Eur. Phys. J. D 63, 391-400 (2011)
https://doi.org/10.1140/epjd/e2011-10465-1

Regular Article

Cluster ion emission from LiF induced by MeV N^q+ projectiles and ²⁵²Cf fission fragments

¹ Centre de Recherche sur les Ions, les Matériaux et la Photonique CIMAP-GANIL (CEA-CNRS-ENSICAEN-UCBN), BP 5133, Boulevard Henri Becquerel, 14070 Caen Cedex 05, France
² Physics Department, Universidade Federal de Santa Catarina, Florianópolis, Brazil
³ Universidade Federal do Rio Grande do Sul, Instituto de Física, Avenida Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil
⁴ Physics Department, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, Gávea, 22453-900 Rio de Janeiro, Brazil

^a e-mail: rothard@ganil.fr

Received: 23 August 2010
Received in final form: 17 February 2011
Published online: 28 June 2011

Abstract

Ion cluster desorption yields from LiF were measured at PUC-Rio with  ≈0.1 MeV/u N^q+ (q = 2,4,5,6) ion beams by means of a time-of-fight (TOF) mass spectrometer. A ²⁵²Cf source mounted in the irradiation chamber allows immediate comparison of cluster emissions induced by  ≈65 MeV fission fragments (FF). Emission of (LiF)_nLi⁺ clusters are observed for both the N beams and the ²⁵²Cf fission fragments. The observed cluster size n varies from 1 to 6 for N^q+ projectiles and from 1 to  ≈40 for the ²⁵²Cf-FF. The size dependence of the Y(n) distributions suggests two cluster formation regimes: (i) recombination process in the outgoing gas phase after impact and (ii) emission of pre-formed clusters from the periphery of the impact site. The corresponding distribution of ejected negative cluster ions (LiF)_nF⁻ closely resembles that of the positive secondary (LiF)_nLi⁺ ions. The desorption yields of positive ions scale as Y(n) ~ q⁵. A calculation with the CASP code shows that this corresponds to a cubic scaling $\mathrm{~}{\mathit{S}}_{\mathit{e}}^{\mathrm{3}}$ with the electronic stopping power S_e, as predicted by collective shock wave models for sputtering and models involving multiple excitons (Frenkel pair sputtering). We discuss possible interpretations of the functional dependence of the evolution of the cluster emission yield Y(n) with cluster size n, fitted by a number of statistical distributions.