Quasifree Mechanism Research Articles

Quasifree Photoionization under the Reaction Microscope

We experimentally investigated the quasifree mechanism (QFM) in one-photon double ionization of He and H2 at 800 eV photon energy and circular polarization with a COLTRIMS reaction microscope. Our work provides new insight into this elusive photoionization mechanism that was predicted by Miron Amusia more than four decades ago. We found the distinct four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that has previously only been observed for He. Furthermore, we provide experimental evidence that the photon momentum is not imparted onto the center of mass in quasifree photoionization, which is in contrast to the situation in single ionization and in double ionization mediated by the shake-off and knock-out mechanisms. This finding is substantiated by numerical results obtained by solving the system’s full-dimensional time-dependent Schrödinger equation beyond the dipole approximation.

Collinear configuration of the helium atom and two-electron ions

Collinear configurations of the helium-like atomic systems, relevant, e.g., for the quasifree mechanism of the double photoionization of helium, are studied, parameterized by the single scalar parameter −1≤λ≤1 (“collinear parameter”) where λ=0 corresponds to the electron–nucleus (e −−n) coalescence and λ=1 corresponds to the electron–electron (e −−e) coalescence. In general, λ>0 corresponds to the n–e–e configuration, and λ<0 to the e–n–e configuration. Simple mathematical representations of the expectation values of the Dirac delta function relevant for the collinear configurations are derived and calculated from fully three-body dynamics without approximation for the two-electron atomic wave functions with nuclear charge 1≤Z≤5. Simple formulas for calculating the expectation values of the kinetic and potential energy operators in collinear configurations are derived. Unusual physical properties of the n-e-e collinear configurations found for certain ranges of λ are presented. The first few angular Fock coefficients for collinear configurations are derived as functions of λ. Highly accurate model wave functions describing the ground states of the two-electron atoms with a collinear arrangement of the particles are constructed. All results are illustrated in tables and figures.

Small recoil momenta double ionization of He and two-electron ions by high energy photons

We calculate various differential and double differential characteristics of ionization by a single photon for H−, He and for the two-electron ions with Z = 3, 4, 5 in the region of the so-called quasi-free mechanism (QFM) domination. We employ highly accurate wave functions at the electron-electron coalescence line where coordinates of both ionized electrons coincide. We trace the Z dependence of the double differential distributions. For all considered targets we discuss the dependence of the photoelectron energy distribution on the photon energy. Our calculation demonstrated the rapid decrease of QFM contribution with increase of the difference in energy of two outgoing electrons, and with decrease of the angle between two outgoing momenta. As a general feature, we observe the decrease of QFM contribution with nuclear charge growth.

Revealing the two-electron cusp in the ground states of He and H2 via quasifree double photoionization

We report on kinematically complete measurements and ab initio non-perturbative calculations of double ionization of He and H2 by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for H2 and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a new method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation.

Photon Momentum Transfer in Single-Photon Double Ionization of Helium.

We theoretically and experimentally investigate the photon momentum transfer in single-photon double ionization of helium at various large photon energies. We find that the forward shifts of the momenta along the light propagation of the two photoelectrons are roughly proportional to their fraction of the excess energy. The mean value of the forward momentum is about 8/5 of the electron energy divided by the speed of light. This holds for fast and slow electrons despite the fact that the energy sharing is highly asymmetric and the slow electron is known to be ejected by secondary processes of shake off and knockout rather than directly taking its energy from the photon. The biggest deviations from this rule are found for the region of equal energy sharing where the quasifree mechanism dominates double ionization.

Separating Dipole and Quadrupole Contributions to Single-Photon Double Ionization.

We report on a kinematically complete measurement of double ionization of helium by a single 1100eV circularly polarized photon. By exploiting dipole selection rules in the two-electron continuum state, we observed the angular emission pattern of electrons originating from a pure quadrupole transition. Our fully differential experimental data and companion abinitio nonperturbative theory show the separation of dipole and quadrupole contributions to photo-double-ionization and provide new insight into the nature of the quasifree mechanism.

