Proton Continuum Research Articles

Neural Network-Based Sum-Frequency Generation Spectra of Pure and Acidified Water Interfaces with Air.

The affinity of hydronium ions (H3O+) for the air-water interface is a crucial question in environmental chemistry. While sum-frequency generation (SFG) spectroscopy has been instrumental in indicating the preference of H3O+ for the interface, key questions persist regarding the molecular origin of the SFG spectral changes in acidified water. Here we combine nanosecond long neural network (NN) reactive simulations of pure and acidified water slabs with NN predictions of molecular dipoles and polarizabilities to calculate SFG spectra of long reactive trajectories including proton transfer events. Our simulations show that H3O+ ions cause two distinct changes in phase-resolved SFG spectra: first, a low-frequency tail due to the vibrations of H3O+ and its first hydration shell, analogous to the bulk proton continuum, and second, an enhanced hydrogen-bonded band due to the ion-induced static field polarizing molecules in deeper layers. Our calculations confirm that changes in the SFG spectra of acidic solutions are caused by hydronium ions preferentially residing at the interface.

Recent progress in two-proton radioactivity

During the last few decades, rare isotope beam facilities have provided unique data for studying the properties of nuclides located far from the beta-stability line. Such nuclei are often accompanied by exotic structures and radioactive modes, which represent the forefront of nuclear research. Among them, two-proton (2p) radioactivity is a rare decay mode found in a few highly proton-rich isotopes. The 2p decay lifetimes and properties of emitted protons hold invaluable information regarding the nuclear structures in the presence of a low-lying proton continuum; as such, they have attracted considerable research attention. In this review, we present some of the recent experimental and theoretical progress regarding the 2p decay, including technical innovations for measuring nucleon–nucleon correlations and developments in the models that connect their structural aspects with their decay properties. This impressive progress should play a significant role in elucidating the mechanism of these exotic decays, probing the corresponding components inside nuclei, and providing deep insights into the open quantum nature of dripline systems.

Hyper‐Raman study of hydrated excess protons in water: Measurement of concentrated HCl solution

AbstractWe report hyper‐Raman (HR) spectra of concentrated hydrochloric acid solutions for the first time. HR signals from the OH stretching region to the low‐frequency region reflect the presence of chloride anions solvated in water. By contrast, it turns out that HR spectroscopy is insensitive to the presence of excess protons in water. HR spectra of HCl solutions do not show any signature of a proton continuum and shoulders on the high/low‐frequency sides of the bending mode of H2O in stark contrast to IR spectroscopy. Moreover, we found that a sharp HR signal around 500 cm−1 appear in 532 nm excited HR spectra. A laser power dependence indicates that this signal comes from a photochemical reaction. The absence of this signal under 1064 nm HR excitation and 532 nm Raman excitation suggests that the resonance (hyper‐)Raman effect enhances the signal from the photoproduct.

Information content of the differences in the charge radii of mirror nuclei

Differences in the charge radii of mirror nuclei have been recently suggested to contain information on the slope of the symmetry energy $L$. To test this hypothesis, we perform statistical correlation analysis using quantified energy density functionals that are consistent with our previous knowledge on global nuclear observables such as binding energies and charge radii. We conclude that the difference in charge radii between a mirror pair, $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$, is an inferior isovector indicator compared to other observables, such at the neutron skin or electric dipole polarizability ${\ensuremath{\alpha}}_{\mathrm{D}}$. In particular, this quantity correlates poorly with both the neutron skin and $L$. We demonstrate that $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$ is influenced by pairing correlations in the presence of low-lying proton continuum in the proton-rich mirror-partner nucleus. Considering the large theoretical uncertainties on $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$, we conclude that the precise data on mirror charge radii cannot provide a stringent constraint on $L$.

Nature of Excess Hydrated Proton at the Water-Air Interface.

Understanding the interfacial molecular structure of acidic aqueous solutions is important in the context of, e.g., atmospheric chemistry, biophysics, and electrochemistry. The hydration of the interfacial proton is necessarily different from that in the bulk, given the lower effective density of water at the interface, but has not yet been elucidated. Here, using surface-specific vibrational spectroscopy, we probe the response of interfacial protons at the water–air interface and reveal the interfacial proton continuum. Combined with spectral calculations based on ab initio molecular dynamics simulations, the proton at the water–air interface is shown to be well-hydrated, despite the limited availability of hydration water, with both Eigen and Zundel structures coexisting at the interface. Notwithstanding the interfacial hydrated proton exhibiting bulk-like structures, a substantial interfacial stabilization by −1.3 ± 0.2 kcal/mol is observed experimentally, in good agreement with our free energy calculations. The surface propensity of the proton can be attributed to the interaction between the hydrated proton and its counterion.

