► Scope Special atoms
and molecules are systems that keep some resemblance with common atoms and
molecules, as in their zero order Hamiltonian, but that contains particles
not common to the world of typical systems, say muons,
positrons, etc. The main difference from the typical systems lies in the mass
of the exotic particles involved. In fact, positrons are positively charged
particles with the same mass of electrons, and muons, that can be
positive or negative, are about 207 times heavier than electrons. The study
of special atoms and molecules is connected to various branches of physics
and chemistry and to the research of new sources of energy as well. The first
basic approximation in the study of typical molecules is the Born-Oppenheimer
approximation (BOA) that states that the motion of the electrons can be
assumed to develop in a clamped nuclei framework. Of course this can not be
assumed in the study of special atoms and molecules, and the development of
new theoretical models beyond the BOA is one of the aims of our laboratory.
This feature links our research to the study of finite nuclear mass effects
in molecules and isotopic systems (isotopomers), as
heavy water, D2O and HDO, so that it can be also classified as
Chemical-Physics beyond the BOA. |
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Being
of our particular interest, positrons (e+) are emitted by 22Na
sources and annihilate with their antimatter particles, the electrons. However
they can live long enough before annihilation to either form interesting stable
complexes, as the positronium hydride (HPs), or to present scattering or meta-stable states. We
developed a method in which the positron is considered as a light atomic
nucleus, so that the powerful methodology of quantum chemistry, with slight
modifications, can be used to generate potential energy surfaces for positron
motion.
We
started some collaboration with experimentalists as well, to build up a
positron beam and perform experiments of scattering of e+ by gaseous
molecules and to use data of positron lifetimes in various substances to build
a theoretical model for Ps formation in matter.
The group is leaded by prof. José R. Mohallem and have as members Drs. Flávia Rolim and Cristina P. Gonçalves and various students. For questions please contact rachid@fisica.ufmg.br.
► Recent representative talks and
publications ● [1] Molecular structure of water-like positron
complexes, presented at the 35 Polish Seminar on Positron Annihilation, Physica Polonica, to appear. ● [2] Adiabatic corrections to the energies of
50 typical medium size molecules containing H atoms, Chem. Phys. Lett. 406
(2005) 371. ● [3]A new algorithm to handle finite nuclear
mass effects in electronic calculations: the ISOTOPE program, J. Comput.
Chem. 25 (2004) 1736. ● [4] A molecular model for positron complexes:
long-range effects on 2γ pair-annihilation rates, J. Phys. B: At. Mol. Opt. Phys. 37 (2004) 1045. ● [5] Point group symmetries of the molecular orbitals of HD+ beyond the
Born-Oppenheimer approximation, Chem. Phys. Lett. 367
(2003) 533. ● [6] MO theory
of isotope symmetry breaking in HDO and HD, Chem. Phys. Lett. 380 (2003)
378. |
Pictorial representation of Ps2O and PsOH, from reference 1. |