RESEARCH:

          The improvement on the experimental techniques in the last years has allowed to observe the spectra of  molecules inmersed in helium droplets, both boson, ⁴He, and fermion, ³He, and it reveals some very important environment quantum properties (see Ref. 1). For instance, it can be highlighted the presence of superfluidity in a microscopic level for complexes formed by ⁴He, its absence for that of  ³He, and the gradual appearance on the mixed clusters through infrarred spectroscopy of OCS [2,3] on helium drops. In the case of complexes formed by bosonic helium the dopant molecule spectrum is similar to that of the isolated molecule, with well defined branches. It suggests that the molecule rotates freely without friction, i. e., the molecule is in a superefluid coating. In the fermionic case it is obtained an structurless band, similar to that shown by normal liquids. Surprisingly, as the number of ⁴He atoms are added to a complex formed by ³He ones the spectrum evolves gradually from the structurless to the defined one, recovering the branches mentioned before with a few tens of atoms.

       Diferencial and exclusive characteristics between bosonic and fermionic droplets appear as a consequence of the isotopic differences between the ³He and the ⁴He atoms. The pure ⁴He complexes are always bounded, regardless of the number of atoms, and they state superfluidity features at 2.2 K. On the contrary, the ³He complexes are only bounded with more than 30 atoms (aprox.), and they only present superfluidity below 3 mK.
 
          So far the superfluidity have been only explained in a partial way. However, some members of Mr. David López Durán researching group, to be precise,  Res. Prof. Pablo Villarreal, Mrs. María Pilar de Lara Castells, and Res. Prof. Gerardo Delgado Barrio, together with coworkers in Italy and USA, have considered, solved, and presented a model that throw light on the problem, taking into account from the dynamic to the spectroscopy of the system [4].

          The most important conclusion is that it has been found one of the causes that would contribute to the differences mentioned before depending on the nature of the helium atom, ³He or ⁴He.  The simulation of Raman spectra of a Br₂(X) molecule inmersed in helium clusters [5,6] reveals how  the existence of spin multiplets in the case of complexes containing ³He atoms produce an overlapping of lines that give rise to a fermionic structurless spectrum.  The gradual adittion of ⁴He atoms decreases the number of spin multiplets until the recovering of the isolated Br₂(X) Raman spectrum in the pure bosonic case, in excellent agreement with Refs. 2 and 3.

References:

1. J. P. Toennies, and A. F. Vilesov, Angewandte Chemie-International Ed. 43, 2622 (2004).
2. S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279, 2083 (1998).
3. J. P. Toennies, A. F. Vilesov, and K. B. Whiley, Phys. Today 54, 31 (2001).
4. M. P. de Lara-Castells, D. López-Durán, G. Delgado-Barrio, P. Villarreal, C. Di Paola, F. A. Gianturco, and J. Jellinek, Phys. Rev. A 71, 33203 (2004).
5. D. López-Durán, M. P. de Lara-Castells, G. Delgado-Barrio, P. Villarreal, C. Di Paola, F. A. Gianturco, and J. Jellinek, Phys. Rev. Lett. 93, 053401 (2004).
6. D. López-Durán, M. P. de Lara-Castells, G. Delgado-Barrio, P. Villarreal, C. Di Paola, F. A. Gianturco, and J. Jellinek, J. Chem. Phys. 121, 2975 (2004).