Kinematically complete experiment

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In accelerator physics, a kinematically complete experiment is an experiment in which all kinematic parameters of all collision products are determined. If the final state of the collision involves n particles 3n momentum components (3 Cartesian coordinates for each particle) need to be determined. However, these components are linked to each other by momentum conservation in each direction (3 equations) and energy conservation (1 equation) so that only 3n-4 components are linearly independent. Therefore, the measurement of 3n-4 momentum components constitutes a kinematically complete experiment.

If the final state involves only two particles (e.g. in the Rutherford experiment on elastic scattering) then only one particle needs to be detected. However, for processes leading to three collision products, like e.g. single ionization of the target atom, then two particles need to be momentum-analyzed (for one of them it is sufficient to measure two momentum components) and measured in coincidence. Any pair of the three final state particles (i.e. the scattered projectile, the ejected electron, and the recoiling target ion) can be detected. The first kinematically complete experiment on single ionization was performed for electron impact. [1] There, the scattered projectile electron and the ejected electron were momentum-analyzed. For ion impact, such an experiment is much more challenging because of the much larger projectile mass. As a result, the projectile scattering as well as the projectile energy loss relative to the initial energy are by many orders of magnitude smaller than for electron impact and are not measurable with standard techniques for fast heavy ions. Furthermore, only with the advent of cold target recoil-ion momentum spectroscopy (COLTRIMS) [2] could the recoil ions be measured with sufficient momentum resolution. The first kinematically complete experiment on single ionization by ion impact was performed by momentum analyzing the recoil ions and the ejected electrons. [3] For proton impact at much smaller energy kinematically complete experiments were also performed by momentum-analyzing the scattered projectiles and the recoil ions. [4] These studies play an important role in the context of the few-body problem (see the article on few-body systems).

Other processes involving more than two final state particles for which kinematically complete experiments were performed include double ionization of the target by electron impact, [5] transfer-ionization (i.e. one target electron is ejected to the continuum while a second electron is captured by the projectile) by ion impact [6] and dissociative capture in p + H2 collisions, [7] where the capture of an electron to the projectile leads to a fragmentation of the target molecule. Studies on double ionization and transfer-ionization revealed the important role of electron-electron correlation effects in processes involving multiple electrons. In dissociative capture pronounced quantum-mechanical interference was observed, from which detailed information about the phase angle, which in turn provides sensitive information on the few-body dynamics, was obtained.

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<span class="mw-page-title-main">Ionization</span> Process by which atoms or molecules acquire charge by gaining or losing electrons

Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.

<span class="mw-page-title-main">Rydberg atom</span> Excited atomic quantum state with high principal quantum number (n)

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<span class="mw-page-title-main">Two-photon physics</span> Branch of particle physics concerning interactions between two photons

Two-photon physics, also called gamma–gamma physics, is a branch of particle physics that describes the interactions between two photons. Normally, beams of light pass through each other unperturbed. Inside an optical material, and if the intensity of the beams is high enough, the beams may affect each other through a variety of non-linear effects. In pure vacuum, some weak scattering of light by light exists as well. Also, above some threshold of this center-of-mass energy of the system of the two photons, matter can be created.

A shape resonance is a metastable state in which an electron is trapped due to the shape of a potential barrier. Altunata describes a state as being a shape resonance if, "the internal state of the system remains unchanged upon disintegration of the quasi-bound level." A more general discussion of resonances and their taxonomies in molecular system can be found in the review article by Schulz,; for the discovery of the Fano resonance line-shape and for the Majorana pioneering work in this field by Antonio Bianconi; and for a mathematical review by Combes et al.

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<span class="mw-page-title-main">David Ceperley</span>

David Matthew Ceperley is a theoretical physicist in the physics department at the University of Illinois Urbana-Champaign or UIUC. He is a world expert in the area of Quantum Monte Carlo computations, a method of calculation that is generally recognised to provide accurate quantitative results for many-body problems described by quantum mechanics.

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<span class="mw-page-title-main">Carlos Lousto</span>

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<span class="mw-page-title-main">Louis F. DiMauro</span>

Louis Franklin DiMauro is an American atomic physicist, the Edward and Sylvia Hagenlocker Professor In the Department of Physics at The Ohio State University, Columbus, Ohio, USA. His interests are atomic, molecular and optical physics. He has been elected a Fellow of the American Association for the Advancement of Science, American Physical Society and Optical Society.

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