arXiv subjects

Dhananjay Nandi

Publications and source records attributed to Dhananjay Nandi.

At least 19 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

Study of low-energy electron-induced dissociation of 1-Propanol

The fragmentation of 1-propanol resulting from dissociative electron attachment has been explored across an energy range of 3.5 to 16 eV. Four distinct ion species are identified: $\text{H}^{-}$, $\text{O}^{-}$, $\text{OH}^{-}$, and $\text{C}_{3}\text{H}_{7}\text{O}^{-}$. The $\text{OH}^{-}$ ion exhibited a prominent peak near 8.7 eV, along with a small hump near 5.6 eV. Complementary channels led to the formation of the $\text{H}^{-}$ and $\text{C}_{3}\text{H}_{7}\text{O}^{-}$ ions. Both these two ions exhibit a sharp peak near 6 eV and broad overlapping resonances between 7 to 12 eV. The observed ion yields of distinct dissociation fragments in this study, when compared with those from previously studied alcohols, suggest site-specific fragmentation of alcohols during dissociative electron attachment. To gain a deeper understanding of the dissociation pathways, Density Functional Theory~(DFT) calculations were conducted, revealing the threshold energies for each channel. These threshold energies aligned well with the experimental uncertainties.

physics.atm-clus↗

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↗

arXiv2024

Structural rearrangements and fragmentation pathways induced by a low-energy electron attachment to ethyl acetate

Exploring the molecular fragmentation dynamics induced by low-energy electrons offers compelling insights into the complex interplay between the projectile and target. In this study, we investigate the phenomenon of dissociative electron attachment to ethyl acetate. The recorded yields of various fragment anions within an incident electron energy range of 1 to 13 eV reveal a diverse array of products with six different mass numbers. Examples include (M$-$H)$^-$, CH$_3^-$, C$_2$H$_5$O$^-$, CH$_3$CO$^-$, CH$_2$CHO$^-$, and CH$_3$COO$^-$, formed through the fracture of single bonds. Interestingly, the generation of other fragments, such as HCCO$^-$, suggests a more intricate structural rearrangement of the nuclei, adding a layer of complexity to the observed dissociation dynamics.

physics.atm-clus↗

arXiv2023

Dissociation dynamics in low energy electron attachment to nitrogen dioxide

Complete dissociation dynamics of low energy electron attachment to nitrogen dioxide around 8.5 eV resonance has been studied using a velocity map imaging (VMI) spectrometer. Besides the three prominent resonant peaks at around 1.4 eV, 3.1 eV, and 8.5 eV, we have found an additional small resonance at the higher energy tail of the 8.5 eV resonance. We have collected the momentum distribution data of O$^-$ ions at different incident electron energies around the 8.5 eV resonance along with the smaller additional resonant peak. A theoretical analysis of these resonances with the momentum imaging experimental data on dissociative electron attachment to nitrogen dioxide in the gas phase is used to provide a detailed picture of the molecular dissociation process.

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

Dissociative electron attachment dynamics of carbon disulfide and violation of axial recoil approximation near the 6-eV resonance

Complete dissociation dynamics of low-energy electron attachment to carbon disulfide have been studied using the velocity slice imaging (VSI) technique. The ion yields of S- and CS- fragment ions as the function of incident electron energy in the range 5 to 11 eV have been obtained. Two resonances for S- ions at around 6.2 eV and 7.7 eV and only one resonance for CS- ions at around 6.2 eV have been obtained in this energy range. The kinetic energy and the angular distributions of these fragment negative ions at different incident electron energies around these resonances have been measured. From the angular distribution of these fragment anions, we have found that the bending of the temporary negative ions causes a significant change in the angular distribution from the expected one.

physics.atm-clus↗

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↗

arXiv2019

Dissociation dynamics in the dissociative electron attachment to ammonia molecule

Complete dissociation dynamics of low energy electron attachment to ammonia molecule has been studied using velocity slice imaging (VSI) spectrometer. One low energy resonant peak around 5.5 eV and a broad resonance around 10.5 eV incident electron energy has been observed. The resonant states mainly dissociate via H$^-$ and NH$_2^-$ fragments, though for the upper resonant state, signature of NH$^-$ fragments are also predicted due to three body dissociation process. Kinetic energy and angular distributions of the NH$_2^-$ fragment anions are measured simultaneously using VSI technique. Based on our experimental observations, we find the signature of A$_1$ symmetry in the 10.5 eV resonance energy whereas, the 5.5 eV resonance is associated with the well known A$_1$ symmetry.

