arXiv subjects

Narayan Kundu

Publications and source records attributed to Narayan Kundu.

9 recordsLinked to original sources

arXiv2026

Strong Evidence for Formation of Hydroxyl Anion via 2-Particle-1-Hole Feshbach Resonances

This study investigates the formation of the hydroxyl anion (OH$^-$) via dissociative electron attachment in 2-propanol as a model system for studying both organic and inorganic molecules. Using high-level CAP-EOM-EA-CCSD calculations and advanced ToF mass spectrometry, we demonstrate that OH$^-$ formation at electron energies between 7 and 11 eV is dominated by two-particle-one-hole (2p-1h) Feshbach resonances. The potential-energy curves reveal a dense manifold of anionic states coupled through numerous avoided crossings, facilitating nonadiabatic population transfer during C-OH bond dissociation. Survival-probability analysis identifies a subset of six long-lived resonances that persist long enough to drive fragmentation, with states 25 and 28 acting as primary drivers by funneling the attached electron into the localized $\sigma^*(\mathrm{C{-}OH})$ antibonding orbital. These theoretical predictions are confirmed by experimental observations of a prominent OH$^-$ yield peaking at 8.6 eV, supporting a site-specific fragmentation mechanism that generalizes to the broader class of molecules.

physics.chem-ph↗

arXiv2026

Electron-Impact Quasi-Resonant Ion-Pair Dissociation of OCS: A Velocity Slice Imaging Study with Partial Wave Analysis

We present velocity map imaging data on intramolecular ion-pair dissociation (IPD) of carbonyl sulfide (OCS) induced by electron impact over the 20 eV to 45 eV energy range. Two distinct IPD pathways were resolved: CO+ + S- (threshold 14.8 +- 0.7 eV) and CS+ + O- (threshold 16.8 +- 0.7 eV). The kinetic energy release spectra display a single peak for S- but split into two components for O-; in both channels the maximum kinetic energies level off once the beam energy exceeds roughly 30 eV, pointing to excitation through discrete superexcited states of quasi-resonant character. Partial wave decomposition of the fragment angular distributions reveals that the momentum-transfer parameter (beta) surpasses unity at every energy studied, invalidating the dipole-Born approximation, and that the dominant partial wave character shifts systematically with beam energy. These patterns are consistent with a mechanism in which the incident electron deposits energy through inelastic scattering, populating hybrid Rydberg-ion-pair superexcited configurations that subsequently undergo state-specific unimolecular dissociation along nonadiabatic pathways. From an applied standpoint, intramolecular ion-pair dissociation matters for astrochemistry and radiation biophysics because it generates reactive anions and cations without photon emission, redistributing excess molecular energy nonadiabatically in environments ranging from interstellar clouds to biological systems.

physics.atm-clus↗

arXiv2026

Formation of Hydroxyl Anion via a 2-Particle 1-Hole Feshbach Resonance in DEA to 2-Propanol: A Joint Experimental and Theoretical Study

Absolute cross sections for the formation of OH- from 2-propanol (CH3CH(OH)CH3) via dissociative electron attachment (DEA) are reported in the incident electron energy range of 3.5-13 eV. Four fragment anions are observed: OH-, C2H2O-, C2H4O-, and C3H7O-. The OH- yield exhibits a pronounced resonance centered at 8.2 eV together with a broader structure extending over the 8-10 eV region. Equation-of-Motion Coupled-Cluster (electron attached) calculations with Singles and Doubles combined with a Complex Absorbing Potential (CAP/EOM-EA-CCSD) assign this feature to a two-particle-one-hole (2p-1h) core-excited Feshbach resonance. Potential energy curves along the C-OH dissociation coordinate reveal that core-excited anion states in this energy range promote efficient cleavage of the hydroxyl group. Analysis of Dyson orbitals and resonance widths demonstrates that only states with repulsive antibonding sigma(C-OH) character and sufficiently long lifetimes contribute significantly to the observed OH- production. These results provide fundamental insight into the DEA dynamics of secondary alcohols and highlight the role of multi-electron-attached resonances in site-specific bond rupture induced by low-energy electrons.

physics.atom-ph↗

arXiv2025

Electron Impact Fragmentation Dynamics of Carbonyl Sulfide: A Combined Experimental and Theoretical Study

In this study, we examine the interactions of low- to intermediate-energy electrons (0$-$45 eV) with carbonyl sulfide (OCS). These collisions lead to the formation of several anionic fragments, including C$^-$, O$^-$, S$^-$, and SO$^-$. When the incident electron energy is below the first ionization potential of the molecule, dissociative electron attachment (DEA) process dominates, primarily yielding O$^-$ and S$^-$ fragments. At higher energies, beyond the ionization potential, ion-pair dissociation (IPD) becomes the dominant process, resulting in the emergence of additional fragments such as C$^-$ and SO$^-$. This leads to an increasingly intricate mechanism, necessitating a detailed analysis to elucidate the ion-pair dissociation pathways. The absolute cross section for S$^-$ ions has been determined using the well-established relative flow technique. Theoretical cross sections are calculated using the multi-configurational time-dependent hartree (MCTDH) method, with each potential energy curve obtained from equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations. The computed values are in excellent agreement with the experimental data. The analysis reveals contributions from both linear and bent anionic resonant states. Due to low count rates, only relative cross section curves have been obtained for the O$^-$ and SO$^-$ ions. At higher energies, the ion pair thresholds are evaluated using the Wannier threshold law, yielding values consistent with those derived from thermochemical data.

