Kinetic Energy Release Distributions Research Articles

Pathways of two-body dissociation of BrCNq+ (q = 2, 3) induced by electron collision.

Pathways of two-body fragmentation of BrCNq+ (q = 2, 3) have been explored by combined experimental and theoretical studies. In the experiment, the BrCN molecule is ionized by 1keV electron impact and the created fragment ions are detected using an ion momentum imaging spectrometer. Six two-body fragmentation channels are identified. By measuring the momentum vectors of the fragment ions, the kinetic energy release (KER) distributions for these channels have been determined. Theoretically, the potential energy curves of BrCNq+ (q = 2, 3) as a function of Br-C and C-N internuclear distances are calculated by the complete active space self-consistent field method. By comparing the measured KER and theoretical predictions, pathways for the fragmentation channels are assigned. The relative branching ratios of the channels are also determined.

Total kinetic energy release in the fast neutron induced fission of actinide nuclei

The total kinetic energy release and fission mass distributions for the fast neutron (En = 3–100 MeV) induced fission of 232Th, 233U, 235U, 237Np, 239Pu, 240Pu, and 242Pu have been measured using the LANSCE facility. The neutron energies were deduced from time-of- flight measurements. The fission fragments were detected using Si PIN diode detectors, giving us the fragment energies. The actinide targets were made by vapor deposition leading to high-quality targets, that were thin and uniform with reduced impurities. Corrections were made to the data for pulse height defect and the fragment energy loss in the target and its backing. The TKE distributions were Gaussian in shape and their mean value as a function of incoming neutron energy could be fitted with second order polynomials. In the case of 233U and 235U, our measurements agree with prior work. Our measurements for 232Th are unique. Our data agree with Viola scaling. The constant position of the heavy mass peak is interpreted as being due to the influence of the N = 88 and Z = 50 shells. The GEF model predictions agree with the data in general as do the CGMF model predictions.

Photodissociation Dynamics of the [O2-H2O]+ Ionic Complex.

We present an experimental study on the photodissociation dynamics of [O2-H2O]+ in the 580-266 nm wavelength range using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces mass selected and internally cold ions for photodissociation. By detecting both the O2+ and H2O+ photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O2+ + H2O and H2O+ + O2 product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O2-H2O]+ are interpreted as (1) the O2(X3Σg-) + H2O+, O2(a1Δg) + H2O+, and O2(X3Σg-) + H2O+ channels are produced from direct dissociation of [O2-H2O]+ in its excited , , and states, respectively; (2) the O2+(X2Πg) + channel is produced from nonadiabatic relaxations of the excited , , and states to the ground state with subsequent dissociation. The latter nonadiabatic processes involve charge-transfer on the potential energy surfaces, and the charge-transfer probabilities are determined from experimental results. The dissociation energy of the ground state to the lowest dissociation limit is experimentally refined as D0 = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O2-H2O]+ and in the ion-molecule reaction O2 + H2O+ → O2+ + H2O.

Infrared Activated Signatures and Jahn-Teller Dynamics of NO-CH4 Collision Complexes.

Bimolecular collision outcomes sensitively depend on the chemical functionality and relative orientations of the colliding partners that define the accessible reactive and nonreactive pathways. Accurate predictions from multidimensional potential energy surfaces demand a full characterization of the available mechanisms. Therefore, there is a need for experimental benchmarks to control and characterize the collision conditions with spectroscopic accuracy to accelerate the predictive modeling of chemical reactivity. To this end, the bimolecular collision outcomes can be investigated systematically by preparing reactants in the entrance channel prior to reaction. Herein, we investigate the vibrational spectroscopy and infrared-driven dynamics of the bimolecular collision complex between nitric oxide and methane (NO-CH4). We recorded the vibrational spectroscopy of NO-CH4 in the CH4 asymmetric stretching region using resonant ion-depletion infrared spectroscopy and infrared action spectroscopy, thus revealing a significantly broad spectrum centered at 3030 cm-1 that extends over 50 cm-1. The asymmetric CH stretch feature of NO-CH4 is explained by CH4 internal rotation and attributed to transitions involving three different nuclear spin isomers of CH4. The vibrational spectra also show extensive homogeneous broadening due to the ultrafast vibrational predissociation of NO-CH4. Additionally, we combine infrared activation of NO-CH4 with velocity map imaging of NO (X2Π, ν″ = 0, J″, Fn, Λ) products to develop a molecular-level understanding of the nonreactive collisions of NO with CH4. The anisotropy of the ion image features is largely determined by the probed rotational quantum number of NO (J″) products. For a subset of NO fragments, the ion images and total kinetic energy release (TKER) distributions show an anisotropic component at low relative translation (∼225 cm-1) indicating a prompt dissociation mechanism. However, for other detected NO products, the ion images and TKER distributions are bimodal, in which the anisotropic component is accompanied by an isotropic feature at high relative translation (∼1400 cm-1) signifying a slow dissociation pathway. In addition to the predissociation dynamics following vibrational excitation, the Jahn-Teller dynamics prior to infrared activation need to be considered to fully describe the product spin-orbit distributions. Therefore, we correlate the Jahn-Teller mechanisms of NO-CH4 to the symmetry-restricted NO (X2Π, ν″ = 0, J″, Fn, Λ) + CH4 (ν″) product outcomes.

