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Naduvalath Balakrishnan

Publications and source records attributed to Naduvalath Balakrishnan.

At least 19 recordsLinked to original sources
arXiv2026

Full-dimensional quantum scattering calculations of rovibrationally excited HD+HD collisions

Full-dimensional quantum scattering calculations are reported for ro-vibrational transitions in HD+HD collisions using a highly accurate interaction potential for the H$_2$-H$_2$ system. Several near-resonant ro-vibrational transitions are identified that conserve the overall rotational angular momentum and nearly conserve the internal energy of the collision partners. Key anisotropic terms that drive the rotational transitions and angular momentum partial waves that contribute to low energy resonant features in the energy dependence of the cross sections are identified. The computed results are in agreement with total cross sections reported in previous experimental results, including resonant features in the energy dependence of the cross section. In particular, low-energy cross sections show a strong resonant feature associated with an $l=3$ partial wave in the incident channel. Rate coefficients for several inelastic rotational and ro-vibrational transitions are reported for temperatures ranging from $0.1$ K to $200$ K and they display a maximum between $1$ K-$10$ K reflecting the important contributions from the $l=3$ shape resonance that occurs around 2.5 K.

physics.atom-ph↗
arXiv2025

Neural network ensemble for computing cross sections for rotational transitions in H$_{2}$O + H$_{2}$O collisions

Water (H$_2$O) is one of the most abundant molecules in the universe and is found in a wide variety of astrophysical environments. Rotational transitions in H$_2$O + H$_2$O collisions are important in modeling environments rich in water molecules but they are computationally intractable using quantum mechanical methods. Here, we present a machine learning (ML) tool using an ensemble of neural networks (NNs) to predict cross sections to construct a database of rate coefficients for rotationally inelastic transitions in collisions of complex molecules such as water. The proposed methodology utilizes data computed with a mixed quantum-classical theory (MQCT). We illustrate that efficient ML models using NN can be built to accurately interpolate in the space of 12 quantum numbers for rotational transitions in two asymmetric top molecules, spanning both initial and final states. We examine various architectures of data corresponding to each collision energy, symmetry of water molecule, and excitation/de-excitation rotational transitions, and optimize the training/validation data sets. Using only about 10\% of the computed data for training, the NNs predict cross sections of state-to-state rotational transitions of H$_{2}$O + H$_{2}$O collision with average relative root mean square error of 0.409. Thermally averaged cross sections, computed using the predicted state-to-state cross sections ($\sim$90\%) and the data used for training and validation ($\sim$10\%) were compared against those obtained entirely from MQCT calculations. The agreement is found to be excellent with an average percent deviation of about $\sim$13.5\%. The methodology is robust, and thus, applicable to other complex molecular systems.

physics.chem-ph↗
arXiv2023

The Li + CaF $\to$ Ca + LiF chemical reaction under cold conditions

The calcium monofluoride (CaF) molecule has emerged as a promising candidate for precision measurements, quantum simulation, and ultracold chemistry experiments. Inelastic and reactive collisions of laser cooled CaF molecules in optical tweezers have recently been reported and collisions of cold Li atoms with CaF are of current experimental interest. In this paper, we report ab initio electronic structure and full-dimensional quantum dynamical calculations of the Li + CaF $\to$ LiF + Ca chemical reaction. The electronic structure calculations are performed using the internally contracted multi-reference configuration-interaction method with Davidson correction (MRCI+Q). An analytic fit of the interaction energies is obtained using a many-body expansion method. A coupled-channel quantum reactive scattering approach implemented in hyperspherical coordinates is adopted for the scattering calculations under cold conditions. Results show that the Li + CaF reaction populates several low-lying vibrational levels and many rotational levels of the product LiF molecule and that the reaction is inefficient in the 1-100 mK regime allowing sympathetic cooling of CaF by collisions with cold Li atoms.

