We measured the Q-value and the scattering angle distributions for non-dissociative state selective single electron capture in collisions of 7.5 keV H$^+$ and 15 keV H$_2^+$ with He. The experimental data are compared with semiclassical close-coupling calculations and predictions from the classical trajectory Monte Carlo simulations. By analogy with Fraunhofer diffraction, we also developed a toy model to reconstruct an imaginary screen that reflects the reaction impact-parameter dependence, in channels where the magnetic quantum number remains unchanged. It is well established that H$_2^+$ acts as a molecular double-slit in scattering processes. By demodulating the Young's double-slit-type interference pattern, we extracted the individual slit diffraction pattern of H$_2^+$ and compared it with that of the H$^+$ atomic single-slit. For ground state electron capture, we found that the single and the double slit diffraction patterns have equal fringe width, whereas for excited state electron capture, diffraction patterns are quite different.

The fragmentation dynamics of CHF33+ ions produced in slow collisions with Ar4+ projectile velocity ( v=0.49a.u.) is investigated using cold-target recoil ion momentum spectroscopy. Five distinct fragmentation pathways are observed, including an incomplete three-body breakup, a complete three-body breakup, and three incomplete four-body dissociation channels. The mechanisms of complete and incomplete three-body fragmentation are analyzed using a Dalitz plot and a Newton diagram, which reveal a signature of sequential dissociation. For the incomplete four-body channels, projected three-dimensional Newton diagrams are employed, allowing for the identification of the underlying stepwise breakup sequence despite the presence of an undetected neutral fragment. For selected channels, potential energy curves of the intermediate ions are calculated to support the sequential behavior inferred from the experimental observations. For all channels, the measured kinetic-energy-release distributions are interpreted within a classical Coulomb-explosion model to understand the underlying charge distribution at the moment of breakup. In addition, the relative yields of multi-electron capture processes are evaluated and analyzed within the framework of the extended classical over-the-barrier model.

Abstract The fragmentation dynamics of CHF 3 3 + ions produced in slow collisions with Ar 4 + projectile velocity ( v = 0.49 a . u . ) is investigated using cold-target recoil ion momentum spectroscopy. Five distinct fragmentation pathways are observed, including an incomplete three-body breakup, a complete three-body breakup, and three incomplete four-body dissociation channels. The mechanisms of complete and incomplete three-body fragmentation are analyzed using a Dalitz plot and a Newton diagram, which reveal a signature of sequential dissociation. For the incomplete four-body channels, projected three-dimensional Newton diagrams are employed, allowing for the identification of the underlying stepwise breakup sequence despite the presence of an undetected neutral fragment. For selected channels, potential energy curves of the intermediate ions are calculated to support the sequential behavior inferred from the experimental observations. For all channels, the measured kinetic-energy-release distributions are interpreted within a classical Coulomb-explosion model to understand the underlying charge distribution at the moment of breakup. In addition, the relative yields of multi-electron capture processes are evaluated and analyzed within the framework of the extended classical over-the-barrier model.

We present an experimental investigation of the state-selective single-electron capture process in 10 and 40 keV NO2+ + {He,H2} collision systems. Using the cold target recoil ion momentum spectroscopy technique, we measure the non-dissociative state populations following electron capture and the corresponding scattering angle distributions. To interpret the observed structures in the scattering angle distributions, we estimated the impact parameter dependence using a 'toy' model inspired by Fraunhofer matter-wave diffraction from a circular aperture. This model estimated the wavefront alteration in the scattering processes and provided a qualitative insight into the role of the molecular double-slit in the state-selective electron capture process.

We investigate the fragmentation dynamics of methane dication (CH_4^2+) produced in collisions with 50-MeV C^6+ ions using the COLTRIMS technique. The method provides complete three-dimensional momentum vectors of the charged fragments, enabling full kinematic reconstruction of the fragmentation process. The dynamics are analyzed using Dalitz plots, Newton diagrams, and the native-frame method to distinguish between concerted and sequential dissociation mechanisms. The data indicate the presence of sequential fragmentation pathways for the CH_4^2+ → CH_2^+ + H^+ + H, CH_4^2+ → CH^+ + H^+ + 2H, and CH_4^2+ → C^+ + H^+ + 3H channels, consistent with dissociation via short-lived dicationic intermediates CH_3^2+, CH_2^2+, and CH^2+, respectively. From the Newton-diagram momentum distributions, we further estimate the half-rotational periods of the intermediate states, providing insight into their rotational dynamics and finite lifetimes prior to fragmentation. The experimental observations are further supported by comparisons with calculated potential-energy curves.

