Coincidence Experiments Research Articles

Fragmentation of Methanol Molecules after Core Excitation and Core Ionization Studied by Negative-Ion/Positive-Ion Coincidence Experiments.

We have studied the fragmentation of the methanol molecule after core excitation and core ionization by observing coincidences between negative and positive ions. Five different negative ions (H-, C-, CH-, O-, and OH-) were observed at both the C 1s and O 1s edges. As negative ion formation occurs after resonant and normal Auger decay of core-hole states, it is necessarily linked with the release of positively charged fragments. Our data show that such fragmentation can happen in many different ways: We found approximately 30 negative-ion/positive-ion/positive-ion coincidence (NIPIPICO) channels. All involve only singly charged positive ions. Fragmentation channels leading to atomic ions are the most probable, but positive molecular ions are also frequently found in the context of anion formation. Coincidence yields as a function of photon energy were determined for the most intense NIPIPICO channels. Adding together the data measured at different photon energies, we could also verify the occurrence of four-ion coincidences, which involved one negative ion (H- or O-) and three positive ions.

Correlations and Entanglement of Microwave Photons Emitted in a Cascade Decay.

We use a three-level artificial atom in the ladder configuration as a source of correlated, single microwave photons of different frequency. The artificial atom, a transmon-type superconducting circuit, is driven at the two-photon transition between ground and second-excited state, and embedded into an on-chip switch that selectively routes different-frequency photons into different spatial modes. Under continuous driving, we measure power cross-correlations between the two modes and observe a crossover between strong antibunching and superbunching, typical of cascade decay, and an oscillatory pattern as the drive strength becomes comparable to the radiative decay rate. By preparing the source in a superposition state using an excitation pulse, we achieve deterministic generation of entangled photon pairs, as demonstrated by nonvanishing phase correlations and more generally by joint quantum state tomography of the two itinerant photonic modes.

Observation of fast and slow interatomic Coulombic decay in argon dimers induced by electron-impact ionization

We investigate the interatomic Coulombic decay (ICD) in argon dimers induced by electron-impact ionization (${E}_{0}=90\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) using a multiparticle coincidence experiment in which the momentum vectors and, consequently, the kinetic energies for electrons and fragment ions are determined. The signature of the ICD process is obtained from a correlation map between ejected electron energy and kinetic energy release (KER) for ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$ fragment ions where low-energy ICD electrons can be identified. Furthermore, two types of ICD processes, termed fast and slow interatomic decay, are separated by the ICD initial-state energies and projectile energy losses. The dependence of the energies of emitted low-energy ICD electrons on the initial-state energy is studied. ICD electron energy spectra and KER spectra are obtained separately for fast and slow decay processes where the KER spectra for the slow decay channel are strongly influenced by nuclear motion. The KER and ICD electron energy spectra are well reproduced by ab initio calculations.

Localized or delocalized K-holes in N2: Photoelectron-Auger electron coincidence experiments with high energy resolution

SynopsisFor K-shell ionization of N2 photoelectron auger electron coincident angular distributions show that pathways with gerade and ungerade K-hole state can interfere. On the other hand high resolution photoelectron spectroscopy seems to indicate that the gerade and ungerade hole state can be separated in energy. The present experiment aims to combine the high energy resolution with full angular detection of Auger and Photoelectron to resolve this seeming contradiction.

Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements

X-ray free electron lasers (XFELs) providing ultrashort intense pulses of X-rays have proven to be excellent tools to investigate the dynamics of radiation-induced dissociation and charge redistribution in molecules and nanoparticles. Coincidence techniques, in particular multi-ion time-of-flight (TOF) coincident experiments, can provide detailed information on the photoabsorption, charge generation, and Coulomb explosion events. Here we review several such recent experiments performed at the SPring-8 Angstrom Compact free electron LAser (SACLA) facility in Japan, with iodomethane, diiodomethane, and 5-iodouracil as targets. We demonstrate how to utilize the momentum-resolving capabilities of the ion TOF spectrometers to resolve and filter the coincidence data and extract various information essential in understanding the time evolution of the processes induced by the XFEL pulses.

Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques.