physics.chem-ph↗
arXiv2022

Stereodynamical control of cold collisions between two aligned D2 molecules

Resonant scattering of optically state-prepared and aligned molecules in the cold regime allows the most detailed interrogation and control of bimolecular collisions. This technique has recently been applied to collisions of two aligned ortho-D2 molecules prepared in the j=2 rotational level of the v=2 vibrational manifold using the Stark-induced adiabatic Raman passage technique. Here, we develop the theoretical formalism for collisions of two aligned molecules and apply our approach to state-prepared D2(v=2,j=2) + D2(v=2,j=2) --> D2(v=2,j=2) + D2(v=2,j=0) collisions. Quantum scattering calculations were performed in full-dimensionality on an accurate H$_2$-H$_2$ interaction potential. Key features of the experimental angular distributions are reproduced and attributed primarily to a partial wave resonance with orbital angular momentum L=4

physics.atom-ph↗
arXiv2022

The role of low energy resonances in the stereodynamics of cold He+D2 collisions

In recent experiments using the Stark-induced Adiabatic Raman Passage (SARP) technique, Zhou et al. measured the product's angular distribution for the collisions between He and aligned D2 molecules at cold collision energies. The signatures of the angular distributions were attributed to a l=2 resonance that governs scattering at low energies. A first principles quantum mechanical treatment of this problem is presented here using a highly accurate interaction potential for the He-H2 system. Instead, our results predict a very intense l=1 resonance at low energies, leading to angular distributions that differ from those measured in the experiment. A good agreement with the experiment is achieved only when the l=1 resonance is artificially removed, for example, by excluding the lowest energies present in the experimental velocity distribution. Our analysis revealed that neither the position nor the intensity of the l=1 resonance significantly changes when the interaction potential is modified within its predicted uncertainties. Energy-resolved measurements may help to resolve the discrepancy.

physics.chem-ph↗
arXiv2021

On the use of stereodynamical effects to control cold chemical reactions: the H + D$_{2}\longleftrightarrow$ D + HD case study

Quantum calculations are reported for the stereodynamic control of the H + D$_{2}\longleftrightarrow$ D + HD chemical reaction in the energy range of 1-50 K. Stereodynamic control is achieved by a formalism similar to that reported by Perreault et al. [Nature Chem. 10, 561 (2018)] in recent experimental works in which the alignment of the molecular bond axis relative to the incident relative velocity is controlled by selective preparations of the molecule in a specific or superposition of magnetic projection quantum numbers of the initial molecular rotational level. The approach presented here generalizes the experimental scheme of Perreault et al. and offers additional degree of control through various experimental preparation of the molecular alignment angle. Illustrative results presented for the H + D$_{2}$ and D + HD reactions show significant control with the possibility of turning the reaction completely on or off with appropriate stereodynamic preparation of the molecular state. Various scenarios for maximizing and minimizing the reaction outcomes are identified with selective preparation of molecular rotational states.

physics.chem-ph↗
arXiv2021

Stereodynamic control of cold rotationally inelastic CO + HD collisions

Quantum control of molecular collision dynamics is an exciting emerging area of cold collisions. Co-expansion of collision partners in a supersonic molecular beam combined with precise control of their quantum states and alignment/orientation using Stark-induced Adiabatic Raman Passage allows exquisite stereodynamic control of the collision outcome. This approach has recently been demonstrated for rotational quenching of HD in collisions with H2, D2, and He and D2 by He. Here we illustrate this approach for HD(v=0,j=2)+CO(v=0,j=0) -> HD(v'=0,j')+CO(v'=0,j') collisions through full-dimensional quantum scattering calculations at collision energies near 1 K. It is shown that the collision dynamics at energies between 0.01--1K are controlled by an interplay of L=1 and L=2 partial wave resonances depending on the final rotational levels of the two molecules. Polarized cross-sections resolved into magnetic sub-levels of the initial and final rotational quantum numbers of the two molecules also reveal a significant stereodynamic effect in the cold energy regime. Overall, the stereodynamic effect is controlled by both geometric and dynamical factors, with parity conservation playing an important role in modulating these contributions depending on the particular final state.