physics.atm-clus↗

arXiv2025

Observation of Resonant Tunneling from Molecular Shape into Vibronic Feshbach Resonances Followed by Mode-Specific Fragmentation

We present a kinematically complete study of dissociative electron attachment (DEA) in linear OCS molecules, focusing on how electrons resonantly attach and trigger dissociation. Near the Franck-Condon regime, DEA is dominated by molecular shape resonances, where transient OCS$^-$ states form with high vibrational amplitudes, spectroscopically evident as broad features in DEA cross-sections. As the electron beam energy increases from 5.5 to 6.0 eV, S$^-$ population shifts from lower to higher-energy highly dense bending vibrational states, reinforcing our findings on dipole-forbidden vibronic intensity borrowing. Our advanced potential energy curve calculations, employing the Equation-of-motion coupled cluster singles and doubles for electron attachment (EA-EOMCCSD) method, reveal that beyond the shape resonance, non-adiabatic resonant tunneling governs the avoided crossings, dynamically generating three mode-specific vibronic Feshbach resonances before complete dissociation into three distinct kinetic energy bands of S$^-$. Our theoretical results probe most of the experimental observations quantitatively and qualitatively. These insights deepen our fundamental understanding of resonance-mediated dissociation in electron-molecule resonant scattering, with broader implications for quantum mechanics, plasma physics, vibrational revival, astrochemistry, and radiation damage research.

physics.atm-clus↗

arXiv2022

Low-energy electron-induced ion-pair dissociation to "Trilobite-resembling" long-range heavy Rydberg system

We have studied electron-induced ion-pair dissociation dynamics of CO using the state-of-art velocity map imaging technique in combination with a time-of-flight-based two-field mass spectrometer. Extracting the characteristics for O$^-$/CO nascent atomic anionic fragments from the low energy (25 - 45 eV) electron-molecule scattering, first-time, we have directly detected the existence of S-wave resonated Trilobite resembling a novel molecular binding energy mechanism, as predicted by Greene \textit{et al.} \cite{greene2000creation}. The energy balance demands ion-pair dissociation (IPD) lie within a long-range (<1000 Bohr radius) heavy Rydberg system. Modified Van Brunt expression capturing the deflection of dipole-Born approximation is used to model the angular distributions (AD) for the anionic atomic fragments. The AD fits reveal that the final states are dominantly associated with $Σ$ symmetries and a minor contribution from $Π$ symmetric states that maps the three-dimensional unnatural oscillation of Born-Oppenheimer's potential.

physics.atom-ph↗

arXiv2022

Observation of rovibrationally coupled bi-modality and speed-dependent orientation in DEA dynamics of OCS: reveals partial correlations among point group symmetries

Dissociative electron attachment (DEA) to gas-phase carbonyl sulfide (OCS) has been studied diligently using the time-of-flight (TOF) based state-of-the-art velocity map imaging (VMI) technique. Three well-resolved DEA resonances are observed at 5.0, 6.5 and 10.0 eV incident electron energies along with a weak structure at 8.0 eV. The velocity slice images (VSI), Kinetic energy (KE) and angular distributions (AD) for the fragmented sulfur anions are obtained using the wedge slicing technique. The KE distributions for the sulfur nascent fragments reveal bi-modality with rovibrational signatures. The ADs substantiate speed-dependent angular anisotropy demand the existence of partial correlations among three different point group symmetries, confirmed through an in-plane bending mode of vibration with the axial recoil breakdown. Theoretical calculations using R-matrix and density functional approaches strongly support the experimental observations.

physics.atom-ph↗

arXiv2021

Effect of slicing in velocity map imaging for the study of dissociation dynamics

Inelastic collision dynamics between isolated gas-phase carbon monoxide molecules and low energetic electrons (< 50 eV) has been studied using state-of-the-art velocity map imaging apparatus and reported previously. These were based on data analysis using the time-gated parallel slicing technique, which has recently revealed the drawback of lower momentum ion exaggeration mainly due to the inclusion of whole Newton sphere's of diameter $\le$ parallel slicing time window. To overcome this drawback, we report implementing a wedge slicing technique so that every momentum sphere contributes equally to the statistics. We also present a comparative study between these two techniques by reanalyzing the data using the time-gated parallel slicing technique. Unlike parallel slicing, the wedge slicing technique better represents the dissociation dynamics, particularly for the ions with low kinetic energy.

physics.atom-ph↗

arXiv2020

Ion-pair dissociation dynamics in electron collision with carbon dioxide probed by velocity slice imaging

Ion-pair dissociation (IPD) to gas phase carbon dioxide molecule has been studied using time of flight (TOF) based mass spectroscopy in combination with the highly differential velocity slice imaging (VSI) technique. The appearance energy of the fragmented anion provides the experimental threshold energy value for ion-pair production. The kinetic energy (KE) distributions and angular distributions (AD) of the fragment anion dispense the detailed insight into the IPD dynamics. The KE distribution clearly reveals that the IPD dynamics may be due to the direct access to the ion-pair states. However, indirect mechanism can't be ruled out at higher incident electron energies. The angular distribution data unambiguously identified the involvement of the ion-pair state associated with Sigma symmetry and a minor contribution from Pi symmetric states. Computational calculations using density functional theory (DFT) strongly support the experimental observations.

physics.atom-ph↗