Understanding the ultrafast dynamics of highly excited electronic states of small molecules is critical for a better understanding of atmospheric and astrophysical processes, as well as for designing coherent control strategies for manipulating chemical dynamics. In highly excited states, nonadiabatic coupling, electron-electron interactions, and the high density of states govern dynamics. However, these states are computationally and experimentally challenging to access. Fortunately, new sources of ultrafast vacuum ultraviolet pulses, in combination with electron-ion coincidence spectroscopies, provide new tools to unravel the complex electronic landscape. Here we report time-resolved photoelectron-photoion coincidence experiments using 8 eV pump photons to study the highly excited states of acetone. We uncover for the first time direct evidence that the resulting excited state consists of a mixture of both ny → 3p and π → π* character, which decays with a time constant of 330 fs. In the future, this approach can inform models of VUV photochemistry and aid in designing coherent control strategies for manipulating chemical reactions.

Breaking through the false coincidence barrier in electron-ion coincidence experiments.

Photoelectron Photoion Coincidence (PEPICO) spectroscopy holds the promise of a universal, isomer-selective, and sensitive analytical technique for time-resolved quantitative analysis of bimolecular chemical reactions. Unfortunately, its low dynamic range of ∼103 has largely precluded its use for this purpose, where a dynamic range of at least 105 is generally required. This limitation is due to the false coincidence background common to all coincidence experiments, especially at high count rates. Electron/ion pairs emanating from separate ionization events but arriving within the ion time of flight (TOF) range of interest constitute the false coincidence background. Although this background has uniform intensity at every m/z value, the Poisson scatter in the false coincidence background obscures small signals. In this paper, temporal ion deflection coupled with a position-sensitive ion detector enables suppression of the false coincidence background, increasing the dynamic range in the PEPICO TOF mass spectrum by 2-3 orders of magnitude. The ions experience a time-dependent electric deflection field at a well-defined fraction of their time of flight. This deflection defines an m/z- and ionization-time dependent ion impact position for true coincidences, whereas false coincidences appear randomly outside this region and can be efficiently suppressed. When cold argon clusters are ionized, false coincidence suppression allows us to observe species up to Ar9+, whereas Ar4+ is the largest observable cluster under traditional operation. This advance provides mass-selected photoelectron spectra for fast, high sensitivity quantitative analysis of reacting systems.

Disentangling Multichannel Photodissociation Dynamics in Acetone by Time-Resolved Photoelectron-Photoion Coincidence Spectroscopy.

For the investigation of photoinduced dynamics in molecules with time-resolved pump-probe photoionization spectroscopy, it is essential to obtain unequivocal information about the fragmentation behavior induced by the laser pulses. We present time-resolved photoelectron-photoion coincidence (PEPICO) experiments to investigate the excited-state dynamics of isolated acetone molecules triggered by two-photon (269 nm) excitation. In the complex situation of different relaxation pathways, we unambiguously identify three distinct pump-probe ionization channels. The high selectivity of PEPICO detection allows us to observe the fragmentation behavior and to follow the time evolution of each channel separately. For channels leading to fragment ions, we quantitatively obtain the fragment-to-parent branching ratio and are able to determine experimentally whether dissociation occurs in the neutral molecule or in the parent ion. These results highlight the importance of coincidence detection for the interpretation of time-resolved photochemical relaxation and dissociation studies if multiple pathways are present.

Imaging a multidimensional multichannel potential energy surface: Photodetachment of H(-)(NH3) and NH4 (.).

Probes of the Born-Oppenheimer potential energy surfaces governing polyatomic molecules often rely on spectroscopy for the bound regions or collision experiments in the continuum. A combined spectroscopic and half-collision approach to image nuclear dynamics in a multidimensional and multichannel system is reported here. The Rydberg radical NH4 and the double Rydberg anion NH4 (-) represent a polyatomic system for benchmarking electronic structure and nine-dimensional quantum dynamics calculations. Photodetachment of the H(-)(NH3) ion-dipole complex and the NH4 (-) DRA probes different regions on the neutral NH4 PES. Photoelectron energy and angular distributions at photon energies of 1.17, 1.60, and 2.33 eV compare well with quantum dynamics. Photoelectron-photofragment coincidence experiments indicate dissociation of the nascent NH4 Rydberg radical occurs to H + NH3 with a peak kinetic energy of 0.13 eV, showing the ground state of NH4 to be unstable, decaying by tunneling-induced dissociation on a time scale beyond the present scope of multidimensional quantum dynamics.