physics.atm-clus↗

arXiv2019

Dissociative electron attachment to pulsed supersonic O$_2$ jet : Violation of $Σ^{+} \rightleftharpoons Σ^{-}$ selection rule and dependence on carrier gas proportion

The formation of $O^{-}$ and $O_{2}^{-}$ ions via dissociative electron attachment to a pulsed supersonic jet of $O_{2}$ molecules containing weakly bound small van der Waals clusters seeded in a beam of argon is reported. The energy dependence of the $O^{-}$ and $O_{2}^{-}$ yield exhibits three peaks near 7, 11 and 16 eV incident electron energies. The 7 eV peak arises from the $^{2}Π_{u}$ state of $O_{2}^{-}$ whereas, the 11 and 16 eV peaks are ascribed to two distinct resonance states:$ ^{2}Σ_{g}^{+} $ and $^{2}Σ_{u}^{+}$ states of $O_{2}^{-}$, respectively, via a violation of the $Σ^{+} \rightleftharpoons Σ^{-}$ selection rule. The dependence of the cross-section of these two new peaks at $\sim$11 and $\sim$16 eV on the proportion of the carrier gas is also investigated and an optimum proportion has been observed experimentally which gives the lowest temperature of 14.86 K and highest Mach number of 72.31 for the pulsed supersonic jet.

physics.atm-clus↗

arXiv2018

Dissociative electron attachment to sulfur dioxide : A theoretical approach

In this article, density functional theory (DFT) and natural bond orbital (NBO) calculations are performed to understand experimental observations of dissociative electron attachment (DEA) to SO$_2$. The molecular structure, fundamental vibrational frequencies with their corresponding intensities and molecular electrostatic potential (MEP) map of SO$_2$ and SO$_2^-$ are interpreted from respective ground state optimized electronic structures calculated using DFT. The quantified MEPs and the second order perturbation energies for different oxygen lone pair (n) to $σ^*$ and $π^*$ interactions of S-O bond orbitals have been calculated by carrying out NBO analysis. The change in the electronic structure of the molecule after the attachment of a low-energy ($\leq$ 15 eV) electron, thus forming a transient negative ion, can be interpreted from the $n\rightarrowσ^*$ and $n\rightarrowπ^*$ interactions. The results of the calculations are used to interpret the dissociative electron attachment process. The dissociation of the anion SO$_2^-$ into negative and neutral fragments has been explained by interpreting the infrared spectrum and different vibration modes. It could be observed that the dissociation of SO_{2}^{-} into S^{-} occurs as a result of simultaneous symmetric stretching and bending modes of the molecular anion. While the formation of O$^-$ and SO$^-$ occurs as a result of anti-symmetric stretching of the molecular anion. The calculated symmetries of the TNI state contributing to the first resonant peak at around 5.2 eV and second resonant peak at around 7.5 eV was observed from time-dependent density functional theory calculations to be an A$_1$ and a combination of A$_1$+B$_2$ states for the two resonant peaks, respectively. These findings strongly support our recent experimental observations for DEA to SO$_2$ [Jana and Nandi, Phys. Rev. A, 97, 042706 (2018)].

physics.atm-clus↗

arXiv2018

Absolute cross-sections of fragment negative ions in electron collisions with difluoromethane

Dissociative electron attachment (DEA) and ion-pair dissociation (IPD) processes of Difluoromethane (CH$_2$F$_2$) have been studied in the incident electron energy range 0 to 45 eV. Three different fragment anions (F$^-$, CHF$^-$ and F$_2^-$) are detected in the DEA range and two anions (F$^-$ and CHF$^-$) are detected in IPD range. Absolute cross-section of the F$^-$ fragment ion is measured for the first time. Three different resonances for both F$^-$ and CHF$^-$ ions and one single resonance peak for the F$_2^-$ ions are observed. Constant increase in ion counts above 8 eV incident electron energy indicates the involvement of IPD process. From the experimental observation, it is speculated that near 11 eV incident electron energy both DEA and IPD processes occur simultaneously.

physics.atm-clus↗

arXiv2016

Dipolar dissociation dynamics in electron collisions with oxygen molecules

The dipolar dissociation of molecular oxygen due to 21-35 eV energy electron collision has been studied using the time sliced velocity map imaging technique. A rough estimation about the threshold of the process and the kinetic energy and angular distribution of the fragment negative ions are measured. The dipolar dissociation found to be occur due to pre-dissociation of a Rydberg state via ion-pair state for lower incident electron energies as well from also direct excitation to the ion-pair states for relatively higher primary beam energy. The location and symmetry of the excited states were determined from the kinetic energy and angular distribution data respectively.

physics.atm-clus↗