High-Level VSCF/VCI Calculations Decode the Vibrational Spectrum of the Aqueous Proton

The hydrated excess proton is a common species in aqueous chemistry, which complexes with water in a variety of structures. The infrared spectrum of the aqueous proton is particularly sensitive to this array of structures, which manifests as continuous IR absorption from 1000 to 3000 cm-1 known as the "proton continuum". Because of the extreme breadth of the continuum and strong anharmonicity of the involved vibrational modes, this spectrum has eluded straightforward interpretation and simulation. Using protonated water hexamer clusters from reactive molecular dynamics trajectories, and focusing on their central H+(H2O)2 structures' spectral contribution, we reproduce the linear IR spectrum of the aqueous proton with a high-level local monomer quantum method and highly accurate many-body potential energy surface. The accuracy of this approach is first verified in the vibrational spectra of the two isomers of the protonated water hexamer in the gas phase. We then apply this approach to 800 H+(H2O)6 clusters, also written as [H+(H2O)2](H2O)4, drawn from multistate empirical valence bond simulations of the bulk liquid to calculate the infrared spectrum of the aqueous proton complex. Incorporation of anharmonic effects to the vibrational potential and quantum mechanical treatment of the proton produces a better agreement to the infrared spectrum compared to that of the double-harmonic approximation. We assess the correlation of the proton stretching mode with different atomistic coordinates, finding the best correlation with ⟨ROH⟩, the expectation value of the proton-oxygen distance ROH. We also decompose the IR spectrum based on normal mode vibrations and ⟨ROH⟩ to provide insight on how different frequency regions in the continuum report on different configurations, vibrational modes, and mode couplings.

Neat Water-Vapor Interface: Proton Continuum and the Nonresonant Background.

Whether the surface of neat water is "acidic" or "basic" remains an active and controversial field of research. Most of the experimental evidence supporting the preferential adsorption of H3O+ ions stems from nonlinear optical spectroscopy methods typically carried out at extreme pH conditions (pH < 1). Here, we use vibrational sum frequency spectroscopy (VSFS) to target the "proton continuum", an unexplored frequency range characteristic of hydrated protons and hydroxide ions. The VSFS spectra of neat water show a broad and nonzero signal intensity between 1700 and 3000 cm-1 in the three different polarization combinations examined. By comparing the SF response of water with that from dilute HCl and NaOH aqueous solutions, we conclude the intensity does not originate from either adsorbed H3O+ or OH- ions. Contributions from the nonresonant background are then critically considered by comparing the experimental results with many-body molecular dynamics (MB-MD) simulated spectra.

Puzzling Two-Proton Decay of ^{67}Kr.

Ground-state two-proton (2p) radioactivity is a rare decay mode found in a few very proton-rich isotopes. The 2p decay lifetime and properties of emitted protons carry invaluable information on nuclear structure in the presence of a low-lying proton continuum. The recently measured 2p decay of ^{67}Kr turned out to be unexpectedly fast. Since ^{67}Kr is expected to be a deformed system, we investigate the impact of deformation effects on the 2p radioactivity. We apply the recently developed Gamow coupled-channel framework, which allows for a precise description of three-body systems in the presence of rotational and vibrational couplings. This is the first application of a three-body approach to a two-nucleon decay from a deformed nucleus. We show that deformation couplings significantly increase the 2p decay width of ^{67}Kr; this finding explains the puzzling experimental data. The calculated angular proton-proton correlations reflect a competition between 1p and 2p decay modes in this nucleus.

Vortices in the three-body electron–positron–proton continuum system induced by the positron-impact ionization of hydrogen

By means of a systematic theoretical study of the transition matrix element T for the ionization collisions in a collinear geometry, we uncover the presence of three isolated zeros at intermediate impact energies. We demonstrate that these zeros actually represent vortices in the generalized velocity field associated to T. One of these vortices is shown to be related to a deep minimum observed more than two decades ago by Brauner and Briggs in 1991 at extremely large impact energies, where the corresponding fully differential cross section was too small to be experimentally accessible. Here we elucidate that it might still be present at much smaller impact energies, thus being amenable to experimental investigation. Furthermore, we discovered that this vortex is paired with a second one of opposite circulation, in accordance with one of the scenarios for their emergence. We study the location of these vortices and find that they seem to be located at, emerge from, or move towards specific points which could be related to particular collision mechanisms, as the direct ionization and capture to the continuum cusps, or the saddle-point and Thomas processes.

