Experimental Momentum Distributions Research Articles

The vibrational proton potential in bulk liquid water and ice

We present an empirical flexible and polarizable water model which gives an improved description of the position, momentum, and dynamical (spectroscopic) distributions of H nuclei in water. We use path integral molecular dynamics techniques in order to obtain momentum and position distributions and an approximate solution to the Schrodinger equation to obtain the infrared (IR) spectrum. We show that when the calculated distributions are compared to experiment the existing empirical models tend to overestimate the stiffness of the H nuclei involved in H bonds. Also, these models vastly underestimate the enormous increase in the integrated IR intensity observed in the bulk over the gas-phase value. We demonstrate that the over-rigidity of the OH stretch and the underestimation of intensity are connected to the failure of existing models to reproduce the correct monomer polarizability surface. A new model, TTM4-F, is parametrized against electronic structure results in order to better reproduce the polarizability surface. It is found that TTM4-F gives a superior description of the observed spectroscopy, showing both the correct redshift and a much improved intensity. TTM4-F also has a somewhat improved dielectric constant and OH distribution function. It also gives an improved match to the experimental momentum distribution, although some discrepancies remain.

High resolution electron momentum spectroscopy of dichlorodifluoromethane: Unambiguous assignments of outer valence molecular orbitals

On account of controversial orbital assignment that appeared in previous works, [J. Chem. Phys. 120, 7933 (2004), and references therein] high resolution electron momentum spectroscopy (EMS) measurements on dichlorodifluoromethane has been carried out using a newly developed high resolution energy-momentum dispersive multichannel spectrometer employing asymmetric noncoplanar geometry at an impact energy of 2500 eV plus binding energy. Four resolved structures and two shoulders were obviously observed in high resolution binding energy spectrum in energy range covering eight outermost valence orbitals, whereas only two broad lobes were resolved in previous EMS studies [J. Chem. Phys. 120, 7933 (2004); Chin. Phys. 14, 2467 (2005)]. The ordering of these orbitals was reassigned unambiguously by simple comparison of experimental momentum distributions with theoretical ones.

One-neutron knockout in the vicinity of theN=32sub-shell closure:Be9(Cr57,Cr56+γ)X

The one-neutron knockout reaction $^{9}\mathrm{Be}$($^{57}\mathrm{Cr}$,$^{56}\mathrm{Cr}$+\ensuremath{\gamma})X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in $^{56}\mathrm{Cr}$ were partially untangled through the detection of the characteristic \ensuremath{\gamma}-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full $\mathit{fp}$ shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

Study of the molecular structure, ionization spectrum, and electronic wave function of 1,3-butadiene using electron momentum spectroscopy and benchmark Dyson orbital theories

The scope of the present work is to reconcile electron momentum spectroscopy with elementary thermodynamics, and refute conclusions drawn by Saha et al. in J. Chem. Phys. 123, 124315 (2005) regarding fingerprints of the gauche conformational isomer of 1,3-butadiene in electron momentum distributions that were experimentally inferred from gas phase (e,2e) measurements on this compound [M. J. Brunger et al., J. Chem. Phys. 108, 1859 (1998)]. Our analysis is based on thorough calculations of one-electron and shake-up ionization spectra employing one-particle Green's function theory along with the benchmark third-order algebraic diagrammatic construction [ADC(3)] scheme. Accurate spherically averaged electron momentum distributions are correspondingly computed from the related Dyson orbitals. The ionization spectra and Dyson orbital momentum distributions that were computed for the trans-conformer of 1,3-butadiene alone are amply sufficient to quantitatively unravel the shape of all available experimental (e,2e) electron momentum distributions. A comparison of theoretical ADC(3) spectra for the s-trans and gauche energy minima with inner- and outer-valence high-resolution photoelectron measurements employing a synchrotron radiation beam [D. M. P. Holland et al., J. Phys. B 29, 3091 (1996)] demonstrates that the gauche structure is incompatible with ionization experiments in high-vacuum conditions and at standard temperatures. On the other hand, outer-valence Green's function calculations on the s-trans energy minimum form and approaching basis set completeness provide highly quantitative insights, within approximately 0.2 eV accuracy, into the available experimental one-electron ionization energies. At last, analysis of the angular dependence of relative (e,2e) ionization intensities nicely confirms the presence of one rather intense pi(-2) pi(*+1) satellite at approximately 13.1 eV in the ionization spectrum of the s-trans conformer.

