Toshio Ichihara

Mass distributions of copper, silver and gold clusters and electronic shell structure

Abstract Mass distribution of copper (Cu)n+, silver (Ag)n+ and gold (Au)n+ clusters were investigated up to cluster size n = 250. The clusters were produced by bombardment of 10 keV Xe ions and were analyzed using a sector-type mass spectrometer. The mass distributions for these metal clusters were similar. The ion intensity of clusters decreased with increasing cluster size, n. Superimposed on this general decrease, two types of anomalies were observed. One anomaly was a regular variation of ion intensity between odd and even numbers of n, where the intensity of odd-n clusters was greater than that of even-n. The other anomaly was a discontinuous variation of cluster ion intensity at peculiar numbers of n. These numbers were 3, 9, 21, 35, 41, 59, 93, 139 and ∼ 200. This behavior can be explained by a one-electron shell model in which electrons are bound in a spherically symmetric potential well.

Mass distributions of negative cluster ions of copper, silver, and gold

Abstract Mass distributions of negative cluster ions of copper (Cu) n − , silver (Ag) n − , and gold (Au) n − obtained by the bombardment of metal sheets with Xe ions were investigated up to cluster size n = 250 and were compared with those of the positive cluster ions. The mass spectra were obtained under the same conditions as previously employed to study positive cluster ions and the behaviour of the mass distributions of the negative cluster ions were very similar to those of the positive cluster ions. The ion intensity of the clusters decreased with increasing cluster size, n , and two types of anomaly were observed. One was a regular variation of the ion intensity between odd and even values of n , where the intensity of odd- n clusters was greater than that of even- n , which is explained by pairing of the electrons. The other anomaly was a discontinuous variation of ion intensity at specific numbers n = 1, 7, 17, 19, 33, 39, 57, 91, 137, and 197. In the mass spectra of positive cluster ions, these were 3, 9, 19, 21, 35, 41, 59, 93, 139, and 199. It is confirmed from this intensity behaviour that a one-electron shell model can be applied to explain the stability of those metal clusters in which s valence electrons are bound in a spherically symmetric potential well.

Correlation between mass distributions of zinc, cadmium clusters and electronic shell structure

Mass distributions of zinc (Zn)n+ and cadmium (Cd)n+ clusters have been investigated up to the cluster size n = 75. The clusters produced by the bombardment of the metals by 10 keV Xe ions have been analyzed using a single focusing mass spectrometer. The mass distributions for these metal clusters are quite similar. The ion intensity of clusters decreases with an increase of n. Superimposed on this general decrease, abundant ion currents are observed for clusters with n = 10, 18, 20, 28, 30, 32, 35, 40, 41, 46, 54, 57, 60 and 69. These numbers can be explained by a shell-closing effect of s-valence electrons which are bound in a spherically symmetric potential well of the cluster. The most probable potential is the finite square well potential with round edges.

Fission-like dissociation of doubly charged silver clusters

The dissociation patterns of doubly charged silver clusters (Ag)2+n to singly charged clusters (Ag)+m have been investigated. They were produced by the Xe+ ion bombardment of a silver sheet and were mass-analyzed using a double focusing mass spectrometer; the resulting ion intensity was < 1% of that of singly charged clusters. The daughter ions, the size of which is larger than n/2, were detected without the interface from those decaying from singly charged clusters by lowering the acceleration voltage below the normal value. The decomposition probabilities as a function fo m-value had a tendency to increase as the difference between m-values and n/2 decreased. Superimposed on the increase, an enhanced ion intensity at certain m-values and an odd-even alternation were observed in which the intensity of odd-m clusters was larger than that of even-m.

Metastable decay of cesium iodide cluster ions

Abstract Metastable decay of the cluster ions [(CsI) n Cs] + with n = 7 to 38 was investigated by using a double focusing mass spectrometer of normal geometry. The energy distributions of the cluster ions, when entering the mass spectrometer, vary in a different manner with n values, due to metastable decay. It is confirmed that the n = 13 and 22 cluster ions are more stable to metastable decay than others and seem to be anchor points in metastable decay chains of the higher n cluster ions.

