Sur la distribution énergétique des ions moléculaires produits par évaporation de champ à partir de pointes liquides

Abstract

We present experimental results on Sn n p+ and Ge n p+ cluster ions (n≤30; p≥3) obtained in Liquid Metal Ion sources (LMIS). We first briefly recall the mass-spectrography data. Then we present counterfield measurements which give the energy distribution of each molecular species. The Sn n + and Ge n + singly charged ions show a two-maximum distribution for n≥6 (we call ΔV the separation between the peaks). We observe that, for increasing n values, the relative magnitude of the second peak increases. On the contrary, the multiply charged ions have an energy distribution with only one peak. It is also observed that, for a given single charged Sn n + or Ge n + cluster ion, the ΔV value increases when D, the tip-cathode distance, decreases. We then discuss a model based on the fragmentation in flight of two kinds of metastable droplets. The first ones (reaction A, corresponding to the first peak) are strongly overcharged and, practically, explode at the tip surface. The second ones (reaction B, corresponding to the second peak) have a lower specific charge. For these last droplets, the excitation energy responsible for their metastability is the sum of their thermal energy (the tip temperature is ∼1000°C) and of their electrostatic energy. They explode after a time τ (which here is evaluated as being ∼10 −11 s), after this time all the excitation energy is converted into atomic motions. This model allows one to understand seceral experimental results: relative magnitude of the two peaks versus n and p Mesures de contre champ pour determiner la distribution energetique des agregats Sn n p+ et Ge n p+ (n≤30; p≤3). Pour n≥6, observation de deux maximums separes de ΔV, le second devenant plus important quand n croit. Pour un ion donne, augmentation de ΔV quand la distance pointe-cathode, D, diminue. Interpretation des resultats a l'aide d'un modele reposant sur la fragmentation en vol de deux types de gouttelettes metastables