Determination of the Polarization Observables $${{\varvec{C}}}_{{\varvec{x}}}$$, $${{\varvec{C}}}_{{\varvec{z}}}$$, and $${{\varvec{P}}}_{{\varvec{y}}}$$ for the Quasi-Free Mechanism in the Reaction $${\varvec{\overrightarrow{\gamma }}}{\varvec{d}}\varvec{\rightarrow } {\varvec{K}}^{\varvec{+}}{\varvec{\overrightarrow{\varLambda }}} {{\varvec{n}}}$$

The study of the excited-nucleon ( $$N^*$$ ) spectrum is fundamental for the quest to understand non-perturbative quantum chromodynamics. Measured kinematical evolutions of experimental observables, such as polarizations and unpolarized cross sections, for exclusive meson photoproduction off proton and neutron, are the foundation of theoretical and phenomenological analyses that identify the $$N^*$$ states excited in these reactions and determine their parameters. While free proton targets are readily available, experiments typically use bound neutrons, such as in the deuteron, to estimate observables for photoproduction off the free neutron. Even though exclusivity allows to select event samples dominated by quasi-free scattering (QF), it is not clear if the obtained observables are unbiased estimates of the observables for photoproduction off the free neutron. Studies assessing the possible sources of bias, such as the neutron binding and final-state interactions (FSI), are sparse and primarily focused on the unpolarized cross sections. The comparison between observables for production off the free and off the bound proton in high-statistics reactions, seems to be the natural way to study and to quantify these effects experimentally. Alternatively, one can map the evolution of the QF observables with target-neutron momentum and potentially extrapolate it to the “free”-neutron point of zero momentum. In this work, we determine the recoil polarization $$P_y$$ of the $$\varLambda $$ hyperon and the polarization transfers $$C_x$$ and $$C_z$$ from circularly-polarized photons to the $$\varLambda $$ for the quasi-free mechanism of the reaction $$\overrightarrow{\gamma }d\rightarrow K^+\overrightarrow{\varLambda } n$$ , i.e., for photoproduction off the bound proton. The data were taken with the CLAS detector at the Thomas Jefferson National accelerator facility (JLab) during the E06-103 (g13a) experiment and cover $$E_{\gamma }$$ between 0.9 and 2.6 GeV and $$\cos \theta _{KCM}$$ between $$-\,0.3$$ and 0.8. For several kinematic bins, the dependence of the polarizations on the target-proton momentum is studied. Our work can be used as a benchmark to develop analysis strategies to reduce potential bias due to FSI and the binding of the neutron in the deuteron in reported polarization observables for quasi-free meson photoproduction off the bound neutron.

The Treiman-Yang Criterion: validating the Trojan Horse Method by experimentally probing the reaction mechanism

Proper selection of the quasi-free (QF) break-up channel in a three-body reaction is a key aspect for the applicability of the Trojan Horse Method (THM). The Treiman-Yang (TY) Criterion is a model-independent experimental test for the dominance of the QF mechanism, and hence constitutes one of the strongest validity tests of the THM. An experiment was performed at LNS to apply the test to the d(10B, 7Be α)n reaction. Here, the criterion is described and some preliminary data from the experiment are shown.

Studies of Final-State Interactions via Helicity Asymmetries in Exclusive Pseudoscalar Meson Photoproduction off Deuteron

Exclusive meson photoproduction off deuteron is a great tool to study final-state interactions (FSI) between the hadrons produced in the scattering of the incident photon off one of the target nucleons and the spectator nucleon. FSI in the reaction \(\gamma d\rightarrow K^+\varLambda n\) allow to access the dynamics of \(\varLambda n\) elastic scattering and provide a method of studying the hyperon-nucleon (YN) interaction. Also, as deuteron is often used as a neutron target, FSI contribute to the reaction dynamics in addition to the quasi-free mechanism and their understanding is critical for the estimate of observables off the free neutron. Here we show preliminary results for beam-helicity asymmetries for the reactions \(\mathbf {\gamma } d\rightarrow K^+\varLambda n\) and \(\mathbf {\gamma } d\rightarrow p\pi ^+\pi ^- n\) and their evolution over the spectator-nucleon momentum. The data were obtained with the CEBAF Large Acceptance Spectrometer at Thomas Jefferson National Accelerator Facility (JLab). We show how to kinematically select an event sample dominated by a specific FSI mechanism. We also discuss a method to extract polarization observables for photoproduction off the free nucleon using the helicity asymmetries for reactions off the deuteron.

