Ming-Teh Hsu

Structures of carbon cluster ions from 3 to 60 atoms: Linears to rings to fullerenes

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures. Fullerenes first appear at C+30 and begin to dominate above C+45. From small to large the isomeric progression is from linear to rings to fullerenes.

On the structure, reactivity and relative stability of the large carbon cluster ions C+62, C+60 and C+58

Abstract The exclusive metastable loss of C 2 from C + 62 , C + 60 and C + 58 generated by laser ablation from graphite is studied using a high resolution reverse geometry mass spectrometer. Kinetic energy release distributions are peaked at low energies (≈0.2 eV) and fall off quasi-exponentially at higher energies. Successful modeling by statistical phase space theory indicates no reverse activation energy for C 2 -loss. A molten droplet of spheroidal geometry for the structure of these ions best fits the data. The modeling also indicates C 2 is bound to C + 62 by 3.0 eV and to C + 60 and C + 58 by ≈4.6 eV. Electronic rather than structural effects are suggested as the cause of the instability of C + 62 relative to C + 60 and C + 58 . Kinetic effects that reflect the instability of C + 62 and to a lesser extent the stability of C + 60 are suggested as the principal reasons why C + 60 can be a dominant feature in the mass spectrum under favorable experimental conditions.

Do small fullerenes exist only on the computer? Experimental results on C=/−20 and C+/−24

Abstract Experimental results on the structure of carbon cluster anions and cations with 20 and 24 atoms are presented. Recent quantum chemical calculations predict either cage or graphitic structures to be lowest in energy for C20 and C24. However experimentally we find C+20 and C−20 are dominated by monocyclic ring structures, with C−20 exhibiting minor amounts of planar bicyclic structures and a linear structure. For C+24 and C−24 both monocyclic rings and planar bicycle rings are observed. No fullerene or “graphitic” structures are observed for these clusters. A number of possible bicyclic structures for C24 are discussed.

Evaporation of covalent clusters: Unimolecular decay of energized size‐selected carbon cluster ions (C+n, 5≤n≤100)

The unimolecular decay of energized size‐selected carbon clusters (C+n, 5≤n≤100) is investigated. The clusters are produced in a laser‐generated plasma on the surface of a graphite rod. Directly extracted cations that decay on a μs time scale are probed in a double‐focusing, reverse‐geometry mass spectrometer. The unimolecular decomposition rates are extracted from metastable fraction measurements. We observe a dramatic discontinuous increase in the decay rate constant as a function of cluster size around mass C+c0 (factor of 5 to 10). Additionally, low rate constants, relative to the neighbors, are found for C+50, C+60, and C+70. The results are rationalized by postulating a phase transition from small ‘‘rigid’’ clusters for n 30. In this model local deviations in rate constant reflect the thermodynamic stabilities of the clusters. A further consequence of this model is that ‘‘magic’’ numbers in the mass spectrum originate primarily from the intrinsic stability of t...

Thermal bimolecular reactions of size selected transition metal cluster ions: Nb+n + O2, n = 1–6

Abstract Niobium cluster ions, Nb + n , are generated using a new laser desorption source. This source, floated at 5 kV, is coupled to a high resolution reverse geometry mas spectrometer. Mass selected cluster ions exiting the mass spectrometer are decelerated to several electronvolts and spatially focused on a small hole in the entrance plate to a high pressure (0–7 Torr), variable temperature (80–600K) drift reactor. The cluster ions gently drift through this 4 cm long cell under the influence of small electric fields. A helium buffer gas is used to thermalize the ions. Trace amounts of O 2 are added for the reaction studies. Ions exiting the drift reactor are analyzed using a quadrupole mass filter and ion counting techniques. The quadrupole mass filter has an upper mass range of about 600 dalton, limiting the current maximum cluster size to n = 6. All reactions of Nb + n with O 2 are found to be fast, proceeding at 40 ± 10% of the collision rate. For n = 2–6, the major (90%) product results from loss of NbO from the Nb + n · O 2 collision complex. For n = 3–6, other (small) competing channels include loss of NbO 2 and Nb atoms from the collision complex. Fast sequential reactions of the reaction products with O 2 are observed. An analysis of the arrival time distribution of Nb + at the detector indicates laser desorption forms ≈ 80% ground state (4d 4 , 5 D) and 20% excited state (4d 3 5s 1 , 5 F). Collisional deactivation by He occurs at a rate constant ⪡ 1 × 10 −15 cm 3 s −1 .

