J. Purnell

Metallo-Carbohedrenes [M8C12+ (M = V, Zr, Hf, and Ti)]: A Class of Stable Molecular Cluster Ions

Findings of magic peaks corresponding to M8C12+ (M = V, Zr, and Hf) formed from reactions of the respective metals with various small hydrocarbons, in conjunction with recent findings for the titanium system, establish metallo-carbohedrenes as a stable general class of molecular cluster ions. A dodecahedral structure of Th point symmetry accounts for the stability of these ionic clusters.

The clustering reactions of benzene with sodium and lead ions

Abstract Thermochemical properties are determined for the association reactions of benzene with Na + and Pb + ions. The values obtained are -28.0 kcal/mol and -31.2 cal/mol K for the enthalpy and entropy changes of the sodium ion, and -26.2 kcal/mol and -21.6 cal/mol K for Pb + , respectively. An ab initio calculation was conducted to determine the bonding and structure of Na + -benzene. The C 6v symmetry structure of Na + -benzene, and the small charge transfer from benzene to Na + suggest that the bonding of the ion complex is largely electrostatic in nature. Like other lead ion complexes, Pb + -benzene displays an unusually strong interaction. Possible bonding of Pb + -benzene is also discussed.

Metallo-carbohedrenes : formation of multicage structures

An unusual structural growth pattern has been found in the system of ZrmCn, in which multicage structures are formed. The experimental evidence shows that the first cage closes at Zr8C12. Surprisingly, subsequent cluster growth does not lead to the enlargement of the cage size as it usually does in the case of pure carbon clusters and water clusters, for example. Rather, multicage structures are developed, that is, a double cage at Zr13C22 and Zr14C21/23, a triple cage at Zr18C29, and a quadruple cage at Zr22C35. This feature distinguishes the class of metallo-carbohedrenes from the regular doped fullerenes.

Ultrafast laser-induced Coulomb explosion of clusters with high charge states

Abstract Iodine and argon ions, as highly charged as I17+ and Ar8+, are found to be formed upon the irradiation of HI clusters and HIArm clusters with an intense femtosecond laser beam. Double peaks are observed for each multicharged species, and identified to result from Coulomb explosion of clusters. Kinetic energy release values are calculated from the peak splittings, and also directly measured from cutoff potentials using a reflectron as an energy analyzer. The determined values are surprisingly large, on the order of several hundred electron volts.

Femtosecond laser-induced Coulomb explosion of ammonia clusters

Abstract Nitrogen atoms, as highly charged as N 5+ , are found to be formed upon the irradiation of (NH 3 ) n clusters with an intense femtosecond laser beam. Kinetic energy release values of the nitrogen atoms, from the Coulomb explosion of ammonia clusters, are found to be on the order of several hundred electron volts as calculated from the peak splittings, and also directly measured from cutoff potentials using a reflection as an energy analyzer. Also, we observe a Coulomb explosion process which results in the production of intact cluster ions, and we propose a mechanism for their formation.

Ultrafast reaction dynamics of electronically excited à state of ammonia clusters

Femtosecond pump–probe techniques combined with a reflectron time‐of‐flight mass spectrometer are utilized to study the ultrafast reaction dynamics of the electronically excited A state of ammonia clusters. All of the detected protonated cluster ions, (NH3)nH+ n=2−6, are observed to display two distinct features with respect to preselected pump–probe time delays; a fast decay, followed by a persistent ion signal leveling off to a finite nonzero value. The fast decay is attributed to a predissociation process; while an intracluster reaction, which leads to formation of long‐lived intermediates (NH3)nNH4, is responsible for the nonzero falling off regime. The results provide conclusive experimental evidence that both an absorption–ionization–dissociation mechanism and an absorption–dissociation–ionization mechanism are operative in the A state of ammonia clusters.

Real-time dynamics of ammonia clusters excited through the à state: formation of the protonated cluster ions

Abstract Femtosecond pump-probe techniques combined with a reflectron time-of-flight mass spectrometer are employed to study the dynamics of ammonia clusters excited to the A state. In this paper we present data on the formation of protonated ammonia cluster ions and consider a reaction scheme for their formation through the dissociation of (NH 3 ) n (A 1 A″ 2 ). We propose that the formation of the radical (NH 3 ) n -,NH, species is due to a reaction involving the dissociation of (NH3)n (A 1 A″ 2 ) to NH, (A 2A,). A competing process in the dissociation of (NH3)n (A 1 A″ 2 ) to NH, (X 2B,), yields fragments that are too translationally “hot” to react to form the (NH 3 ) n − 2 NH 4 species. We also quantitatively model these reactions and report the lifetimes associated with the dissociation processes of (NH 3 ) n , and concomitant formation of (NH 3 ) n − 2 NH 4 . Lifetimes of 250–900 fs are observed for the dissociation of (NH3)3_23 excited to the A state.

FORMATION AND METASTABLE DECOMPOSITION OF UNPROTONATED AMMONIA CLUSTER IONS UPON FEMTOSECOND IONIZATION

The formation and metastable dissociation mechanism of unprotonated ammonia cluster ions, (NH3)+n, produced by multiphoton ionization (MPI) at 624 nm and a nominal pulse width of 350 fs, are investigated through a reflectron time‐of‐flight (TOF) mass spectrometric technique. Detection of the unprotonated ions after femtosecond and nanosecond multiphoton ionization under various intensity conditions is explained. The role of the energy of the ionizing photons, and the observation of these ions after femtosecond MPI is examined. The formation of the unprotonated series is found to be a function of intensity in the case of ionization on the nanosecond time scale, but not so for the femtosecond time domain. The results can be explained in terms of ionization mechanisms and ionizing pulse durations. The findings of the present study suggest that the unprotonated ions are trapped behind the barrier to intracluster proton transfer and/or concomitant NH2 loss. The studies of metastable decomposition also reveal t...

Studies of the metastable decay of met-cars: The vanadium and niobium systems

Abstract The decay fractions and metastable decomposition channels were measured for VmC+n and NbmC+n clusters using a reflectron time-of-flight mass spectrometer. These measurements for the VmC+n clusters reveal stable (8, 12) and (8, 13) met-car species. However, under the present experimental conditions, the NbmC+n clusters display dominantly cubic structures, but the decay fraction and metastable decomposition measurements also show the presence of a stable (8, 12) species in the parent cluster spectrum. This evidence indicates that cubic structures and met-cars are being formed simultaneously for the niobium system.

The production of metallocarbohedrenes by the direct laser vaporization of the carbides of titanium and zirconium

Abstract Metallocarbohedrenes (Met-Cars) of titanium and zirconium have been produced by the direct laser vaporization of their respective pure carbides. Time-of-flight mass spectra of both ionic and neutral metallocarbohedrenes formed in the laser-induced plasma are presented and compared to spectra of the same systems generated under laser vaporization/molecular beam conditions. Potential mechanisms of formation of these clusters are presented and discussed.