Silicon clusters produced by femtosecond laser ablation: Non-thermal emission and gas-phase condensation

Neutral silicon clusters Si_n (up to n = 7) and their cations Si_n+ (up to n = 10) have been produced by femtosecond laser ablation of bulk silicon in vacuum and investigated using time-of-flight mass spectrometry. Two populations of the Si_n+ clusters with different velocity and abundance distributions in the ablation plume have been clearly distinguished. Possible mechanisms of cluster formation (Coulomb explosion, gas-phase condensation, phase explosion) are discussed

A STUDY OF ULTRA-FAST ION REACTIONS WITH FEMTOSECOND TIME RESOLUTION : FIELD DISSOCIATION OF METAL HELIDE IONS BY ATOMIC TUNNELING

An ion reaction time amplification scheme for the time-of-flight spectrometer is proposed. With this scheme ion reactions can be studied with a time resolution as good as 10-14 sec. We have studied field dissociation by atomic tunneling of metal helide ions such as RhHe2+ using this scheme. Field dissociation of RhHe2+ can occur by atomic tunneling within a few 10-13 sec in a field of ~4.8 V/Å

EVIDENCE OF POST FIELD IONIZATION AND OBSERVATION OF NOVEL FEATURES IN THE ENERGY DISTRIBUTION OF FIELD EVAPORATED IONS

When field evaporation is carried out under the condition where more than one charge States coexist, the energy distribution of higher charge state ions exhibits a low energy tail similar to that of gas ions in field ionization. Ions in the tail can only be produced by post field ionization. A double peak structure has been found in the energy distribution of Mo2+ where the low energy peak dominates at high field and the high energy peak dominates at low field, the origin of which is not yet understood. High temperature field evaporation produces Mo2+ ; this has interesting implications to Coulomb explosion and theory of field evaporation. The stability of this ion is probably the result of an effective screening of the two positive charges by the electronic charges in the ion

FIELD DISSOCIATION BY ATOMIC TUNNELING OF COMPOUND IONS : ISOTOPE AND ORIENTATIONAL EFFECTS

Dissociation of HeRh2+ in high electric field occurs by tunneling of a neutral He out of the ion. This tunneling process occurs most readily when it is rotated 180° from its desorbed orientation. This rotation time is measured to be 790±21 fs. The tunneling effect is supported by a very strong isotope effect observed when 4He is replaced by 3He, and also by a WKB calculation of barrier penetration probabilities

Applications of Laser Techniques to Field Ion Microscopy

Laser light has been used in field ion microscopy for rnainly three purposes. First, for studying photon stimulated effects on field ionization and field desorption. Second, pico-second pulsed-laser technique has been used in time-of-flight atomprobe operation with some advantages. Beside being able to field evaporate semiconductors and to improve the rnass and energy resolution; rnany interesting effects have been found. Third, laser light has been use for heating the very small part of the tip apex either in continuous time of in ns pulses. In the latter case, the effects of rapid heating and quenching of material surfaces, the atomic motion and surface reconstruction produced by the heating of ns laser pulses can be studied. Our studies in field ion emission, ion physics and surface science are reviewed

PULSED-LASER STIMULATED FIELD EVAPORATION OF SILICON A PHOTOEXCITATION EFFECT AND CLUSTER ION FORMATION*

L'évaporation de champ stimulée par impulsion laser produit des ions moléculaires de 3 à 16 atomes par amas. Les nombres magiques sont 4, 5, 6 et 13 qui correspondent à des unités petites et à forte symétrie des atomes dans un cristal de Si. Un effet de photoexcitation a également été observé.Pulsed-laser stimulated field evaporation produces cluster ions of 3 to 16 atoms per cluster. The magic numbers are 4, 5, 6 and 13 which correspond to small, higly symmetric units of atoms in a Si crystal. A photoexcitation effect has also been found