Shaped Charge Jet Penetration of Alon® Ceramic Assessed by Proton Radiography and Computational Simulations

This work describes the use of proton radiography and continuum simulations to investigate the mechanics of a copper jet penetrating unconfined ALON® transparent ceramic. Use of proton radiography enabled characterization of the jet and ceramic material at 21 time steps, in situ, throughout the penetration process. These radiographs provide time-evolution data pertaining to the material densities, cavity growth, and material failure. The data were compared to legacy analytical penetration models and to a simulation of the event computed using a continuum multi-physics code. These comparisons revealed additional insights into the penetration mechanics as well as strengths and weaknesses of the computational algorithms and material models used in the simulations.

Observation of Hydronium Ions at the Air−Aqueous Acid Interface: Vibrational Spectroscopic Studies of Aqueous HCl, HBr, and HI

The air−liquid interface of aqueous hydrogen-halide solutions is examined using vibrational sum frequency generation spectroscopy. Infrared and Raman spectroscopies are used to compare the effects of the ions on the water structure in the bulk solutions to that of the interface. The addition of HCl, HBr, and HI to water is found to cause a significant disruption in the hydrogen-bonding network at the air−liquid interface, similar to that which is observed for sodium halides. However, a convolution of additional effects are observed for acids at the air−aqueous interface: interfacial H3O+ (and H5O2+) which gives rise to surface potential changes, an increase in interfacial depth from interfacial concentration gradients, and a decrease in the number of dangling OH bonds relative to the neat water surface. Additionally, and moreover, sum frequency spectra reveal a surface proton continuum at frequencies below 3000 cm-1, indicating that hydronium ions (and Zundel ions) exist at the air−aqueous interface.

Y-scaling in proton continuum scattering

Data for inclusive proton scattering spectra at kinematics suited to incoherent quasifree scattering from single nucleons bound within complex nuclei are analysed in the y-scaling format found to unify many electron scattering spectra into universal nuclear responses. Proton continuum spectra are analyzed for beam energies from 392 MeV to 19.2 GeV. Scaling is somewhat successful for proton scattering on light nuclei or at fixed angles, with differences from electron scattering that indicate modifications of proton scattering on nucleons within nuclei from scattering in free space. Comparisons to inclusive continuum meson scattering in the same y-scaling format are made, and a new hadronic scaling relation is demonstrated in light nuclei.

Coupling-in-the-continuum effects in Coulomb dissociation of halo nuclei

Higher-order effects in the Coulomb dissociation of halo nuclei are investigated by comparing the first-order perturbation theory to the numerical resolution of a time-dependent Schr\odinger equation. The calculations are performed for the breakup on a lead target of $^{11}\mathrm{Be}$ and $^{8}\mathrm{B}$. The populations of the different partial waves composing the $^{10}\mathrm{Be}$-neutron or $^{7}\mathrm{Be}$-proton continuum reveal that couplings in the continuum remain significant even at high impact parameters and high projectile-target relative velocities. Although the total breakup cross section is fairly well described by a first-order approximation, its partial-wave components reached by the first-order transitions are significantly depleted toward other partial waves after the closest approach. The information extracted by assuming the validity of the first-order approximation is affected by an energy distortion. Another distortion is caused by the presence of a resonance as exemplified by the $5/{2}^{+}$ resonance of $^{11}\mathrm{Be}$. Such effects may partly explain discrepancies between direct and indirect measurements of the astrophysical S factor of the $^{7}\mathrm{Be}(\text{p},\ensuremath{\gamma})^{8}\mathrm{B}$ reaction at stellar energies.

Measuring molecular wave functions using laser Coulomb explosion imaging with ultraviolet lasers

A high-frequency laser (λ<40 nm) can ionize the H2+ molecule via a one-photon transition to the electron–nuclei continua. From exact non-Born–Oppenheimer simulations of dynamics of H2+ in intense laser fields we show that it is possible to reconstruct the shape of the initial, stationary vibrational wave function |ψv(R)|2 by measuring kinetic energy spectra of either protons or electrons. The reconstruction procedure from proton spectra is based on a perturbative one-photon transition to the electron–proton continua and formally is similar to the previously discussed laser Coulomb explosion imaging technique. For sufficiently high explosion energy, proton spectra are directly proportional to |ψv(q2/EN)|2, where EN is the nuclear energy. Alternatively, one can measure kinetic energy of electrons, from which the proton energy can be deduced via the conservation of energy for a one-photon transition to the electron–proton continuum, and thus the shape of the first peak in the electron spectrum is also directly proportional to the initial vibrational probability distribution. We compare these two methods and also compare them with the previously proposed imaging methods based on Coulomb explosions induced by multiphoton transitions. We find that high-frequency lasers allow us to perform the imaging of a stationary wave function with longer and less intense pulses than pulses required in the previously discussed long-wavelength regime. Imaging based on electron spectra requires longer pulses, since the spread of photon energy in short pulses does not allow us to determine nuclear energy, in an accurate way, via conservation of energy. In general, the photo-electron spectra from H2+ are found to be very different from atomic spectra, the latter having much narrower peaks, while the former are greatly enlarged due to sharing the final energy with nuclei. The photo-electron and nuclear spectra show a very clear imprint of the nuclear wave function, e.g. if the molecule is prepared in the v-state that has v nodes, both spectra will also have v nodes.