We study the fragmentation dynamics of CO molecules in slow- (0.16 a.u. Xe4+) and intermediate- (1.41 a.u. He2+) velocity electron capture collisions. The kinetic energy release distributions of the C++O+ fragmentation channel show the variation of energy depositions during the collisions for the associated capture paths. We have also observed molecular-axis orientation-dependent variations in the angle-differential cross sections for the double-electron capture and transfer ionization processes. A simple mathematical model based on Young's optical double-slit experiment has been invoked. The variations are attributed to the molecular double-slit-type interference phenomena arising from the interference between the two outgoing de Broglie waves formed by electron capture. The asymmetry in the interference pattern due to the scattering phase gives insight into the role of the quantum phenomena in characterizing such collision dynamics. Our study thus provides general evidence of the wave-particle duality in low to highly perturbative ion-molecule collisions.

We report on the combined experimental and theoretical studies of the single-electron capture collisions of Ar8+ projectiles with the H2 molecules at 1, 2, and 4 keV/u collision energies. The nondissociative recoil H2+ molecular ions are measured in coincidence with the charge-changing Ar7+ projectiles. The relative cross sections of the different state-selective capture channels are obtained from the experimentally measured Q-value spectra. The 1s electron transfers to the highly excited 4d + 4 f, 5s, 5p, 5d + 5 f + 5g, 6s + 6p, and 6d + 6 f + 6g + 6h states are resolved experimentally. The differential scattering angle distributions for the dominant 1s to 5s, 5p, and 5d + 5 f + 5g transitions are compared with the two-center atomic orbital closecoupling methods. A collision energy-dependent dynamical coupling effect is also observed for the 1s -> 5p0 and 1s -> 5p +/- 1 transitions. The dominant oscillatory structures in the scattering-angle distributions are attributed to Stueckelberg-type oscillations. In contrast, the less visible undulations in the smaller scattering angles imprint the signature of the quantum matter-wave scattering of the projectiles. The quantum nature of the oscillations in the angular distributions is further validated by classical calculations. Our study thus illustrates the highly excited quantum state-selective electron capture process and sheds light on the scattering-angle-range-dependent collision dynamics for highly charged ion-molecule collisions in the highly perturbative regime.

We present an experiment with numerical simulations for the state-selective single-electron-capture process in low-energy 1-8-keV/u Ar8+-He collisions. The relative cross sections for the state-selective transitions are obtained from the measured Q-value spectra. Subshell-resolved differential scattering angle distributions (DSADs) for the dominant (n = 4) 1s to 4s, 4p, 4d, and 4 f transitions and weak (n = 5) 1s to 5s and 5p transitions at 1 keV/u and some mixed state transitions for 5- and 8-keV/u collision energy are measured. The experimentally obtained state-selective DSADs are compared with the two-center atomic orbital close-coupling calculations and qualitatively reproduced the oscillatory structures, which are quantum in nature. Undulations in the small scattering angles for 1s to 4s and 4p0 transitions are found to be arising from quantum matter-wave scattering, further satisfied by the simple mathematical model based on the optical Fraunhofer diffraction theory of light. The double differential Q-value versus scattering angle distributions give insight into the double-electron-capture mechanisms measured at 5- and 8-keV/u collision energies. Our findings shed light on the excited-state electronic dynamics to characterize the highly charged ion-atom collisions in the highly perturbative regime.

We present a direct observation where fragmentation of the CO22+ dication, upon highly charged ion impact, leads to the formation of molecular oxygen. We assert that molecular bending and bond stretching modes of the dication represent the underlying mechanisms driving the generation of O2+. We conducted ab initio quantum chemistry calculations for the electronic state of the dication and found that the 5A1 state is responsible for the bond-rearrangement reaction. The branching ratios of this channel for multiple projectile beams of varying charge and velocity have been reported and are found to be independent of the projectile’s charge and velocity.

