He-doped clusters


Using the Br2 molecule as dopant, we explain in Phys. Rev. Lett. 93 (2004) 053401 the origin of the enormous difference, experimentally found, in the spectra of molecules immersed in helium nanodroplets depending on the fermion or boson nature of the environment. This is claimed as manifestation of superfluidity of boson 4He at a microscopic scale. For instance, the IR spectrum of OCS in a fermion 3He nanodroplets shows a broad and unstructured profile which resembles the spectra of heavy molecules in liquids. On the contrary, in a boson ambient, the spectrum is, apart from some shifts and broadenings, pretty close to that of the isolated molecule. In fact, the molecule seems to freely rotate within the boson nanodroplet. Our model mimics electronic structure calculations where the He atoms act as electrons while the dopant plays the role of nuclei. Depending on the boson or fermion character of the environment, we use Hartree or Hartree-Fock methodologies (or a combining SCF procedure for dealing with mixtures). By the way, in pure boson environments, the ground level of the different sized complexes corresponds to a Bose condensate in which all the helium atoms occupy the same orbital (Phys. Rev. A 71 (2005) 033203).

In this framework, one looks for the distortion induced by the environment on the impurity .For dopant diatomic molecules, and since we dispose of wave-functions, it is simple to simulate IR or Raman spectra for polar or non-polar molecules, respectively. In the fermion case, the different spin multiplets become quasi-degenerated in energy, in such a way that even at extremely low temperatures all of them contribute to the corresponding branches of the spectrum. Moreover, some lines, which are forbidden by selection rules in the boson case, become allowed in a fermion scenario. As a result, and in agreement with the experiment, one finds very congested spectra in fermion environments. For the ICl molecule -which behaves as linear OCS- and as occurs when it is isolated, only P and R branches can appear in the IR spectra of its complexes with boson 4He (Phys. Rev. A 74 (2006) 053201), while additional Q branches are present in complexes with 3He (Phys. Scripta 76 (2007) C96).­

Presently we are working in several directions. We are assessing the applicability of the model as regards the main approaches involved, i e., the Born-Oppenheimer approximation for the vibrational motion of the dopant, and the separation of its rotation from the orbital angular momentum of the helium atoms (J. Chem. Phys. 128 (2008) 164313). Also, in order to get quantitative results, high level ab initio methodologies of FCI type are being implemented. Using a Jacobi-Davidson procedure of diagonalization, the first attempts considering small pure fermion environments and Br2 (J. Chem. Phys. 125 (2006) 221101) or Cl2 (J. Chem. Phys. 131 (2009) 194101) as dopants are very encouraging. Extensions to treat aggregates of different dopants with helium, as Cs2 in its triplet ground state (J. Phys. Chem. A 113 (2009) 14718) which presumably remains attached in the surface instead of embedded in the cluster, as well as to consider environments of molecular hydrogen are envisaged in the near future


Main people involved:

    M.P. de Lara-Castells, D. López-Durán, G. Delgado-Barrio,O.Roncero,R.Prosmiti, R. Pérez de Tudela

    F. A. Gianturco,C. Di Paola, J. Jellinek

J. C. Rayez, T. Stoecklin

A. O. Mitrushchenkov