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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) n Li+ 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) n F− closely resembles that of the positive secondary (LiF) n Li+ ions. The desorption yields of positive ions scale as Y(n) ∼ q 5. A calculation with the CASP code shows that this corresponds to a cubic scaling ∼S 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.

Experimental results on fast ion collision with icy surfaces having astrophysical interest are presented. 252Cf fission fragments projectiles were used to induce ejection of ionized material from H2O, CO2, CO, NH3, N2, O2 and Ar ices; the secondary ions were identified by time-of-flight mass spectrometry. It is observed that all the bombarded frozen gas targets emit cluster ions which have the structure XnR±, where X is the neutral ice molecule and R± is either an atomic or a molecular ion. The shape of the positive or negative ion mass spectra is characterized by a decreasing yield as the emitted ion mass increases and is generally described by the sum of two exponential functions. The positive ion water ice spectrum is dominated by the series (H2O)nH3O+ and the negative ion spectrum by the series (H2O)nOH− and (H2O)nO−. The positive ion CO2 ice spectrum is characterized by R+ = C+, O+, CO+, O2+ or CO2+ and the negative one by R− = CO3−. The dominant series for ammonia ice correspond to R+ = NH4+ and to R− = NH2−. The oxygen series are better described by (O3)nOm+ secondary ions where m = 1, 2 or 3. Two positive ion series exist for N2 ice: (N2)nN2+ and (N2)nN+. For argon positive secondary ions, only the (Ar)nAr+ series was observed. Most of the detected molecular ions were formed by one-step reactions. Ice temperature was varied from ∼20 K to complete sublimation.

Electronic sputtering of lithium fluoride by swift heavy ions was studied as a function of electronic energy loss (dE/dx) e . The single crystal targets were irradiated with swift heavy ion beams (166Z P 692; 46E P /M P (MeV/u) 611). This allowed varying the deposited energy by a factor of 20 (1.86dE/dx (keV/nm) 632). The sputtered secondary ions were measured from well controlled LiF targets without surface contaminations, by means of the time-of-flight technique (TOF-SIMS). The mass spectrum reveals an important contribution of clusters (over single ions), which increases with (dE/dx) e . Another observation for the strongest perturbation at high dE/dx (>8 keV/nm) is that the secondary ion yields saturate: Y(dE/dx) = constant. In contrast, at lower dE/dx (