physics.chem-ph↗
arXiv2021

Rainbow scattering in rotationally inelastic collisions of HCl and H$_2$

We examine rotational transitions of HCl in collisions with H$_2$ by carrying out quantum mechanical close-coupling and quasi-classical trajectory calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H$_3$Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state suggesting that classification of rainbow scattering into rotational and $l$-type rainbows is effective for H$_2$+HCl collisions. While the quasi-classical trajectory method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.

physics.chem-ph↗
arXiv2020

Stereodynamics of rotationally inelastic scattering in cold He+HD collisions

Stereodynamics of cold collisions has become a fertile ground for quantized studies of molecular collisions and control of the collision outcome. A benchmark process for stereodynamic control is rotational transition in He+HD collisions. This process was recently probed experimentally by Perreault et al. by examining quenching from $j=2$ to $j'=0$ state in the $v=1$ vibrational manifold. Here, through explicit quantum scattering calculations on a highly accurate ab initio interaction potential for He+H$_2$, we reveal how a combination of two shape resonances arising from $l=1$ and $l=2$ partial waves controls the stereodynamic outcome rather than a single $l=2$ partial wave attributed in the experiment. Further, for collision energies below 0.5 cm$^{-1}$, it is shown that stereodynamic preference for integral cross section follows a simple universal trend.

physics.chem-ph↗
arXiv2020

Non-adiabatic quantum interference effects and chaoticity in the ultracold Li + LiNa $\to$ Li$_2$ + Na reaction

Electronically non-adiabatic effects play an important role in many chemical reactions. How these effects manifest in cold and ultracold chemistry remain largely unexplored. Here, through first principles non-adiabatic quantum dynamics calculations of the Li + LiNa $\to$ Li$_2$ + Na chemical reaction, it is shown that non-adiabatic dynamics induces quantum interference effects that dramatically alter the ultracold rotationally resolved reaction rate coefficients. The interference effect arises from a conical intersection between the ground and an excited electronic state that is energetically accessible even for ultracold collisions. These unique interference effects might be exploited for quantum control applications as a quantum molecular switch. A statistical analysis of rotational populations of the Li$_2$ product reveals a Poisson distribution implying an underlying classically chaotic dynamics. The Poisson distribution is robust and amenable to experimental verification and appears to be a universal property of ultracold reactions involving alkali metal dimers.

physics.chem-ph↗
arXiv2020

Stereodynamic control of overlapping resonances in cold molecular collisions

Stereodynamic control of resonant molecular collisions has emerged as a new frontier in cold molecule research. Recent experimental studies have focused on weakly interacting molecular systems such as HD collisions with H$_2$, D$_2$ and He. We report here the possibility of such control in strongly interacting systems taking rotational relaxation in cold collisions of HCl and H$_2$. Using explicit quantum scattering calculations in full six dimensions it is shown that robust control of the collision dynamics is possible even when multiple (overlapping) shape-resonances coexist in a narrow energy range, indicating that cold stereochemistry offers great promise for many molecules beyond simple systems. We demonstrate a striking case where two prominent peaks in overlapping resonances are switched-off simultaneously by suitable alignment of the HCl molecule.

physics.chem-ph↗
arXiv2019

Stereodynamical control of a quantum scattering resonance in cold molecular collisions

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD(v=1,j=2) by collisions with ground state para-H2, the process is dominated by a single L=2 partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.

quant-ph↗
arXiv2018

Unravelling the stereodynamics of cold HD-H2 collisions

Measuring inelastic rates with partial wave resolution requires temperatures close to a Kelvin or below, even for the lightest molecule. In a recent experiment Perreault et al. [1] studied collisional relaxation of excited HD molecules in the v = 1, j = 2 state by para- and ortho-H2 at a temperature of about 1 K, extracting the angular distribution of scattered HD in the v = 1,j = 0 state. By state-preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. Here, we report a first-principles simulation of that experiment which enables us to attribute the main features of the observed angular distribution to a single L = 2 partial-wave shape resonance. Our results demonstrate important stereodynamical insights that can be gained when numerically-exact quantum scattering calculations are combined with experimental results in the few-partial-wave regime.

