Theoretical and experimental study of negative LiF clusters produced by fast ion impact on a polycrystalline 7LiF target.

The emission of negative cluster ions produced by the impact of approximately 60 MeV (252)Cf fission fragments on a (7)LiF polycrystalline target is analyzed. The negative ion mass spectrum is dominated by the ((7)LiF)(n)F(-) series, n = 1 to approximately 30. The desorption yield distribution of the ((7)LiF)(n)F(-) members has a maximum at n = 2 and then decreases as the sum of two exponentials whose decay parameters are k(Fast) = 0.9 and k(Slow) = 0.08. These k values are the same as those observed for the positive series and close to others obtained for condensed gas targets. Relative cluster ion stabilities, deduced from the experimental ion abundances for the (LiF)(n)F(-) series, are proposed to be correlated with theoretical structures according to their internal energy by using the deviation plot (D-plot) methodology. A pool of candidate cluster structures was generated using a genetic algorithm and further analyzed and optimized using density functional theory (DFT) with the hybrid functional B3LYP (DFT/B3LYP) and Moller-Plesset perturbation theory (MP2). For the small clusters (n = 1 to 2), the most stable structures are found to be linear, whereas the larger clusters (n = 4 to 6) present cubic or polyhedral structures. Fragmentation energies, ionization potentials, and relative stabilities are reported for the most abundant families of the (LiF)(n)F(-) and (LiF)(n)(-) series.

Characterization of (NH3)(n=1-6)NH+ clusters produced by 252Cf fragments impact onto a NH3 condensed target.

This paper reports the first characterization of the (NH(3))(n)NH+ cluster series produced by a 252Cf fission fragments (FF) impact onto a NH(3) ice target. The (NH(3))(n=1-6)NH+ members of this series have been analyzed theoretically and experimentally. Their ion desorption yields show an exponential dependence of the cluster population on its mass, presenting a relative higher abundance at n = 5. The results of DFT/B3LYP calculations show that two main series of ammonium clusters may be formed. Both series follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of the respective {NH(3)NH}+ and {NH(2)NH(2)}+ cores. The energy analysis (i.e., D-plot and stability analysis) shows that the calculated members of the (NH(3))(n-1){NH(2)NH(2)}+ series are more stable than those of the (NH(3))(n-1){NH(3)NH}+ series. The trend on the relative stability of the members of more stable series, (NH(3))(n-1){NH(2)NH(2)}+, shows excellent agreement with the experimental distribution of cluster abundances. In particular, the (NH(3))4{NH(2)NH(2)}+ structure is the most stable one, in agreement with the experiments.