Determination of potential energy curves for ground and metastable excited state transition metal ions interacting with helium and neon using electronic state chromatography

Reduced zero field mobilities of Cr+, Co+, and Ni+ in their ground state configuration (3dn) and their metastable excited state configuration (3dn−14s1) have been measured in He and Ne as a function of temperature. Parameters for a n−6−4 interaction potential are obtained by fitting the mobility vs temperature curve. Bond dissociation energies, vibrational frequencies, and equilibrium bond lengths are extracted from these curves and compared with data from traditional equilibrium measurements and ab initio theory. Dramatic changes in bonding are observed for the 3dn and 3dn−14s1 configurations; the 3dn configuration yielding very much stronger bonds and very much shorter equilibrium bond lengths than the 3dn−14s1 configuration. The presence of multiple potential curves in the interaction of Co+(3F,3d8) and Ni+(2D,3d9) with He/Ne make interpretation of the ground state mobility data ambiguous. The analysis of each system is discussed in some detail.

Observation of small doubly charged niobium clusters

Abstract Small doubly charged niobium clusters are observed in a molecular beam emerging from a laser vaporization/supersonic expansion source if electron impact ionization is utilized in the expansion region. Doubly charged clusters from n = 2 to n = 15 are readily detected. The doubly charged dimer cation has the same charge to mass ratio as the very abundant singly charged monomer. However, the doubly charged dimer can be detected with collision-induced charge exchange reactions and collision-induced dissociation. The kinetic energy released into the fragment ions of the cluster decay process allows us to estimate the charge separation in the parent (4.3 A for the dimer and 5.7 A for the trimer). The results strongly imply that predictions of the stability of doubly charged clusters on the basis of binding energy of the neutral and Coulomb repulsion energy alone do not suffice and that particularly bonding conditions can yield metastable states with substantial lifetimes.

Structures and stabilities of carbon cluster ions

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29 ≤ n ≤ 45 indicating 3 or 4 different structures. Fullerenes first appear at C30+ and begin to dominate above C45+. From small to large the isomeric progression is from linear to rings to fullerenes. Preliminary results on some theoretically calculated carbon cluster structures and their simulated mobilities are presented.

Generation and Reactivity of State Selected Atomic and Size Selected Transition Metal Cluster Ions

In this chapter, two distinct studies in the general field of gas phase transition metal ion chemistry are presented. In the first, atomic metal ions of the first row transition series are generated by electron impact on various volatile organometallic reagents. These ions are then mass selected and injected at low energies into a pressure and temperature variable drift reactor. After drifting through the reactor, filled with He buffer gas, the ions exit, are mass analyzed by a quadrupole and detected. By pulsing the ion beam as it enters the drift reactor, arrival time distributions (ATD) at the detector can be obtained. Many of the ATD’s were found to be bimodal or trimodal, indicating the atomic metal ion beams contained multiple components that interacted significantly differently with the He buffer gas. Analysis of the data showed the largest splittings in the ATD’s were due to ion electronic states with different electronic configurations (either 3dn or 4s3dn-1). In favorable cases electronic states within the 4s3dn-1 configuration could be resolved. Clustering of state selected M+ with He was studied. It was found that only M+ ions with 3dn configurations formed the M+-He adduct. Equilibrium constant measurements yielded values of ΔH between 1 and 4 kcal/mol and ΔS between 13 and 18 cal/mol°K. Deactivation of excited states was also observed only for metal ions with 3dn configuration excited states and 4s3dn-1 ground states. Deactivation rate constants were measured for two systems and a mechanism proposed. In the second study, size selected Nbn + ions (1 ≤ n ≤ 6) were injected into the drift cell and reacted with O2, NO, C2H6, C3Hg and C6H6 along with deuterated analogues. Several reactions of NbO+ are also reported. Both total rate constants and product branching ratios were measured. Examples were chosen to highlight the size selective behavior of the clusters and to point out that truly surprising reactions are observed. Thermodynamic and mechanistic conclusions are drawn where appropriate.