The four-body fragmentation of acetylene molecular ions (C2Hq+2 , 4 q 8) upon impact with 1 MeV Ar8+ projectile ions with C2H2 molecules, using cold target recoil ion momentum spectrometer (COLTRIMS), is studied by measuring momenta of fragments in a recoil-ion projectile-ion coincidence measurement. Chargesymmetric (-asymmetric) channels in which carbon ions share the same (different) charge are observed. Within the fragmentation of a parent ion, where there is a possibility of both the symmetric and asymmetric charge distribution of carbon ion, charge-symmetric breakup channels are favored over charge-asymmetric ones. Kinetic energy release (KER) distributions are presented and probable electronic states are associated with particular features in the KER distribution by using high-level ab initio quantum chemical calculations. The ab initio calculations also indicated that the difference between KER values for linear and bent structures (cis- and trans-configurations) is of the order of 0.1 eV. The projectile's final charge state after the collision and the reaction windows calculation within the framework of the extended classical overbarrier model examine the role of various electron-capture-associated processes in the formation of the molecular ions in a particular charge state. We observed that as the acetylene ion's charge state increases, the contribution from capture stabilization also increases, indicating the population of captured electrons into the lower energy states of the projectile. The momentum correlation plots in fragments' momentum space were used to visualize the fragmentation mechanism, and the observation of the double distribution of momenta of carbon ions for charge-asymmetric channels showed the direct signatures of fragmentation from two different molecular orientations with respect to the spectrometer axis.

We present an experimental study of multiple-electron capture-induced fragmentation dynamics of Ar2m+ (4 ≤m≤ 7) dimer ions in 4 keV/u Ar8+–Ar2 collisions. The fragment recoil ion pairs and the charge-changing projectiles are coincidentally measured using a double coincidence technique. The branching ratios between the different charge-sharing fragmentation channels show an inherent enhancement of the asymmetric channels. The kinetic energy release (KER) distributions for the associated electron capture process show a shift in the mean KER values toward the higher side with increasing capture stabilization. The interplay between the different projectile autoionization processes sheds light on the energy depositions to the system during collisions. The Coulomb potential energy curves give a physical insight into the role of the projectile final states in the dimer fragmentation dynamics. The dimer-axis orientation-dependent cross sections for the asymmetric fragmentation channels reveal a forward–backward asymmetry that arises from the geometry of the collision system. Our findings thus give insight into the impact parameter-controlled fragmentation dynamics of multiply charged Ar2m+ dimer ions in highly charged ion–dimer slow collisions.

We present an experiment combined with numerical simulations to study single and double electron capture processes induced during the collisions between the He target and the He2+ projectile in the energy range 2-20 keV/u. According to our experimental observations, measuring the angular distribution of the scattered projectile He2+ reveals a clear oscillatory structure. The latter records an imprint of the collision dynamics, specifically, a signature of the matter-wave scattering and the internal electronic structure of the collisional system He2+-He. We found that this feature is sensitive to the incident projectile's energy and the nature of the involved process. With the help of four-body semiclassical close coupling and classical trajectory Monte Carlo methods, we show that the observed structure is of a quantum nature. This is further supported by a simple mathematical model based on Fraunhofer-type diffraction of light, to which the oscillations are attributed. Our findings thus provide insights into the role of quantum phenomena in characterizing collision dynamics

We present a recoil ion-projectile ion coincidence study on the three body fragmentation of CO23+ ion into C+ + O+ + O+ fragments upon impact with Ar3+, Ar6+ and Ar8+ projectile ions having the same velocity of v(p) = 0.49 a.u. (atomic units) using the recoil ion momentum imaging technique. The kinetic energy release (KER) distributions are presented and the effect of different electron-capture associated processes such as pure capture, electron capture followed by target-ionization or projectile auto-ionization is discussed based on the analysis of KER corresponding to different final projectile charge states post-collision. As the capture stabilization increased, the relative yields in the KER distribution are found to decrease in the high KER side for low charged Ar3+ impact, while an increase is observed for Ar6+ and Ar8+ impacts. We discuss the interplay between electron-capture, projectile auto-ionization, and target ionization for the dominance of a particular process. We also observe a low-KER feature around 15 eV which was observed for electron and proton impacts, that has not yet been observed for highly charged ion impacts.