Experimental and theoretical investigations into the electronic structure of cyclohexene by electron momentum spectroscopy

The orbital electron momentum distributions and binding energy spectrum of the valence shell of cyclohexene (C6H10) were investigated by a binary (e, 2e) electron momentum spectrometer at an impact energy of 1200 eV plus the binding energy. The experimental momentum distributions are compared with the theoretical momentum distributions obtained by using Hartree–Fock and density functional theory methods. The agreements between theory and experiment for the shape and intensity are generally good. Some discrepancies between experimental data and theoretical calculations of the momentum distributions could be due to the possible distorted-wave effects and the conformation variety of cyclohexene. The pole strengths of the main ionization peaks from the orbitals in the inner valence region are estimated. © 2005 Elsevier B.V. All rights reserved.

One-neutron knockout of 23O

Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope 23O. The 22O fragments produced in one-nucleon removal from 23O at 938 MeV/nucleon in a carbon target were detected in coincidence with de-exciting γ rays, allowing to discern between 22O ground-state and excited-states contributions. From the comparison of exclusive experimental momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated in an Eikonal model for the knockout process, and spin and parity assignment of Iπ = 1/2+ was deduced for the 23O ground state. This result solved the existent experimental discrepancy.

An investigation of the outer valance orbital electron density distribution of ethylene by electron momentum spectroscopy at 800 eV

Electron density distributions in momentum space of the outer valence orbitals of ethylene are measured by electron momentum spectroscopy (EMS) in a non-coplanar symmetric geometry. The impact energy was 800 eV plus binding energy (8–22 eV) and energy resolution of the EMS spectrometer was 0.95 eV. The measured experimental momentum distributions of the outer valence orbitals are compared with Hartree-Fock and Density Functional Theory (DFT) calculations. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree-Fock and DFT calculations when large and diffuse basis sets are used, except for 1b3g orbital. The experimental momentum profile of 1b3g orbital clearly show remarkable “turn up” in the low momentum region comparing with theoretical calculation and experimental results at impact energy of 1200 eV.

Ambiguities in statistical calculations of nuclear fragmentation

The concept of freeze out volume used in many statistical approaches for disassembly of hot nuclei leads to ambiguities. The fragmentation pattern and the momentum distribution (temperature) of the emanated fragments are determined by the phase space at the freeze-out volume where the interaction among the fragments is supposedly frozen out. However, to get coherence with the experimental momentum distribution of the charged particles, one introduces Coulomb acceleration beyond this freeze-out. To be consistent, we investigate the effect of the attractive nuclear force beyond this volume and find that the possible recombination of the fragments alters the physical observables significantly casting doubt on the consistency of the statistical model.

An investigation of valence shell orbital momentum profiles of difluoromethane by binary (e,2e) spectroscopy

The electron binding energy spectra and momentum profiles of the valence orbitals of difluoromethane, also known as HFC32 (HFC-hydrofluorocarbon) (CH(2)F(2)), have been studied by using a high resolution (e,2e) electron momentum spectrometer, at an impact energy of 1200 eV plus the binding energy, and by using symmetric noncoplanar kinematics. The experimental momentum profiles of the outer valence orbitals and 4a(1) inner valence orbital are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods with various basis sets. In general, the shapes of the experimental momentum distributions are well described by both the Hartree-Fock and DFT calculations when large and diffuse basis sets are used. However, the result also shows that it is hard to choose the different calculations for some orbitals, including the methods and the size of the basis sets employed. The pole strength of the ionization peak from the 4a(1) inner valence orbital is estimated.

Investigation of the highest occupied molecular orbital of cyclohexene by electron momentum spectroscopy

We report here the first measurements of the frontier highest occupied molecular orbital (HOMO) momentum profile and the complete valence shell binding energy spectra of cyclohexene by a binary ( e , 2 e ) spectrometer using the non-coplanar symmetric kinematics. The experimental orbital electron momentum distribution is well described by the theoretical calculations.

The outer valance orbital electron densities of cyclopentane by binary (e,2e) spectroscopy.

The binding energy spectra and electron distributions in momentum space of the valence orbitals of cyclopentane (C(5)H(10)) are studied by Electron Momentum Spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 1.2 eV. The experimental momentum profiles of the outer valence orbitals are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree-Fock and DFT calculations when the large and diffuse basis sets are used.

