On the decay of hot metallic clusters by evaporation

Abstract

Starting from the Weisskopf theory decay rates for the evaporation of cluster atoms from hot liquid alkali metal clusters are derived. The crucial input quantity is the level density which is determined from empirical properties of the bulk, namely from the specific heat and the thermal expansion coefficient. The resulting rate expression is compared with decay rate formulas given by Engelking, Klots and Gspann. Furthermore, critical (appearance) sizes of multiply charged clusters are calculated by equating the rates for neutral monomer and light charged particle emission. Also shrinking and cooling rates of large hot clusters are determined by treating multiple emission of cluster atoms, thus establishing a time scale for the decay of clusters theoretically.

Efficiency of Thermionic Emission from C60

“…Highly excited gas-phase fullerenes have been observed to emit delayed electrons, − a continuous spectrum of photons, − and C 2 fragments. ,− These phenomena are commonly interpreted as the molecular analogues of thermionic emission, thermal (blackbody-like) radiation, , and evaporation , because they occur under conditions where the excitation energy, estimated to range from roughly 15 to 60 eV, is likely to be randomized over all internal degrees of freedom. , The degree to which they compete with each other is not well-known; neither their absolute nor their relative rates have been determined so far . The experimentalist trying to measure absolute rates for any of these reactions, or relative rates for any pair of them, faces several challenging tasks: controlling the excitation energy of an ensemble of free fullerenes, determining their number, and detecting the products with known efficiencies.…”

An inelastic threshold in electron - alkali cluster collisions

“…The latter implies a statistical sharing of the excitation energy between all the vibrational degrees of freedom (cluster heating) followed by the evaporation of a small fragment (see, e.g., [30,31] and references therein). In alkali cluster studies, such events follow, e.g., the decay of optically-induced electronic collective resonance states, or plasmons [32].…”

Multiply charged clusters

“…Hence, n app will be larger than the Rayleigh limit, especially if the multiply charged cluster ions that are prepared in the experiment are vibrationally highly excited. However, theoretical modeling shows that the rates for monomer evaporation and asymmetric fission have different temperature dependences; the cluster size where the two rates are equal will increase with increasing cluster temperature [7].…”

“…The former exhibit roughly symmetric fission while the latter exhibit highly asymmetric fission. The difference arises from the difference in charge distribution [7]: nuclei are homogeneously charged while, in metallic clusters, the charge resides on the surface. Fission of nonmetallic clusters, though, may be more symmetric, especially for large sizes, as shown in an experimental study of antimony clusters [12], and a molecular dynamics study of argon clusters [13].…”