Pooja Chahal

RESEARCH, TEACHING, or OTHER INTERESTS

Astronomy and Astrophysics, Physical and Theoretical Chemistry, Physics and Astronomy

Scopus Publications

Quantum rotational dynamics of linear C<inf>5</inf> at low interstellar temperatures for H<inf>2</inf> collision
Pooja Chahal, Apoorv Kushwaha, and T. J. Dhilip Kumar
AIP Publishing
The quantum dynamics of carbon chains through H2 and He collisions in the interstellar medium (ISM) is an important step toward accurate modeling of their abundance in non-local thermodynamic equilibrium conditions. The C5(Σg+1) molecule is the longest pure carbon chain detected in the ISM to date. While He collisions are computationally easy to perform, the collision with much more abundant H2 is both complicated and computationally demanding. Using templates for approximating p-H2 collisional rates, such as scaling He rates and using a reduced 4D → 2D potential energy surface (PES), has limited applicability. On the other hand, any such approximation does not exist for o-H2. Therefore, a full rotational dynamics of C5 with both p- and o-H2 is performed considering both molecules as rigid-rotors. The PES is calculated using CCSD(T)-F12a/AVTZ, and a neural network fitting model has been carefully chosen to strictly obey spectroscopic accuracy and augment the PES. The augmented PES is then expanded into radial terms using the bispherical harmonics function, and close coupling calculations have been done to get the cross sections and, subsequently, rate coefficients for various rotational transitions of C5.


Inelastic scattering of PO⁺ by H<inf>2</inf> at interstellar temperatures
Pooja Chahal, Apoorv Kushwaha, and T J Dhilip Kumar
Oxford University Press (OUP)
ABSTRACT Phosphorous species are of great interest in interstellar chemistry since they are the basic blocks for building life here on Earth. Modelling the abundance and environment of recently detected PO$^{+}$ under non-local thermodynamic equilibrium (LTE) requires rotational spectra of the molecule along with accurate collisional rates with the most abundant species, hydrogen and helium. A new 4D ab initio potential energy surface (PES) of PO$^{+}$ – H$_{2}$ collision is calculated using CCSD(T)/CBS(DTQ) methodology considering rigid rotor approximation. The region containing the minima of the PES is augmented using neural networks (NNs) model while very high potentials ($\\gt 2500$ cm$^{-1}$) and asymptotic region have been approximated using Slater and R$^{-4}$ functions, respectively. The close coupling calculations have been performed using molscat software for both ortho and para-H$_{2}$. The rate coefficients have been reported for transitions $j-j^{\\prime }=$$1-0$, $2-1$, $3-2$, and $5-4$ through which PO$^{+}$ has been experimentally detected in interstellar medium (ISM). The rate coefficients for even and odd transitions of PO$^{+}$ with para-H$_{2}$ are compared with that of helium and are found to be 1.1–2.0 times higher. For even transitions ($\\Delta j = 2$), the ortho-H$_{2}$ rates are 10 per cent higher than para-H$_{2}$ rates. However, the trend reverses in the case of odd transitions ($\\Delta j = 1$) when higher J transitions are considered at low temperatures. At higher temperatures, the ortho rates cross the para-H$_{2}$ rates and become larger than the latter. The new rate coefficients with both ortho and para-H$_{2}$ will enable accurate modelling of the PO$^{+}$ abundance in the ISM under non-LTE conditions.


Rotational dynamics of CNCN by p-H<inf>2</inf> and o-H<inf>2</inf> collision at interstellar temperatures
Apoorv Kushwaha, Pooja Chahal, and T. J. Dhilip Kumar
AIP Publishing
The rotational dynamics of isocyanogen (CNCN) is studied for its collision with para (p-) and ortho (o-) hydrogen (H2) in the temperature range of 1–100 K. These temperatures correspond to the cold dense molecular clouds in the interstellar medium where molecular hydrogen is the primary collider. An ab initio 4D potential energy surface (PES) is constructed keeping the two molecules under rigid rotor approximation. The PES is generated using the CCSD(T)-F12b/AVTZ level of theory. The 4D PES is further fitted into a neural network (NN) model, which can augment the surface and account for missing data points within spectroscopic accuracy. This NN-fitted PES is then expanded over a bispherical harmonics function to get radial terms, which are expressed into analytic functions. Thereafter, the cross sections (σ) are computed for rotational transitions of CNCN (j → j′) using the close-coupling and centrifugal sudden methods for both p-H2 (jc = 0) and o-H2 (jc = 1) collision till 194 cm−1. In addition, p-H2 (jc = 0, 2) cross sections are also computed using the centrifugal sudden approximation method. The collisional rates are achieved by taking the Boltzmann distribution of σ over the translational energy of H2 till 100 K. Finally, the CNCN–H2 rates are compared to CNCN–He and NCCN–H2 collisional rates. Comparing even and odd transitions for the CNCN–H2 rates show a propensity toward higher rates for even transitions especially for o-H2 collisions considering low-order transitions.


Quantum rotational dynamics of l-C<inf>4</inf>(³Σ⁻<inf>g</inf>) by H<inf>2</inf> at low temperatures employing a machine learning augmented potential energy surface
Pooja Chahal, Apoorv Kushwaha, and T. J. Dhilip Kumar
Royal Society of Chemistry (RSC)
Rotational transitions due to collision of linear carbon cluster with hydrogen: PES aided with neural networks.


PO⁺-He collision: Ab initio potential energy surface and inelastic rotational rate coefficients
Pooja Chahal and T J Dhilip Kumar
Oxford University Press (OUP)
ABSTRACT Computations involving quantum dynamics are performed to attain cross-sections corresponding to rotational de-excitation and excitation rates of the PO+ species including four rotational lines recently detected in the interstellar molecular clouds. New ab initio potential energy surface (PES) for PO+−He collision is constructed by using CCSD(T) method and basis set extrapolated to complete basis set limit (CBS) considering a rigid rotor approximation. The PES is then trained to create neural network (NN) model to construct an augmented surface with angular coordinates at 1° intervals. The PES has a global minimum located at $\\theta =110{}^{\\circ }$ and R = 3.1 Å. An analytical fitting is performed on the NN surface to obtain the first 41 radial coefficients needed to solve the equations of the coupled-channel method. The essentially precise close coupling approach is used to compute the rotational (de-)excitation cross-sections till 1400 cm−1 with rotational states converged up to 26. Further, these cross-sections are thermally averaged to get the rate coefficients for various rotational transitions till 200 K. The propensity rule favours the odd transitions (Δj = 1) for the current study. The rate for the transition 5 → 4 is found to be higher than transition 1→ 0 by a factor of 3.1 at T = 20 K that decreases to 2.1 at T = 100 K.


Non-adiabatic interactions in H⁺ + C<inf>3</inf> system: An ab initio study
Pooja Chahal and T.J. Dhilip Kumar
Elsevier BV



Rotational Excitation of NCCN by p-H<inf>2</inf>(j<inf>c</inf> = 0) at Low Temperatures
Apoorv Kushwaha, Ritika, Pooja Chahal, and Thogluva Janardhanan Dhilip Kumar
American Chemical Society (ACS)

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