Three-body fragmentation dynamics of BrCNq+ (q=3–6) induced by 1-keV electron impact

The three-body fragmentation dynamics of $\mathrm{BrC}{\mathrm{N}}^{q+} (q=3--6)$ produced by 1-keV electron impact has been investigated using the ion momentum imaging technique. Up to 11 three-body Coulomb explosion channels were identified and analyzed. The corresponding kinetic energy release (KER) distributions were obtained and compared with the predictions of the Coulomb explosion model. By means of the Dalitz plot, Newton diagram, and native frame method, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{Br}}^{l+}+{\mathrm{C}}^{+}+{\mathrm{N}}^{+} (l=1--3)$. The KER for the intermediate dications $\mathrm{Br}{\mathrm{C}}^{2+}$ and $\mathrm{C}{\mathrm{N}}^{2+}$ from the sequential mechanism were determined and the electronic states of the intermediate molecular ion $\mathrm{C}{\mathrm{N}}^{2+}$ were discussed. For the channels leading to higher charge states of the carbon and nitrogen ions, only the concerted fragmentation mechanism was observed.

Cooling dynamics of energized naphthalene and azulene radical cations.

Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az+) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording "molecular cloud in a box" conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ ⇌ Az+ quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az+ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D0 ← D2 transition, efficiently quenches dissociation for vibrational energies up to ≈1eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation.

Fragmentation Dynamics of CO2q+ (q = 2, 3) in Collisions with 1 MeV Proton

The fragmentation dynamics of the CO2q+ (q = 2, 3) molecular ions formed under the impact of 1 MeV protons is studied using a recoil ion momentum spectrometer equipped with a multi-hit time- and position-sensitive detector. Both two-body and three-body fragmentation channels arising from the doubly and triply ionized molecular ions of CO2 are identified and analyzed. Kinetic energy release (KER) distributions have been obtained for various channels. With the help of Dalitz plots and Newton diagrams concerted and sequential processes have been assigned to observed fragmentation channels. In addition, angular correlations are used to determine the molecular geometry of the precursor molecular ion. It is found that the symmetric breakup into C+ + O+ + O+ involves asymmetric stretching of the molecular bonds in CO23+ prior to dissociation via concerted decay implying the fact that collisions with 1 MeV proton induces an asynchronous decay in CO2.

Photoionization Dynamics and Proton Transfer within the Adenine-Thymine Nucleobase Pair

Studying the stability of hydrogen-bonded nucleobase pairs, at the heart of the genetic code, is of utmost importance for an in-depth understanding of basic mechanisms of life and biomolecular evolution. We present here a VUV single photon ionization dynamic study of the nucleobase pair adenine-thymine (AT), revealing its ionization and dissociative ionization thresholds via double imaging electron/ion coincidence spectroscopy. The experimental data, consisting of cluster mass-resolved threshold photoelectron spectra and photon energy-dependent ion kinetic energy release distributions, allow the unambiguous distinction of the dissociation of AT into protonated adenine AH+ and a dehydrogenated thymine radical T(-H) from dissociative ionization processes of other nucleobase clusters. Comparison to high-level ab initio calculations indicates that our experimental observations can be explained by a single hydrogen-bonded conformer present in our molecular beam and allows the estimation of an upper limit of the barrier of the proton transfer in the ionized AT pair.

Fragmentation dynamics of tetrachloromethane molecule induced by highly charged Ar7+-ion impact.