POSSIBLE EXOTIC STRUCTURE IN LIGHT PROTON-RICH NUCLEI

Radioactive ion beams were produced through the projectile fragmentation induced by 69MeV/nucleon 36 Ar primary beam on a 9 Be target. Measurements of reaction cross section (σR) for some proton-rich nuclei on carbon target at intermediate energies around 30MeV/nucleon have been performed on RIBLL of HIRFL by transmission method and transmission plus transportation method. The experimental σR values for 23Al and 27P are abnormally large compared with their neighboring nuclei and that of 17F has an enhancement compared with the neighboring isotopes. It suggests anomalously large matter root-mean-square radii and proton halo structure in 23 Al and 27 P . The experimental σR values and momentum distribution of fragmentation reaction product for 31 Cl , 32 Cl , 33 Cl , 28 S , 29 S were performed also on RIBLL by transmission and transportation method. These data are in analyzing. The calculation of relativistic density-dependent Hartree shows that the nuclei 23 Al , 27 P , 31 Cl may have proton-halo structure and 17F, 32Cl may have proton-skin structure. New experiment on the search for new nuclides 25 P and 26 S in proton drip line is also described. The significance of these measurements and possible proton halo and skin for light proton-rich nuclei has been discussed.

Using Diffusion Monte Carlo to Evaluate the Initial Conditions for Classical Studies of the Photodissociation Dynamics of HCl Dimer

An approach for evaluating the initial conditions for classical studies of the photodissociation dynamics of molecular clusters is described. This approach is based on an approximate separation of the momentum and coordinate space probability distributions, evaluated using a wave function that is obtained from a Diffusion Monte Carlo simulation. Using the initial conditions that are generated by this approach, the photodissociation dynamics of HCl dimer is studied. Excellent agreement between the calculated and experimental angular momentum and vibrational energy distributions for the remaining HCl molecule and the kinetic energy distribution of the dissociated hydrogen atom are obtained. The kinetic energy distribution for the chlorine atom and angular distributions of the remaining hydrogen and chlorine atoms are also obtained and dynamical information contained in these distributions is discussed.

Orbital electron densities of ethane: Comparison of electron momentum spectroscopy measurements with near Hartree–Fock limit and density functional theory calculations

Electron density distributions in momentum space of the valence orbitals of ethane (C2H6) are measured by electron momentum spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 0.95 eV. The measured experimental momentum distributions of the valence orbitals are compared with Hartree–Fock and density functional theory (DFT) calculations. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree–Fock and DFT calculations when large and diffuse basis sets are used. A strong “turn up” of the experimental cross section is observed for the HOMO 1eg orbital in the low momentum region, compared with the theoretical calculations. The pole strengths for the main ionization peaks in the inner-valence region are estimated.

Coulomb and nuclear breakup effects in the single neutron removal reaction197Au(17C,16Cγ)X

We analyze the recently obtained new data on the partial cross sections and parallel momentum distributions for transitions to ground as well as excited states of the 16C core, in the one-neutron removal reaction 197Au(17C,16C gamma)X at the beam energy of 61 MeV/nucleon. The Coulomb and nuclear breakup components of the one-neutron removal cross sections have been calculated within a finite range distorted wave Born approximation theory and an eikonal model, respectively. The nuclear contributions dominate the partial cross sections for the core excited states. By adding the nuclear and Coulomb cross sections together, a reasonable agreement is obtained with the data for these states. The shapes of the experimental parallel momentum distributions of the core states are described well by the theory.

Single-nucleon knockout reactions at fragmentation beam energies

Single-nucleon knockout reactions show considerable promise as a spectroscopic tool for mapping the migration of the dominant single-particle strength in rare isotopes, produced as fragmentation beams with energies > 40 MeV/nucleon. In this paper the theoretical assumptions which underpin the eikonal theory treatment of these reactions are reviewed. Certain sensitivities to input parameters are discussed, as are the possible importance of classes of higher order corrections. The eikonal model prediction for the elastic breakup component of the nucleon removal cross sections is also compared with that from fully-quantum mechanical calculations using the discretised continuum coupled channels method. An asymmetry, manifest in the experimental longitudinal momentum distributions of the ground state residues in weakly-bound-nucleon removal, is well reproduced. 1. INTRODUCTION An ability to map the evolution of the positions and order of nuclear single particle states in rare nuclei of high isospin will be an important component in developing our understanding and the reliability of structure models in these regions. The low intensities of secondary beams of these species, combined with the small number of excited states available in lighter systems, mean that conventional (but in inverse kinematics) lightion single-nucleon transfer reaction and gamma spectroscopic methods are non-trivial. Nucleon-knockout reactions appear to offer an alternative and complementary technique which is particularly well adapted for rare isotope beams produced using fragmentation. From a reaction theory point of view, it has been extremely fortuitous that many of the first generation facilities for the production of rare unstable nuclei have been based on nuclear fragmentation. The high secondary beam energies, and fast collisions with a secondary target, have allowed a rapid development and application of reaction methods, based on the sudden/adiabatic and/or eikonal-like approximations [1–8]. These lead to considerable simplifications and also practical and accurate calculation schemes which have relatively simple and physically transparent input parameters. This has allowed new experimental results to gain feedback rapidly from theory, and vice versa. Having said this, very little of the extensive literature to date has discussed possible schemes to obtain detailed spectroscopic information from data. To a large extent,