Study of the dissociation of a charge‐reduced phosphopeptide formed by electron transfer from an alkali metal target

Doubly protonated phosphopeptide (YGGMHRQET(p)VDC) ions obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions via collision-induced dissociation (CID). The resulting ions were analyzed and detected by using an electrostatic analyzer (ESA). Whereas doubly charged fragment ions resulting from collisionally activated dissociation (CAD) were dominant in the CID spectrum with the Xe target, singly charged fragment ions resulting from electron transfer dissociation (ETD) were dominant in the CID spectrum with the Cs target. The most intense peak resulting from ETD was estimated to be associated with the charge-reduced ion with H2 lost from the precursor. Five c-type fragment ions with amino acid residues detached consecutively from the C-terminal were clearly observed without a loss of the phosphate group. These ions must be formed by N--Calpha bond cleavage, in a manner similar to the cases of electron capture dissociation (ECD) and ETD from negative ions. Although the accuracy in m/z of the CID spectra was about +/-1 Th because of the mass analysis using the ESA, it is supposed from the m/z values of the c-type ions that these ions were accompanied by the loss of a hydrogen atom. Four z-type (or y--NH3, or y--H2O) ions analogously detached consecutively from the N-terminal were also observed. The fragmentation processes took place within the time scale of 4.5 micros in the high-energy collision. The present results demonstrated that high-energy ETD with the alkali metal target allowed determination of the position of phosphorylation and the amino acid sequence of post-translational peptides.

Mass distributions of positive and negative cluster ions of zinc and cadmium

Abstract Mass distributions of positive and negative cluster ions of zinc, (Zn) n + , (Zn) n − , and cadmium, (Cd) n + , (Cd) n − , have been studied up to n = 75. The clusters were generated by the bombardment of metal sheets with 10 keV Xe + ions and were analysed in a single-focussing mass spectrometer. The ion intensity of the clusters decreases with increasing size but, superimposed upon the decrease, discontinuities are observed in both the positive and negative spectra. In the positive spectra, discontinuities are observed at n = 18, 20, 28, 32, 35, 40, 41, 46, 54, 57, 60, and 69 and at n = 27, 29, 31, 34, 39, 40, 45, 46, 53, 54, 56, 60, and 68 in the negative spectra. These discontinuities may be explained by the shell-closing effect of the s -valence electrons which are probably bound in a finite square well potential with round edges.

Fragmentation of doubly charged noble metal clusters

The dissociation patterns of doubly charged noble metal clusters (M)n++ to two singly charged clusters, (M)mEmphasis>1/+ and (M)mEmphasis>2/+ have been investigated using a double focusing mass spectrometer. They are compared with the dissociation patterns from singly charged clusters. The dissociation probabilities to (M)3+ and (M)9+ were large and the odd-even alternations were observed in both patterns.

Cluster abundance mass spectra of (Pb)n+ and (Pb)n−

Abstract Secondary ion mass spectra of (Pb) n + and (Pb) n − clusters have been obtained. The yield as a function of cluster size shows structures which are different between the positive and negative cluster ions. This fact shows that the stability of the clusters is related to the total number of valence electrons in the clusters.

Generation of argon and Ar-Al complex clusters, (Ar)+n, (Ar)nAl+ from bubbles in aluminium metal by argon ion bombardment

Abstract We report the mass spectra of argon clusters (Ar) + n and Ar-Al complex clusters (Ar) n Al + , generated by argon ion bombardment onto an aluminum metal sheet. The mass spectra have been obtained using a deflection-type mass spectrometer. The size distributions of (Ar) + n clusters bear a striking resemblance to those produced by free jet expansion of argon gas with subsequent ionization of the clusters. The (Ar) + n and (Ar) n Al + cluster ions are produced when argon bubbles in the aluminum metal are smashed by argon ion bombardment. Argon bubbles are known to be formed in aluminum by bombardment with energetic Ar + ions.