Pair neutron transfer inNi60+Sn116probed viaγ-particle coincidences

We performed a γ-particle coincidence experiment for the Ni60+Sn116 system to investigate whether the population of the two-neutron pickup channel leading to Ni62 is mainly concentrated in the ground-state transition, as has been found in a previous work [D. Montanari et al., Phys. Rev. Lett. 113, 052501 (2014)]. The experiment has been performed by employing the PRISMA magnetic spectrometer coupled to the Advanced Gamma Tracking Array (AGATA) demonstrator. The strength distribution of excited states corresponding to the inelastic, one- and two-neutron transfer channels has been extracted. We found that in the two-neutron transfer channel the strength to excited states corresponds to a fraction (less than 24%) of the total, consistent with the previously obtained results that the 2n channel is dominated by the ground-state to ground-state transition.Received 2 February 2016DOI:https://doi.org/10.1103/PhysRevC.93.054623©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDirect reactionsLow & intermediate energy heavy-ion reactionsNuclear reactionsTransfer reactionsProperties59 ≤ A ≤ 89TechniquesRadiation detectorsSpectrometers & spectroscopic techniquesNuclear Physics

Effects of molecular rotation after ionization and prior to fragmentation on observed recoil-frame photoelectron angular distributions in the dissociative photoionization of nonlinear molecules

Experimental angle-resolved photoelectron-photoion coincidence experiments measure photoelectron angular distributions (PADs) in dissociative photoionization (DPI) in the reference frame provided by the momenta of the emitted heavy fragments. By extension of the nomenclature used with DPI of diatomic molecules, we refer to such a PAD as a recoil-frame PAD (RFPAD). When the dissociation is fast compared to molecular rotational and bending motions, the emission directions of the heavy fragments can be used to determine the orientation of the bonds that are broken in the DPI at the time of the ionization, which is known as the axial-recoil approximation (ARA). When the ARA is valid, the RFPADs correspond to molecular-frame photoelectron angular distributions (MFPADs) when the momenta of a sufficient number of the heavy fragments are determined. When only two fragments are formed, the experiment cannot measure the orientation of the fragments about the recoil axes so that the resulting measured PAD is an azimuthally averaged RFPAD (AA-RFPAD). In this study we consider how the breakdown of the ARA due to rotation will modify the observed RFPADs for DPI processes in nonlinear molecules for ionization by light of arbitrary polarization. This model is applied to the core C $1s$ DPI of ${\mathrm{CH}}_{4}$, with the results compared to experimental measurements and previous theoretical calculations done within the ARA. The published results indicate that there is a breakdown in the ARA for two-fragment events where the heavy-fragment kinetic energy release was less than 9 eV. Including the breakdown of the ARA due to rotation in our calculations gives very good agreement with the experimental AA-RFPAD, leading to an estimate of upper bounds on the predissociative lifetimes as a function of the kinetic energy release of the intermediate ion states formed in the DPI process.

Reaction d + 2H → 3He + n as a source of quasi-monoenergetic neutrons for studying the properties of neutron detectors

The responses of two liquid scintillators to neutron irradiation are investigated. Monoenergetic neutrons are obtained in the reaction d + 2H → 3He + n in different configurations of a coincidence experiment involving the detection of both neutrons and the 3He nucleus. In the experiment, the 15 MeV deuteron beam from the U-120 accelerator at the Skobeltsyn Institute of Nuclear Physics, Moscow State University, hits a target of deuterated polyethylene, and the secondary neutrons obtained in the reaction irradiate the EJ-301 and EJ-315 liquid scintillators under study.

A joint theoretical and experimental study on diiodomethane: Ions and neutrals in the gas phase.

The chemical physics of halomethanes is an important and challenging topic in several areas of chemistry in particular in the chemistry of the atmosphere. Among the class of halomethanes, the diiodomethane molecule has attracted some interest in the last years, but despite this, the information on its radical cation [CH2I2](⋅+) is still limited. In this work, we measured and calculated the appearance energy (AE) of the ionic fragments I2(⋅+) and CH2(⋅+) and correlated the different fragmentation channels to the electronic states of the cation via photoelectron-photoion coincidence (PEPICO) experiments. In the case of the CH2/I2(⋅+) channel, the experimentally determined AE is in excellent agreement with the adiabatic theoretical value while a discrepancy is observed for the CH2(⋅+)/I2 channel. This discrepancy can be understood accounting for a fragmentation involving the formation of two I atoms (CH2(⋅+)/2I channel), which, as explained by time dependent density functional theory (TD-DFT) calculations, occurs when [CH2I2](⋅+) excited states are involved.