Investigation of orbital momentum profiles of methylpropane (isobutane) by binary (e,2e) spectroscopy

Momentum profiles of the valence orbitals of methylpropane, also known as isobutane (CH3CH(CH3)CH3), have been studied by using a high resolution binary (e,2e) electron momentum spectrometer (EMS), at an impact energy of 1200 eV plus the binding energy, and using symmetric noncoplanar kinematics. The coincidence energy resolution of the EMS spectrometer is 0.95 eV full width at half-maximum. The experimental momentum profiles of the valence orbitals are compared with the theoretical momentum distributions calculated using Hartree–Fock (HF) and density functional theory (DFT) methods with the two basis sets of 6-31G and 6-311++G**. The B3LYP functionals are used for the DFT calculations. In general, the experimental momentum distributions are well described by the HF and DFT calculations. The pole strengths of the main ionization peaks from the orbitals in the inner valence are estimated.

An investigation of the frontier orbital electron density of the antibacterial agent urotropine by electron momentum spectroscopy

The outermost valence (HOMO) orbital electron density distribution for the antibacterial agent urotropine has been studied by electron momentum spectroscopy. The experimental results are compared with quantum chemical calculations within the plane-wave impulse approximation and the target Hartree–Fock or target Kohn–Sham approximations. For the HOMO of urotropine density functional theory calculations with the B3PW91 and B3LYP functionals and 6-311++G** and AUG-CC-PVTZ basis sets provide quite good quantitative agreement with the experimental results, while Hartree–Fock calculations fail to adequately describe the experimental momentum distributions. This indicates the importance of taking into account electron correlation for accurate predictions of the frontier (HOMO) orbital electron density and thus the reactive properties of this molecule.

Valence electron momentum spectroscopy of n-butane

The valence electronic structure and momentum-space electron density distributions of n-butane have been studied by means of high-resolution (e,2e) electron momentum spectroscopy based on noncoplanar symmetric kinematics. Ionization spectra for the range of binding energies 6 to 32 eV and momenta described by azimuthal angles φ=0°, 2°, 4°, 6°, 8°, and 10° have been recorded and compared to the results of one-particle Green’s function calculations, performed using the third-order algebraic–diagrammatic construction [ADC(3)] approximation and series of basis sets of improving quality. Experimental electron momentum profiles have been determined from a set of 11 measurements and compared to theoretical results. It has been shown that despite the complex structure of the spectral bands and the conformational versatility of n-butane, the experimental electron momentum distributions are accurately described by the momentum-space form of orbital densities obtained from Becke three-parameter Lee–Yang–Parr (B3LYP) density functional calculations. Significant broadening of the spectral lines and the s-type angular dependence of their intensities above 24 eV have been explained by the breakdown of the one-electron picture of ionization for the 3ag molecular orbital.

Nuclear Structure Studies with the7Li(e,e′p)Reaction

Experimental momentum distributions for the transitions to the ground state and first excited state of {sup 6}He have been measured via the reaction {sup 7}Li( e,thinspe{prime}p){sup 6}He . They are compared to theoretical distributions calculated with variational Monte Carlo (VMC) wave functions which include strong state-dependent correlations in both {sup 7}Li and {sup 6}He . These VMC calculations provide a parameter-free prediction that reproduces the measured data, including its normalization. The deduced spectroscopic factor for the two transitions is 0.58{plus_minus}0.05 , in perfect agreement with the VMC value of 0.60. This is the first successful comparison of experiment and {ital ab initio} theory for spectroscopic factors in p -shell nuclei. {copyright} {ital 1999} {ital The American Physical Society}