W. Kohn, {Phys. Rev.} {\bf 87}, 977 (1952)] derived an explicit expression for a class of short-range model potentials from a given effective range expansion with the $s$-wave scattering length $a_s$ being negative. For $a_s >0$, they calculated another class of short-range model potentials [R. Jost and W. Kohn, { Dan. Mat. Fys. Medd} {\bf 27}, 1 (1953)] using a method based on an adaptation from Gelfand-Levitan theory [I. M. Gel'fand and B. M. Levitan, { Dokl. Akad. Nauk. USSR} {\bf 77}, 557-560 (1951)] of inverse scattering. We here revisit the methods of Jost and Kohn in order to explore the possibility of modeling resonant finite-range interactions at low energy. We show that the Jost-Kohn potentials can account for zero-energy resonances. The $s$-wave phase shift for positive scattering length is expressed in an analytical form as a function of the binding energy of a bound state. We show that, for small binding energy, both the scattering length and the effective range are strongly influenced by the binding energy; and below a critical binding energy the effective range becomes negative provided the scattering length is large. As a consistency check, we carry out some simple calculations to show that Jost-Kohn potentials can reproduce the standard results of contact interaction in the limit of the effective range going to zero.

physics.atom-ph↗

arXiv2019

Suppressing deleterious effects of spontaneous emission in creating bound states in cold atom continuum

In a previous paper [B. Deb and G. S. Agarwal, Phys. Rev. A 90, 063417 (2014)], it was theoretically shown that, magneto-optical manipulation of low energy scattering resonances and atom-molecule transitions could lead to the formation of a bound state in continuum (BIC), provided there is no spontaneous emission. We find that even an exceedingly small spontaneous decay from exited molecular states can spoil the BIC. In this paper, we show how to circumvent the detrimental effect of spontaneous emission by making use of vacuum-induced coherence (VIC) which results in the cancellation or suppression of spontaneous emission. VIC occurs due to the destructive interference between two spontaneous decay pathways. An essential condition for VIC is the non-orthogonality of the transition dipole moments associated with the decays. Furthermore, the interference between decay pathways requires that the spacing between the two decaying states must be comparable to or smaller than the square root of the product of the two spontaneous linewidths. We demonstrate that these conditions can be fulfilled by microwave dressing of two appropriately chosen molecular excited states, opening a promising prospect for the experimental realization of BIC of cold atoms.

quant-ph↗

arXiv2018

Spectroscopic properties of the molecular ions BeX^+ (X=Na, K, Rb): Forming cold molecular ions from an ion-atom mixture by stimulated Raman adiabatic process

In this theoretical work, we calculate potential energy curves, spectroscopic pa- rameters and transition dipole moments of molecular ions BeX^+ (X=Na, K, Rb) composed of alkaline ion Be and alkali atom X with a quantum chemistry approach based on the pseudopotential model, Gaussian basis sets, effective core polarization potentials, and full configuration interaction (CI). We study in detail collisions of the alkaline ion and alkali atom in quantum regime. Besides, we study the possibility of the formation of molecular ions from the ion-atom colliding systems by stimu- lated Raman adiabatic process and discuss the parameters regime under which the population transfer is feasible. Our results are important for ion-atom cold collisions and experimental realization of cold molecular ion formation.

physics.atom-ph↗

arXiv2016

Formation of a molecular ion by photoassociative Raman processes

We show theoretically that it is possible to form a cold molecular ion from a pair of colliding atom and ion at low energy by photoassociative two-photon Raman processes. We explore the possibility of stimulated Raman adiabatic passage (STIRAP) from the continuum of ion-atom scattering states to an ionic molecular state. We provide physical conditions under which coherent population transfer is possible in stimulated Raman photoassociation. Our results are important for experimental realization of PA in ion-atom cold collisions.

physics.atom-ph↗