Preliminary Studies for Three Experiments at Treiman-Yang Criterion

Nuclear reactions with three bodies in their final state may proceed through different reaction mechanisms. The Feynman graph technique has been widely used to describe such reactions. However, it is very difficult in general to select the graphs that dominate in given process. The Treiman-Yang criterion is one of the most powerful experimental tests for verifying the pole approximation prediction when describing a quasi-free reaction mechanism. We propose the theoretical study of the H 2 (B 10 ,α Be 7 )ns, H 1 (B 11 ,α 1α2)αs, He 3 (Be 9 ,α 1α2)αs reactions at different energies. The preliminary study helps to check the existence of a QF channel by using the TY creterion. The Trojah Horse Method (THM) is a powerful indirect technique that allows to determine the astrophysical factor for rearrangement reactions. The THM involves obtaining the cross section of the binary process 0+T→1+2 at astrophysical energies by measuring the Trojan Horse (TH) reaction (the two-body to three-body process) 0+N→1+2+s in the quasifree (QF) kinematics mode, where the Trojan Horse particle, N = (s+T), which has a dominant cluster structure, is accelerated at energies above the Coulomb barrier. The advantage of the THM is that the extracted cross section of the binary subprocess does not contain the Coulomb barrier suppresion factor and electron screening does not play a role in the cross section. After penetrating the barrier, the TH-nucleus N undergoes breakup leaving particle T

Mechanisms for hadron continuum scattering from complex nuclei

The continuum spectra of hadrons scattered from complex nuclei are influenced by several features of the strong interaction, even at beam energies and momentum transfers where an incoherent quasifree mechanism is formally valid. These hadrons may suffer multiple scattering, they may interact with nucleons in correlated motion with other bound nucleons, and the spectra may become contaminated with events including pion production. In this work, nuclear spectra at energy losses less than those for free beam-nucleon scattering are compared as ratios to similar spectra for deuterium, an idea recently used for simpler electron scattering spectra, and as ratios to scattering from free nucleons, as the single-nucleon responses in scaling formats. A large array of data is organized to distinguish among the possibilities leading to hadron continuum spectra from complex nuclei, and the relative contributions of single-nucleon responses and other mechanisms are distinguished. It is concluded that for a range of three-momentum transfers near 500 MeV/c it is correlations among bound nucleons that generate the hadron spectra.

Quasifree mechanism in theLi6+Li6→3αreaction at low energy

The $^{6}\mathrm{Li}+^{6}\mathrm{Li}$ $\ensuremath{\rightarrow}3\ensuremath{\alpha}$ reaction was measured in a kinematically complete experiment at 3.1 and 2.5 MeV to investigate the presence of quasifree processes at sub-Coulomb energies. Results from the present experiment show a very clear evidence of quasifree mechanisms with an $\ensuremath{\alpha}$-particle spectator either in the target or in the projectile. To check the occurrence of quasifree processes, ``energy sharing'' and ``angular correlation'' analyses have been performed in the framework of the plane wave impulse approximation, both leading to consistent results. The huge peak in the angular correlation spectra, previously ascribed to the ``anomalous quasifree'' process, corresponds instead to the population of the 22.2-MeV $({2}^{+},\ensuremath{\Gamma}=800$ keV) state of $^{8}\mathrm{Be}$.

Recoil momenta distributions in the double photoionization

We calculate the distributions in recoil momenta for the high energy double photoionization of helium caused by quasifree mechanism. The distributions obtain local maxima at small values of the recoil momenta. This agrees with earlier predictions and recent experimental data. Angular correlations which reach the largest value for "back-to-back" configuration of photoelectrons are also obtained.

