Antiprotonic helium atom

In the last method the antiproton forms an exotic metastable antiprotonic helium atom, which then reacts with deliberately introduced positrons (or positronium atoms). 

Ito, Y., Widmann, E. and Yamazaki, T. (1993). Possible formation of antihydrogen atoms from metastable antiprotonic helium atoms and positrons/positroniums. Hyperfine Interactions 76 163-173. 

Kartavstev, O.I. (1996). Variational calculations of antiprotonic helium atoms. Russian J. Nucl. Phys. 59 1541—1550. 

Morita, N., Kumakura, M., Yamazaki, T., Widmann, E., Masuda, H., Sugai, I., Hayano, R.S., Maas, F.E., Torii, H.A., Hartmann, F.J., Daniel, H., von Egidy, T., Ketzer, B., Muller, W., Schmidt, W., Horvath, D. and Eades, J. (1994). First observation of laser-induced resonant annihilation in metastable antiprotonic helium atoms. Phys. Rev. Lett. 72 1180-1183. 

Yamazaki, T. (1992). A possible way to promote antihydrogen formation via metastable antiprotonic helium atoms. Z. Phys. A 341 223-225. 

Formation When an p slows down in He, its kinetic energy eventually falls below the He ionization threshold (Jo = 24.6 eV), at which point it replaces one of the electrons in a He atom to form pHe+. The antiprotonic helium atom thus formed with an initial kinetic energy around 5 eV reaches thermal equilibrium within nanosecond without suffering destruction. 

The p and He2+ are thus regarded as two atomic centers in a diatomic molecule. Because of the dual character as an exotic atom and an exotic molecule Antiprotonic Helium is often called antiprotonic helium atom-molecule, or for short, atomcule. Since the Is electron motion, coupled to a large-(n, l) p orbital, is faster by a factor of 40 than the p motion, the three-body system pHe+ is solved by using the Born-Oppenheimer approximation, as fully discussed by Shimamura [6]. 

Abstract. The antiprotonic helium,pe He2+ (= pHe+), is a peculiar metastable atom, interfacing between matter and antimatter. A series of metastable states axe composed of the He nucleus, one electron in the ground Is configuration and one antiproton orbiting with large quantum numbers (n, l), where n l e 38. They possess... 

Primordial exotic atom The metastable states are located in the primordial zone (n no = /M /me), where the exotic particle and the atomic electron coexist in the same spatial region. With the exception of antiprotonic helium, the primordial zone of exotic atoms has never been identified and remains an untouched object of investigation. 

Unique interface between matter and antimatter Whereas particles and antiparticles cannot coexist stably, Antiprotonic Helium is an exceptional case, where an intruder antiparticle (p) coexists with the normal matter (helium medium) for microseconds. Here, the property of the orbiting p (charge, mass, magnetic moment and other QED characteristics) can be probed. It is an interesting irony that the property of the proton cannot always be studied so precisely, because there is no atomic system in which a proton is orbiting. 

Fig. 2. Atomic and molecular views of Antiprotonic Helium. The large (n, l) states in the atomic yrast region in the atomic model axe also assigned as the molecular states of corresponding rotational and vibrational quantum numbers (J,v) = (l,n — l — 1) in the one-dimensional potential for each J. The radiative transitions with Av = 0, as shown by arrows, are favoured because of the maximum overlapping of the radial densities. In this sense, the atomcule system has a dual character by itself...

The metastablity of antiprotonic helium is known to be affected when foreign atoms and molecules are added to the helium media, as revealed from delayed annihilation time spectra (DATS) in the early stage [2,24,25], However, DATS alone is a macroscopic quantity in which all the microscopic informations cannot be differentiated. Laser resonance techniques have made it possible to investigate microscopically the (n, /[-dependent lifetime shortening effects on the surrounding physico-chemical conditions of antiprotonic helium. 

Bakalov et al. treated the trajectories of the helium atom in collision with pHe+ in a semiclassical way, and calculated the pressure shifts and broadening. They obtained numerical values for a numer of transitions, as presented in Table 2. For the precisely known transitions (39,35) —> (38,34) and (37,34) —> (36,33) their theoretical values with realistic collision trajectories (not the linear approximation) turned out to be in excellent agreement with the experimental values. The theoretical treatment of Bakalov et al. was the first quantum chemistry type calculation on the interaction of antiprotonic helium with other atoms and molecules. 

Exotic ground-state hydrogen atom Antiprotonic Helium is an exotic... 

Some possible mechanisms for the quenching of pHe+ states were discussed in [29]. First, it is to be noted that the antiprotonic helium resembles a hydrogenlike atom from the physico-chemical point of view, since the pHe+ system has only one electron. The proton in this system is a high-lying state [pHe +]( q with a net charge - -1, but an effective charge around 1.6, depending on The other view of antiprotonic helium is that it is a kind of diatomic molecule with the two centers p and He +. One of the plausible processes is exotic molecule formation ... 

The upcoming Antiproton Decelerator (AD) [1] at CERN allows the formation and precision spectroscopy of antiprotonic atoms. Among the three approved experiments, the ASACUSA collaboration [2] will as part of its program continue experiments with antiprotonic helium that were previously performed by the PS205 collaboration [3] at the now closed Low Energy Antiproton Ring (LEAR) of CERN. Antiprotonic helium consisting of an alpha particle, an antiproton, and an electron (He++ —p — = pHe+), was found to have lifetimes in the... 

The development of a full angular momentum, three dimensional, smooth exterior complex dilated, finite element method for computing bound and resonant states in a wide class of quantum systems is described. Applications to the antiprotonic helium system, doubly excited states in the helium atom and to a model of a molecular van der Waals complex are discussed. 2001 by Academic Press. 

There are several reasons that makes it interesting to study three-body atomic and molecular systems. The recent experimental and theoretical studies on antiprotonic helium[3,4] is an example of an exotic atomic system which was studied in order to find possible differences between antimatter and matter. 

The antiprotonic helium system was used as a model when developing our nonzero angular momentum 3D finite element method. This is an example of a system for which the wave function cannot exactly be decomposed into an angular and a radial part. Besides the helium like atoms it is the experimentally most accurately known three-body system. 

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