The ion impact multiple ionization and subsequent dissociation of CCl4 is studied using a beam of Ar7+ ion having the energy of about 1MeV in a linear time- of-flight mass spectrometer, coupled with a position-sensitive detector. The complete, as well as incomplete Coulomb explosion pathways, for CCl4 2+ and CCl4 3+ ions are identified and studied. The kinetic energy release distributions of channels, kinetic energies, and momentum distributions of fragmented ions, as well as neutrals, are also calculated. Possible modes of fragmentation pathways, i.e., concerted and/or sequential, for all the identified channels are studied using Newton diagrams, Dalitz plots, and kinetic energy distributions. The dynamical information and fragmentation pathways were analyzed with the Dalitz plot and Newton diagram for the three-body dissociation channel. The nature of the fragmentation process is further investigated with simulated Dalitz plots and Newton diagrams using the simple classical mechanical model.

Fragmentation of SO2 q+ (q = 2-4) induced by 1keV electron collision.

We report an investigation on the fragmentation dynamics of SO2 q+ (q = 2-4) induced by 1keV electron collision utilizing an ion momentum imaging spectrometer. Six complete Coulomb explosion channels were observed using the time-of-flight correlation map. The kinetic energy release distributions for these channels were obtained and compared with those available in the literature. The fragmentation mechanisms of the three-body dissociation channels were analyzed by the Dalitz plots and Newton diagrams. Both concerted breakup and sequential fragmentation pathways were identified in the channel SO2 3+ → O+ + O+ + S+, whereas only the concerted breakup mechanism was confirmed for the channels SO2 4+ → O+ + O+ + S2+ and SO2 4+ → O2+ + O+ + S+. Using the Coulomb explosion model, we determined the molecular geometry from the concerted fragmentation channels, and the obtained bond lengths and angles from the higher kinetic energy release peaks are close to that of the neutral SO2 obtained by high-level quantum chemical calculation. The present results indicate that the electron impact experiment is a potential tool for the Coulomb explosion imaging of small molecules.

Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds

After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence – radiative relaxation via thermally populated electronic excited states – efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

UV photodissociation dynamics of the acetone oxide Criegee intermediate: experiment and theory.

The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH3)2COO] is characterized following electronic excitation to the bright 1ππ* state, which leads to O (1D) + acetone [(CH3)2CO, S0] products. The UV action spectrum of (CH3)2COO recorded with O (1D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH3)2COO leads predominantly to the O (1D) product channel. A higher energy O (3P) + (CH3)2CO (T1) product channel is not observed, although it is energetically accessible. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O (3P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O (1D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH3)2COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (>100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH3)2CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH3)2CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CH2OO upon UV excitation.

Multiphoton breakdown of acetylene; formation of organic building block fragments.

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C2H2+ and the fragment ions H+, C+, CH+, CH2+, C2+ and C2H+. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation.

Several small Polycyclic Aromatic Hydrocarbons (PAHs) have been identified recently in the Taurus Molecular Cloud (TMC-1) using radio telescope observations. Reproducing the observed abundances of these molecules has been a challenge for astrochemical models. Rapid radiative cooling of PAHs by Recurrent Fluorescence (RF), the emission of optical photons from thermally populated electronically excited states, has been shown to efficiently stabilize small PAHs following ionization, augmenting their resilience in astronomical environments and helping to rationalize their observed high abundances. Here, we use a novel method to experimentally determine the radiative cooling rate of the cation of 1-cyanonaphthalene (C10H7CN, 1-CNN), the neutral species of which has been identified in TMC-1. Laser-induced dissociation rates and kinetic energy release distributions of 1-CNN cations isolated in a cryogenic electrostatic ion-beam storage ring are analysed to track the time evolution of the vibrational energy distribution of the initially hot ion ensemble as it cools. The measured cooling rate is in good agreement with the previously calculated RF rate coefficient. Improved measurements and models of the RF mechanism are needed to interpret astronomical observations and refine predictions of the stabilities of interstellar PAHs.

Design and characterization of a recoil ion momentum spectrometer for investigating molecular fragmentation dynamics upon MeV energy ion impact ionization

We present the development and performance of a newly built recoil ion momentum spectrometer to study the fragmentation dynamics of ionized molecules. The spectrometer is based on the two-stage Wiley-McLaren geometry and satisfies both time and velocity focusing conditions. An electrostatic lens has been introduced in the drift region to achieve velocity imaging and higher angular collection. The spectrometer is equipped with a 2D position-sensitive detector with multi-hit coincidence electronics. Ionic fragments with kinetic energy ∼8eV can be detected with 4π collection. The overall performance of the spectrometer has been tested by carrying out three-dimensional ion imaging measurements for diatomic (N2) and polyatomic (CH2Cl2) molecules under the impact of 1MeV protons. Three-dimensional momentum and kinetic energy release distributions were derived from the measured position and time-of-flight spectra. The observed features of the various fragmentation channels as well as the measured kinetic energy release distributions are in complete agreement with the available data.