We report the development of a supersonic jet assembly to study electron transfer collisions with atoms, molecules, and van der Waals clusters. A comparative study of Ar monomer and dimer cations is presented for different capture-associated channels with a 2.5 keV/u O2+ projectile beam. For the Ar+ + Ar+ fragmentation channel, the interatomic relaxation channels are discussed. The vacancies of the dimer single site or double site show the dependence on capture mechanisms. In the Ar2+ + Ar+ fragmentation channel, double capture, in addition to the single ionization process, dominates. The orientation effect reflects the maximum yield at around 50 and 130 degrees, and angular distributions are nearly symmetric about the axis perpendicular to the dimer axis.

We study the electron capture-induced fragmentation of N-2 molecules upon impact with Ar8+ projectiles at collision velocities 0.4 and 1.0 a.u. The recoil ions and the charge-changing projectiles are recorded in coincidence by combining a cold target recoil ion momentum spectrometer with a projectile final charge state analyzer. The relative contributions between projectile autoionizations and radiative decays are measured for the individual dissociation pairs. The reaction windows and corresponding final projectile states for multiple-electron capture processes are estimated using the extended classical over-the-barrier model. The various dissociation pairs' kinetic energy release (KER) is measured for the associated capture processes. For the dissociating N-2(m+) (2 <= m <= 6) molecular ions, the mean KER values shifted to higher values as capture stabilization increased. The populations of the higher KER regions are explained by the recapture of the loosely bound electrons into the target highly excited states. A simple classical capture model is employed to understand the role of the different projectile states on the KER distributions (KERDs). Our findings thus demonstrate the collision velocity dependence of the projectile's final state populations and its impact on the KERDs in the highly perturbative regime.

Absolute double differential cross sections (DDCS) of electrons emitted from uracil and 5-bromouracil (BrU) in collisions with protons of energy 200 keV have been measured for various forward and backward emission angles over wide range of electron energies. The measured DDCS are compared with the continuum distorted wave-eikonal initial state (CDW-EIS) calculations. The optimized structure of the BrU was estimated along with the population analysis of all the occupied orbitals using a self-consistent field density. A comparison between the measured DDCS data for the two molecules show that the cross section of low energy electrons emitted from BrU is substantially larger than that for uracil. The BrU-to-uracil DDCS ratios obtained from the present measurements indicate an enhancement of the electron emission by a factor which is as large as 2.0 to 2.5. These electrons being the major agent for damaging the DNA/RNA of the malignant tissues, the present results are expected to provide an important input for the radiosensitization effect in hadron therapy. It is noteworthy to mention that the CDW-EIS calculations for Coulomb ionization cannot predict such enhancement. A large angular asymmetry is observed for uracil with a broad structure, which is absent in case of BrU.

We report the development and performance of a cold target recoil ion momentum spectrometer (COLTRIMS) setup at TIFR, which is built to study various atomic and molecular processes involving the interaction of slow, highly charged ions from an electron cyclotron resonance based ion accelerator. We give a detailed description of the experimental setup, as well as report some initial results on the electron-capture process in collisions of Ar8+ ions with helium and carbon monoxide targets. Here, we present the longitudinal momentum transfer and the sub-shell resolved Q-value spectrum in the case of 2, 4, and 6 keV/u Ar8+ beams in collision with helium. A longitudinal momentum resolution of 0.27 a.u. is achieved in the present system. We also report the state-selective scattering angle distributions for all the collision systems under investigation. We further discuss the fragmentation of the CO2+ molecular ions for different electron capture channels for the 5 keV/u Ar8+ beam. The combination of the COLTRIMS, along with the beam cleaner, the electrostatic deflectors, and the charge state analyzer, is shown to have certain advantages.

We have studied the electron emissions from the smallest hydrocarbon molecule, CH_4 , in collisions with 94 MeV Si^13+ -ions. The absolute double differential cross sections (DDCS) of electron emissions are measured in the energy range of 3–400 eV and in the angular range 20 ^∘ to 160 ^∘ The measured data have been compared with the continuum distorted wave-eikonal initial state (CDW-EIS) model. The CDW-EIS calculations performed with the complete neglect of differential overlap (CNDO) and the molecular orbital (MO) description of the target are presented. Calculations using both target descriptions show a reasonably good agreement with the measured data. However, the CDW-EIS (MO) shows better agreement with the measured cross sections as compared to the CDW-EIS (CNDO). The single differential cross section, total cross sections have also been deduced from the measured DDCS. The KLL Auger hypersatellite peak due to the double K-vacancy in C-atom has been observed. The ratio of double-to-single K-vacancy production cross section is found to be substantially large, i.e., 33