Trojan Horse Particle Invariance: An Extensive Study

In the last decades, the Trojan Horse method (THM) has played a crucial role for the measurement of several particle (both neutron and charged one) induced cross sections for reactions of astrophysical interest. To better understand its cornerstones and its applications to physical cases, many tests were performed to verify all its properties and the possible future perspectives. The Trojan Horse nucleus invariance proves the relatively simple approach allowed by the pole approximation and sheds light in the involved reaction mechanisms. Here we shortly review the complete work for the binary 2H(d,p)3H, 6Li(d,α)4He, 6Li(p,α)3He, 7Li(p,α)4He reactions, by using the quasi free reactions after break-ups of different nuclides. Results are compared assuming the 6Li and 3He break-up in the case of the d(d,p)t, 6Li(d,α)4He reactions and considering the 2H and 3He break-up for 6Li(p,α)3He, 7Li(p,α)4He reactions. These results, regardless of the Trojan Horse particle or the break-up scheme, confirms the applicability of the standard description of the THM and suggests the independence of binary indirect cross section on the chosen Trojan Horse nuclei for a whole spectra of different cases. This gives a strong basis for the understanding of the quasi-free mechanism which is the foundation on which the THM lies.

Effects of small recoil momenta in one-photon two-electron ionization

We calculate the distributions in recoil momenta and the energy distributions for the high-energy nonrelativistic double photoionization of helium caused by the quasifree mechanism of the process. The distributions obtain local maxima at small values of the recoil momenta. This is in agreement with the earlier predictions and with recently obtained experimental data. We obtained also the angular correlations, which reach the largest value in the ``back-to-back'' configuration of the photoelectrons. Our analysis is valid in all high-energy nonrelativistic region. Particular equations are true up to the photon energies making several kilo-electron-volts. We present numerical results for the photon energies in the region of 1 keV, employed in the recent experiments.

Quasi free mechanism in single photon double ionization of Helium

We have kinematically complete measured photo double ionization of Helium at Eγ=800 eV and observed recoiling ions with nearly 0 momentum that agree well with theoretical predictions of a new photo double ionization mechanism.

Back-to-back emission of the electrons in double photoionization of helium

We calculate the double differential distributions and distributions in recoil momenta for the high energy non-relativistic double photoionization of helium atom. We show that the results of recent experiments provide the pioneering experimental manifestation of the quasifree mechanism for the double photoionization which was predicted long ago in our papers. This mechanism provides a surplus in distribution over the recoil momenta at small values of the latter, corresponding to nearly “back-to-back” emission of the photoelectrons. Also, in agreement with previous analysis we demonstrate that this surplus is due to the quadrupole terms of the photon-electron interaction. We present the characteristic angular distribution for the back-to-back electron emission. The confirmation of the quasifree mechanism existence opens a new area for exciting experiments, which are expected to increase our understanding of the electron dynamics and of the bound states structure.

A new approach to select the quasifree mechanism in the Trojan horse method

The identification of the quasifree mechanism is very important in the application of the Trojan horse method (THM). We present here a new phenomenological approach for selecting quasifree events in Trojan horse experiments. This approach is based on the assumption that an intermediate break-up process exists. Under certain conditions, the break-up can be considered as quasifree and events can be selected from nonnegative real number solutions of energy and momentum conservation equations. We use experimental data of Wen Q G et al (2008 Phys. Rev. C 78 035805) to preliminarily test the validity of this approach in the THM.

Near-threshold photoproduction of ϕ mesons from deuterium

We report the first, kinematically-complete measurement of the differential cross section of ϕ-meson photoproduction from deuterium near the production threshold for a proton using the CLAS detector and a tagged-photon beam in Hall B at Jefferson Lab. The measurement was carried out by a triple coincidence detection of a proton, K+ and K− near the theoretical production threshold of 1.57 GeV. The extracted differential cross sections dσdt for the initial photon energy range of 1.65–1.75 GeV are consistent with predictions based on a quasifree mechanism. Our finding is different from recent LEPS results on ϕ-meson photoproduction from deuterium in a similar incident photon energy range, but in a different momentum transfer region.

Correlation and polarization characteristics of the 40Ca(p, dπ+)39K reaction

The 40Ca(p, dπ+)39K reaction was studied theoretically at a proton energy of 650 MeV within the distorted wave impulse approximation (DWIA) and under the assumption of a quasi-free mechanism of the elementary process pp → dπ+ of pion production. The peripheral nature of the reaction on the nucleus was shown. The influence of the effects of distortions in the initial and final states of the reaction on the differential cross section and other spin-dependent characteristics are discussed.