This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications. © 1998 John Wiley & Sons, Inc., Mass Spec Rev 17, 1–35, 1998

Fourier transform ion cyclotron resonance mass spectrometry: A primer

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

Ultraviolet photoelectron spectra (UPS) were recorded for mass‐selected negative clusters of copper (1–411 atoms), silver (1–60 atoms), and gold (1–233 atoms), using photodetachment lasers at 6.4 and 7.9 eV photon energy. The results provide a direct estimate of the vertical electron affinity (EA) of these clusters and information on the evolution of the d bands of copper and gold as a function of cluster size. The large even/odd alternation of EA in small clusters of these metals in earlier work is found to largely disappear as the cluster size exceeds 40 atoms. The ellipsoidal shell model is shown to be consistent with the observed EA behavior of all three metals, the predicted spherical shell closing at cluster 58 being evident for silver and gold. The UPS data show a smooth evolution of the d band toward that of the bulk metal.

Ultraviolet photoelectron spectra of coinage metal clusters

Negative ion photoelectron spectra of Cu−n, Ag−n(n=1–10), and Au−n (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu−2, Ag−2, and Au−2, the M2 X 1Σ+g←M−2 X 2Σ+u transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σ+u triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu−3 and Ag−3 spectra to the transition from the linear ground state of the anion (M−31Σ+g) to an excited linear state of the neutral (M3 2Σ+u). The Au−3 spectrum is attributed to a two‐photon process, photodissociation followed by photodetachment of the Au− or Au−2 fragment. For larger clusters, we measure the threshold and vertical detachment energies...

Photoelectron spectroscopy of metal cluster anions : Cu−n, Ag−n, and Au−n

Reaction rates and saturation values were determined for H2 dissociative chemisorption on positive niobium cluster ions in an FT‐ICR apparatus. Clusters with 8,10,12, and 16 atoms were found to be particularly unreactive, in remarkable agreement with the reactivity patterns observed previously for neutral niobium clusters. Saturation coverage for most clusters was found to occur near a hydrogen/niobium ratio of 1.3, although some clusters (8–12,16, and 19) reached effectively inert compositions at considerably lower coverages. Several examples were found of clusters having two isomeric forms with different reactivities. One form of Nb+19 was found to readily react with H2, whereas a second form representing one‐third of the original sample of 19 atom clusters was completely inert to H2 chemisorption under the same FT‐ICR conditions. The geometrical shape of these niobium clusters must therefore have a critical effect on reactivity.

Fourier transform ion cyclotron resonance studies of H2 chemisorption on niobium cluster cations

Using a new magnetically focused time‐of‐flight photoelectron spectrometer, the ultraviolet photoelectron spectra (UPS) of mass‐selected negative copper clusters have been measured at photon energy of 4.66 eV for all clusters in the range from 6 through 41 copper atoms. These UPS data provide the first detailed probe of the 4s valence band structure of such medium size negative copper clusters, and extend previous approximate measures of the electron affinity of the corresponding neutral species. The results are in accord with the predictions of the ellipsoidal shell model for monovalent metal clusters. In particular, clusters 8, 20, and 40 (which correspond to spherical shell closings in this simple model) are found to have unusually low electron affinities and large HOMO–LUMO gaps. Subshell closings at 14 and 34 also appear special in this new UPS data.

Ultraviolet photoelectron spectroscopy of copper clusters

FT‐ICR techniques were used to probe the surface chemistry of isolated silicon cluster ions in the 7–65 atom size range. Dissociative chemisorption reactions with NH3 were observed to proceed with rates which varied widely with cluster size. One particular cluster, Si+39, was found to be remarkably inert. Clusters with 20, 25, 33, and 45 atoms were found to be unreactive as well, while those with 18, 23, 30, 36, 43, or 46 atoms were quite reactive. Similarly oscillating reaction patterns were observed with CH3OH, whereas highly reactive free radical scavengers such as O2 and NO showed little selectivity. These results suggest the silicon clusters in this size range have well‐defined structures which vary in ability to catalyze dissociative chemisorption at the surface.

FT‐ICR probes of silicon cluster chemistry: The special behavior of Si+39

The electronic structure of small Ga_xAs_y clusters (x+y≤10) are calculated using the local density method. The calculation shows that even‐numbered clusters tend to be singlets, as opposed to odd‐numbered clusters which are open shell systems. This is in agreement with the experimental observations of even/odd alternations of the electron affinity and ionization potential. In the larger clusters, the atoms prefer an alternating bond arrangement; charge transfers are observed from Ga sites to As sites. This observation is also in agreement with recent chemisorption studies of ammonia on GaAs clusters. The close agreement between theoretical calculations and experimental results, together with the rich variation of electronic properties of GaAs clusters with composition makes GaAs clusters an ideal prototype system for the study of how electronic structure influences chemical reactivity.

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Electronic structure of small GaAs clusters

Silicon clusters in the size range from 5 to 66 atoms were generated by laser vaporization in a supersonic nozzle and injected into the ion trap of a specially‐designed Fourier transform ion cyclotron resonance apparatus. On the positively charged clusters ammonia chemisorption reaction rates were found to vary by over three orders of magnitude as a function of cluster size, with clusters of 21, 25, 33, 39, and 45 atoms being particularly unreactive, and cluster 43 being the most reactive. For the negative cluster ions, 43 was the only cluster found to react substantially. Although the reaction behavior of many clusters clearly indicated that several structural isomers were present with different reaction rates, the strikingly low net reactivity of such clusters as 39 and 45 provides evidence that they have effectively crystallized into a single specially stable form.

R. T. Laaksonen

Papers

Fourier transform ion cyclotron resonance studies of H2 chemisorption on niobium cluster cations

Ultraviolet photoelectron spectroscopy of copper clusters

FT‐ICR probes of silicon cluster chemistry: The special behavior of Si+39

Electronic structure of small GaAs clusters

Ammonia chemisorption studies on silicon cluster ions