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.

 

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, Turawa, Poland, Sep. 2004; Acta 

   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.