Effect of time walk in the use of single channel analyzer/discriminator for saturated pulses in the 4πβ–γ coincidence experiments

Using the TAC technique, the timing properties of a 4πβ–γ coincidence counting system were experimentally studied with an emphasis on saturated pulses. Experiments were performed for several discriminators (integral mode of TSCA) each with different kinds of timing techniques. Timing spectra were measured at various applied voltage to the 4π proportional detector covering the entire region of the plateau.Most of timing discriminators show good timing property when the pulses remain the linear region, but suddenly deteriorate after the pulses was saturated, and the timing spectra expands seriously up to a few μs in some types of timing discriminator. To overcome this problem, two techniques were proposed.

Light and heavy transfer products inXe136+U238multinucleon transfer reactions

Background: Multinucleon transfer reactions (MNT) are a competitive tool to populate exotic neutron-rich nuclei in a wide region of nuclei, where other production methods have severe limitations or cannot be used at all. Purpose: Experimental information on the yields of MNT reactions in comparison with theoretical calculations are necessary to make predictions for the production of neutron-rich heavy nuclei. It is crucial to determine the fraction of MNT reaction products which are surviving neutron emission or fission at the high excitation energy after the nucleon exchange. Method: Multinucleon transfer reactions in Xe136+U238 have been measured in a high-resolution γ-ray/particle coincidence experiment. The large solid-angle magnetic spectrometer PRISMA coupled to the high-resolution Advanced Gamma Tracking Array (AGATA) has been employed. Beamlike reaction products after multinucleon transfer in the Xe region were identified and selected with the PRISMA spectrometer. Coincident particles were tagged by multichannel plate detectors placed at the grazing angle of the targetlike recoils inside the scattering chamber. Results: Mass yields have been extracted and compared with calculations based on the grazing model for MNT reactions. Kinematic coincidences between the binary reaction products, i.e., beamlike and targetlike nuclei, were exploited to obtain population yields for nuclei in the actinide region and compared to x-ray yields measured by AGATA. Conclusions: No sizable yield of actinide nuclei beyond Z=93 is found to perform nuclear structure investigations. In-beam γ-ray spectroscopy is feasible for few-neutron transfer channels in U and the −2p channel populating Th isotopes.7 MoreReceived 10 June 2015DOI:https://doi.org/10.1103/PhysRevC.92.024619©2015 American Physical Society

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

AbstractWe present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

Selectivity in the photofragmentation of halo-pyrimidines

SynopsisThe fragmentation of 2Br-, 2Cl- and 5Br-pyrimidine following direct valence photoionization or inner shell excitation and decay has been studied by electron-ion coincidence experiments. The results show that the fragmentation is strongly selective on the final singly charged ion state and the dominant fragmentation patterns correlate to the nearest appearance potential.

Development of a method to measure molecular frame EELS cross sections

SynopsisWe have developed an apparatus for electron-ion coincidence experiments to examine high-energy electron scattering cross sections in molecular frame. By using the apparatus a preliminary experiment is performed for inner-shell excitations of N2 at incident electron energy of 1.5 keV. The experimental result for the 1s → π* excitation clearly shows that the electron-scattering cross section for the fixed-in-space N2 molecule has successfully been obtained.

Coincidence spectroscopy: Past, present and perspectives

Coincidence spectroscopy is a powerful spectroscopic method addressing the recording of more than one particle involved in an ionization process simultaneously. In the current manuscript, an overview of the available flavors of coincidence experiments is given, and new results on the Auger decay of CH3F are discussed. An outlook of possible future experiments employing the approach is also briefly discussed.

State-Resolved Quantum Dynamics of Photodetachment of HCO2(-)/DCO2(-) on an Accurate Global Potential Energy Surface.

Full-dimensional quantum dynamics studies of the photodetachment of HCO2(-) and DCO2(-) are reported using a wave-packet method on an accurate global potential energy surface of the neutral HOCO/HCO2 system. The calculated photoelectron spectra reproduced both the positions and widths of the main HCO2 and DCO2 peaks observed in experiment. Specifically, both the (2)A1 and (2)B2 resonance peaks of the neutral radicals were identified in our simulations thanks to the adiabatic PES that captures both the (2)A1 and (2)B2 minima. The narrow widths and isotope effect of the lowest resonances are indicative of tunneling-facilitated predissociation. Furthermore, the dissociation product CO2 was found to be excited in both its symmetric stretching and bending modes, which are coupled via a strong Fermi resonance, but rotationally cold, in good agreement with the recent photoelectron-photodetachment coincidence experiments.