Research Projects

Small carbon chains molecules are being detected in the interstellar medium (ISM) and C₅ being the longest carbon chain detected to date. The carbon chain chemistry of ISM has been examined in the past decades, as these nascent species are the bases for the complex organic molecules that may have reached Earth and acted as seeds for life. The scattering collisional dynamics of small carbon chains Cn (n=2-6) with the most abundant species H₂ present in the ISM result in accurate modeling of the abundance in non-local thermodynamic equilibrium (non-LTE) conditions. Cold collisions with H₂ result in ro-vibrational energy transfer yielding (i) inelastic collisions, (ii) charge transfer collisions, and (iii) ultracold collisions through meta-stable van der Waals complexes. This proposal aims to build potential energy surfaces (PES) to compute accurate quantum scattering dynamical observables for H₂ collision with Cn systems based on kernel methods like supervised neural networks machine learning model. This approach will help to arrive at the PES which is prohibitively expensive by traditional methods and save computational time without compromising on the quality of the results. The collision rates and other dynamical attributes between H₂ and Cn collisions computed for a wide range of temperatures relevant to cold regions will lead us to determine the physical and chemical conditions in many regions of the ISM and to understand the mechanistic insights of the reaction dynamics at the atomic and molecular level. The He collisions are computationally convenient to perform whereas the collision with much more abundant H₂ is both complicated and computationally demanding. Also, for the open shell systems due to the presence of non-zero spin-spin coupling (such as C₄ which is triplet ground state), fine structure splitting takes place and each rotational level will further get split. Therefore, fine structure splitting needs to be considered for the open shell molecules. Using scaled He rates to approximate para-H₂ collisional rates, by reduced 4-dimensional to 2-dimensional PES, has limitations due to large error and limited applicability. On the other hand, no such reduction model exist for ortho-H₂. Therefore, a full rotational collisional scattering dynamics study of Cn with H₂ is required. The PES will be calculated using coupled-cluster (CC) or configuration interaction (CI) method employing large basis sets like aug-cc-pVQZ, benchmarking to complete basis sets limit and machine learning neural network algorithm will be chosen obeying spectroscopic accuracy to augment the ab initio computed PES. The augmented PES will then be converted into radial potential using the bispherical harmonics function by multipolar expansion coefficients (Clebsch–Gordan coefficients), and solve close-coupling equations and scattering matrix in order to arrive at the state-to-state cross-sections and, then rate coefficients for various ro-vibrational transitions of Cn. The different rotational (de-)excitation transition rate coefficients for the ortho- and para-H₂ will be compared and also with the available experimental data. Bound state calculations of van der Waals complexes and pressure broadening cross-sections will be obtained for the resonance lifetime calculations. From above dynamical attributes, abundance ratio of Cn molecules, column densities and their mechanism of formation and decomposition can be predicted.