Dissociation of doubly charged alkali metal clusters

We have studied the competition experimentally observed between fission and neutral atom evaporation, as dissociation channels of excited doubly charged sodium clusters, using the Density Functional Theory and the jellium model. The fission barrier has been obtained from an Extended Thomas-Fermi calculation including density gradient corrections to the kinetic energy of the electronic cloud. We discuss the influence of the coefficient of the density gradient term on the barrier height.     

Fission of doubly charged iron and aluminium clusters

We present a statistical fragmentation study of doubly charged iron and aluminum clusters of less than 60 atoms. At low excitation energies we find that the evaporation of one charged monomer is the most probable decay channel (asymmetric fission). When the excitation energy increases there is a competition between evaporation of a charged monomer and a charged dimer. For higher energies the number of channels increases and the multifragmentation mode appears at about 2.5 eV/atom. Presented by M.E. Madjet at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Coulomb- and antiferromagnetic-induced fission in doubly charged cubelike fe-s clusters

We report the observation of symmetric fission in doubly charged Fe-S cluster anions, [Fe(4)S(4)X(4)](2-)-->2[Fe(2)S(2)X(2)](-) (X=Cl,Br), owing to both Coulomb repulsion and antiferromagnetic coupling. Photoelectron spectroscopy shows that both the parent and the fission fragments have similar electronic structures and confirms the inverted energy schemes due to the strong spin polarization of the Fe 3d levels. The current observation provides direct confirmation for the unusual spin couplings in the [Fe(4)S(4)X(4)](2-) clusters, which contain two valent-delocalized and ferromagnetically coupled Fe2S2 subunits.     

Decay pathways of small gold clusters

The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au ⁺ _n, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation. For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters, dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer evaporation. In this respect, Au ⁺ ₉ shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au ⁺ ₇ and Au ⁺ ₁₁. This nonamer anomaly is typical for copper-group cluster ions M ⁺ ₉ (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M ^- ₇. It is discussed in terms of the well-known electronic shell closing at n _e = 8 atomic valence electrons. Received 2 November 2000     

Comparison of CID, ETD and metastable atom-activated dissociation (MAD) of doubly and triply charged phosphorylated tau peptides

The fragmentation behavior of the 2+ and 3+ charge states of eleven different phosphorylated tau peptides was studied using collision-induced dissociation (CID), electron transfer dissociation (ETD) and metastable atom-activated dissociation (MAD). The synthetic peptides studied contain up to two known phosphorylation sites on serine or threonine residues, at least two basic residues, and between four and eight potential sites of phosphorylation. CID produced mainly b-/y-type ions with abundant neutral losses of the phosphorylation modification. ETD produced c-/z-type ions in highest abundance but also showed numerous y-type ions at a frequency about 50% that of the z-type ions. The major peaks observed in the ETD spectra correspond to the charge-reduced product ions and small neutral losses from the charge-reduced peaks. ETD of the 2+ charge state of each peptide generally produced fewer backbone cleavages than the 3+ charge state, consistent with previous reports. Regardless of charge state, MAD achieved more extensive backbone cleavage than CID or ETD, while retaining the modification(s) in most cases. In all but one case, unambiguous modification site determination was achieved with MAD. MAD produced 15-20% better sequence coverage than CID and ETD for both the 2+ and 3+ charge states and very different fragmentation products indicating that the mechanism of fragmentation in MAD is unique and complementary to CID and ETD.     

Collision induced dissociation of doubly charged silver clusters Agn 2+, n=21,22,23

Doubly charged silver clusters Ag_n ²⁺, n=21, 22, 23, are produced by electron bombardment of an Ag_n ⁺ ensemble stored in a Penning trap. After mass selection the clusters are subjected to collision induced dissociation. The fragmentation channels are determined by time-of-flight mass spectrometry after ejection of the resulting ion ensemble from the trap. Monomer evaporation is the only decay path observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ground-state triply and doubly heavy baryons in a relativistic three-quark model

Mass spectra of the ground-state baryons consisting of three or two heavy (b or c  ) and one light (u,d,s)$(u,d,s)$ quarks are calculated in the framework of the relativistic quark model and the hyperspherical expansion. The predictions of masses of the triply and doubly heavy baryons are obtained by employing the perturbation theory for the spin-independent and spin-dependent parts of the three-quark Hamiltonian.