S. A. Buzza

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.

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.

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.

Further direct evidence for stepwise dissociation of acetone and acetone clusters

The (pre)dissociation of acetone and acetone clusters after excitation to states corresponding to upper {S1,T1} and 3s Rydberg states of the acetone monomer are investigated through femtosecond pump–probe experiments coupled with molecular beam time‐of‐flight mass spectrometry techniques. Upon excitation to either state, [(CH3)2CO]n* dissociates rapidly. Acetyl fragments, [(CH3)2CO]n−1CH3CO+ may arise from ionization of an excited species formed by (pre)dissociation of intact precursors or by dissociation after the intact species has been ionized. The method employed to separate these two channels is discussed herein; the resulting transients are fit to a kinetic model to elucidate intermediate lifetimes and dissociation mechanisms. The present experiments establish that a stepwise dissociation mechanism is operative upon excitation to the 3s Rydberg state for the acetone monomer and dimer, with their corresponding acetyl fragments having lifetimes on the order of picoseconds. Larger cluster species, [(CH...

Femtosecond multiphoton ionization of ammonia clusters

Herein, we report on femtosecond time-resolved experiments in ammonia clusters. The mechanisms of their ionization and the subsequent formation of the protonated ammonia cluster ions are studied using a femtosecond pump-probe technique at 620 nm. It is found that an intermediate corresponding to [ital C][prime] states of the monomer is responsible for the ionization of ammonia clusters. Femtosecond pump--probe studies show that the lifetime of the intermediate to the formation of the protonated cluster ions (NH[sub 3])[sub [ital n]]H[sup +] ([ital n]=1--5) is the same as that leading to the formation of the unprotonated cluster ions (NH[sub 3])[sub [ital m]][sup +] ([ital m]=2--5). The results provide the first direct experimental proof that formation of the protonated cluster ions takes place through an absorption--ionization--dissociation mechanism.

Generation of metal–carbon and metal–nitrogen clusters with a laser induced plasma technique

During the course of investigating dehydrogenation reactions induced by transition metals, we find that using a carrier gas containing hydrocarbons and ammonia instead of pure helium, in conjunction with a laser vaporization device, enables the facile production of metal–carbon and metal–nitrogen clusters in both the neutral and ionic forms. With only a change in the nature of the carrier gas, a variety of new classes of clusters can be produced.

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.

Femtosecond excitation dynamics of acetone: Dissociation, ionization, and the evolution of multiply charged elemental species

Recent femtosecond pump–probe experiments have suggested that a stepwise dissociative mechanism is operative for acetone excited to Rydberg states and upper regions of the mixed singlet/triplet state. The present work focuses on the excitation of acetone and acetone clusters to the 3d (or perhaps 4s) electronic intermediate state in order to further explore the operative dissociation mechanisms and the effects of solvation (clustering). As reported herein, results from femtosecond pump–probe experiments suggest that the availability of additional vibrational modes in clusters, where internal energy may be dispersed, increases the fraction of acetyl intermediates which remain behind the barrier to dissociation into methyl and CO fragments. At progressively higher laser fluences, multiply charged elemental carbon and oxygen ions abruptly appear. Interestingly, the extent of their formation is observed to depend on both laser intensity and the relative time delay between the pump and probe laser beams respon...

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...