Shell corrections of superheavy nuclei in self-consistent calculations

Shell corrections to the nuclear binding energy as a measure of shell effects in superheavy nuclei are studied within the self-consistent Skyrme-Hartree-Fock and Relativistic Mean-Field theories. Due to the presence of low-lying proton continuum resulting in a free particle gas, special attention is paid to the treatment of single-particle level density. To cure the pathological behavior of shell correction around the particle threshold, the method based on the Green's function approach has been adopted. It is demonstrated that for the vast majority of Skyrme interactions commonly employed in nuclear structure calculations, the strongest shell stabilization appears for Z=124, and 126, and for N=184. On the other hand, in the relativistic approaches the strongest spherical shell effect appears systematically for Z=120 and N=172. This difference has probably its roots in the spin-orbit potential. We have also shown that, in contrast to shell corrections which are fairly independent on the force, macroscopic energies extracted from self-consistent calculations strongly depend on the actual force parametrisation used. That is, the A and Z dependence of mass surface when extrapolating to unknown superheavy nuclei is prone to significant theoretical uncertainties.

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction \({\rm e}\;{\rm p} \rightarrow{\rm e}\;{\rm PS}\;{\rm X}\) where PS denotes a pseudoscalar meson \(\pi^0\), \(\eta\), \(\eta'\), or \(\eta_c\) and X either a proton or resonance or continuum state into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in pp and \( {\rm p} \bar{{\rm p}}\) forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering.

Measurements of (n, p) reactions at medium energy using a magnetic spectrometer

The technical aspects of measurements of (n, p) reactions on 1H and 2H at 58 MeV using a magnetic spectrometer are described. By using a veto and trigger counter system at the target position and a ray-trace type wire counter at the focal plane of the spectrometer, low background measurements were realized. The shape of the proton continuum spectrum at the n-n final-state interaction region from the 2H(n, p)2n reaction at 0° was explained by an impulse-approximation calculation taking the experimental finite energy resolution into account.

Tensor analyzing powers of the proton continuum spectra from deuteron breakup at 56 MeV

Cross sections and tensor analyzing powers, A xx and A yy of the continuum spectra in 58Ni and 90Zr( d , pX) reactions were measured at 56 MeV in the angular range between 6° and 30°. A prominent bump due to deuteron breakup was observed in the proton spectra. The peak energy of the bump is about one half of the incident deuteron energy. Tensor analyzing powers of magnitude 0.1–0.2 were observed at the high-energy side of the breakup spectra. The cross section and tensor analyzing powers were calculated based on a projectile breakup model using the S- and D-state momentum distributions of the deuteron derived from the Reid soft-core wave functions. In the calculation the tensor analyzing powers originate from the deuteron D-state alone. The experimental data at forward angles were roughly explained by the calculations except for the low-energy end of the spectra. The observed tensor analyzing powers at forward angles seem to reflect the effect of the deuteron D-state.

Cross sections for theLi6(He3, p)reaction at energies below 2 MeV

Absolute cross sections for the /sup 6/Li(/sup 3/He,p) reaction to the ground, 2.94-, 16.63-, and 16.92-MeV states in /sup 8/Be, as well as for the total underlying proton continuum, have been obtained for incident /sup 3/He energies between approx.0.5 and 1.85 MeV. Differential and total (integrated) reaction cross sections with accuracies of 10 --20 % are presented. Thermonuclear reaction-rate parameters are calculated. Interpretation of the observed proton and alpha particle continuous spectra is discussed.

Deuteron break-up in the fields of nuclei at 56 MeV

Abstract Proton continuum spectra induced by 56 MeV deuterons have been measured for 14 elements ranging from 12C to 209Bi. Each spectrum shows a prominent bump at forward angles due to the break-up of the incident deuteron. The peak energy of the bump is about half the deuteron energy for light elements and increases slightly for heavier elements. Projectile break-up calculations reproduce well the spectral shapes and the angular dependence of the bump spectra. The characteristics of the bump spectra are determined by the proton momentum distribution in the deuteron. The break-up cross section increases with target mass and is nearly proportional to ( A 1 3 + 0.8) 2 . The target mass dependence indicates that all partial waves smaller than the grazing angular momentum contribute to the deuteron break-up cross section at this energy. The total break-up cross section amounts to 24–35% of the deuteron total reaction cross section.