physics.chem-ph↗
arXiv2015

The Geometric Phase Appears in the Ultracold Hydrogen Exchange Reaction

Quantum reactive scattering calculations for the hydrogen exchange reaction H + H$_2$($v=4$, $j=0$) $\to$ H + H$_2$($v'$, $j'$) and its isotopic analogues are reported for ultracold collision energies. Due to the unique properties associated with ultracold collisions, it is shown that the geometric phase effectively controls the reactivity. The rotationally resolved rate coefficients computed with and without the geometric phase are shown to differ by up to four orders of magnitude. The effect is also significant in the vibrationally resolved and total rate coefficients. The dynamical origin of the effect is discussed and the large geometric phase effect reported here might be exploited to control the reactivity through the application of external fields or by the selection of a particular nuclear spin state.

physics.chem-ph↗
arXiv2008

Quantum calculations of H2-H2 collisions: from ultracold to thermal energies

We present quantum dynamics of collisions between two para-H2 molecules from low (1 mK) to high collision energies (1 eV). The calculations are carried out using a quantum scattering code that solves the time-independent Schrodinger equation in its full dimensionality without any decoupling approximations. The six-dimensional potential energy surface for the H4 system developed by Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] is used in the calculations. Elastic, inelastic and state-to-state cross sections as well as rate coefficients from T = 1 K to 400 K obtained from our calculations are compared with available experimental and theoretical results. Overall, good agreement is obtained with previous studies.

physics.chem-ph↗
arXiv2008

Formation of molecular oxygen in ultracold O + OH reaction

We discuss the formation of molecular oxygen in ultracold collisions between hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based on hyperspherical coordinates is employed for the calculations. Elastic, inelastic and reactive cross sections as well as the vibrational and rotational populations of the product O2 molecules are reported. A J-shifting approximation is used to compute the rate coefficients. At temperatures T = 10 - 100 mK for which the OH molecules have been cooled and trapped experimentally, the elastic and reactive rate coefficients are of comparable magnitude, while at colder temperatures, T < 1 mK, the formation of molecular oxygen becomes the dominant pathway. The validity of a classical capture model to describe cold collisions of OH and O is also discussed. While very good agreement is found between classical and quantum results at T=0.3 K, at higher temperatures, the quantum calculations predict a larger rate coefficient than the classical model, in agreement with experimental data for the O + OH reaction. The zero-temperature limiting value of the rate coefficient is predicted to be about 6.10^{-12} cm^3 molecule^{-1} s^{-1}, a value comparable to that of barrierless alkali-metal atom - dimer systems and about a factor of five larger than that of the tunneling dominated F + H2 reaction.

physics.chem-ph↗
arXiv2008

Quantum dynamics of the O + OH -> H + O2 reaction at low temperatures

We report quantum dynamics calculations of the O + OH -> H + O2 reaction on two different representations of the electronic ground state potential energy surface (PES) using a time-independent quantum formalism based on hyperspherical coordinates. Calculations show that several excited vibrational levels of the product O2 molecule are populated in the reaction. Rate coefficients evaluated using both PESs were found to be very sensitive to the energy resolution of the reaction probability, especially at temperatures lower than 100 K. It is found that the rate coefficient remains largely constant in the temperature range 10-39 K, in agreement with the conclusions of a recent experimental study [Carty et al., J. Phys. Chem. A 110, 3101 (2006)]. This is in contrast with the time-independent quantum calculations of Xu et al. [J. Chem. Phys. 127, 024304 (2007)] which, using the same PES, predicted two orders of magnitude drop in the rate coefficient value from 39 K to 10 K. Implications of our findings to oxygen chemistry in the interstellar medium are discussed.

physics.atom-ph↗