Photodissociation spectroscopy via a rovibrational resonance in intense UV pulses

Photodissociation dynamics via a rovibrational resonance in intense UV pulses is investigated theoretically, using a showcase ${\text{CH}}^{+}$ molecule promoted to the $\text{C}\phantom{\rule{0.16em}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ valence excited electronic state with a potential barrier, thus giving access to study shape resonance controlled by the pulse frequency. Simulations of the kinetic energy release (KER) and angular distribution of the photofragments (ADP) spectra show dramatic differences for the cases when the pulse is tuned on and off the rovibrational resonance. It shows that as the increasing pulse intensity for the transitions to shape resonance, the KER spectra develop into new and higher energy peaks overlying on the broadened background, which is explained by the involvement of other resonances with higher partial waves through electronic Rabi flopping between the ground and excited electronic states. Those nonlinear contribution increases drastically the probability of photofragmentation along the laser polarization in the ADP spectra. The coincident KER-ADP spectra reveal clearly the correlated dynamics in the vicinity of the dissociation barrier. The present work opens possibilities for the manipulation of ultrafast photodissociation dynamics with the help of resonance states in the continuum.

Ionization and ultrafast hydrogen migration of methylamine in few-cycle intense near-infrared laser fields

Double ionization and ultrafast hydrogen migration of methylamine induced by a few-cycle intense near-infrared laser pulse are investigated by the coincidence momentum imaging method. By reference to the results obtained by DFT calculations, the peak profiles in the kinetic energy release distributions have been assigned to the singlet and triplet decomposition channels of CH3NH22+. The anisotropy in the angular distribution of the ejection direction of the fragment ions is interpreted based on the symmetries of the (7a’) HOMO orbital. By the pump–probe measurements, it is revealed that the ultrafast hydrogen migration in CH3NH2+ proceeds within ∼ 30 fs.

Fragmentation dynamics of diatomic molecules under proton impact: Kinetic energy release spectra of CO$$^{{q+}}$$ and NO$$^{{q+}}$$ (q = 2, 3) molecular ions

We report on the fragmentation dynamics of triply charged, diatomic, molecular ions of NO and CO. Dissociative fragmentation after multiple ionization of NO and CO is studied under the impact of 200 keV proton beam using recoil-ion momentum spectrometer. Kinetic Energy Release distributions (KERDs) for various fragmentation channels were obtained. We have also calculated the potential energy curves (PECs) for ground and several excited states of NO^{3+} and CO^{3+} molecular ion. The obtained KERDs are discussed in the background of the calculated PECs as well as the simple Coulomb excitation model. Coulomb breakup of the unstable precursor molecular ion shows a clear preference for the N^{2+} + O^+ (and C^{2+} + O^+) fragmentation channel.

Femtosecond multiphoton ionization of dialkyl carbonate green solvents at 400 nm using photoelectron-photoion coincidence imaging

Femtosecond laser based multiphoton ionization experiments have been carried out for the first time on dialkyl carbonate green solvent molecules i.e. dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate (DMC, DEC, EMC). The dissociation products produced consequent to ionization have been studied by detecting electrons and ions in coincidence mode using velocity map imaging technique. Based on detailed comparative analysis of the mass spectra of DMC, DEC and EMC, we propose possible mechanisms involving a combination of direct bond cleavage as well as isomerization and reorganization to generate the final dissociation products. Further, photoelectron kinetic energy release distributions measured in coincidence with various fragment ions correlated with the possible fragmentation pathways and compared for all three molecules offers a validation of the proposed mechanisms. As an offshoot we obtain experimental estimates for the ionization potentials of DEC and EMC which are hitherto unreported.

Observation of Competitive Nonadiabatic Photodissociation Dynamics of H2S+ Cations.

A comprehensive understanding of dissociation mechanisms is of fundamental importance in the photochemistry of small molecules. Here, we investigated the detailed photodissociation dynamics of H2S+ near 337 nm by using the velocity map ion imaging technique together with the theoretical characterizations by developing global full-dimensional potential energy surfaces (PESs). Rotational state resolved images were acquired for the S+(4S) + H2 product channel. Significant changes in product total kinetic energy release distributions and angular distributions have been observed within a small excitation photon energy range of 5 wavenumbers. Analysis based on the full-dimensional PESs reveals that two nonadiabatic pathways determined by the transition state connecting two minima on the 12A' state are responsible for the dramatic variation of observed product distributions. The current study has directly witnessed the competitive photodissociation mechanisms controlled by a critical energy point on the PES, thereby providing in-depth insight into the nonadiabatic dynamics in photochemistry.