Mass-selected Cluster Ions Research Articles

Collision-induced dissociation of mass-selected phosphorus cluster cations

Phosphorus cluster cations of various sizes have been generated by laser ablation of red phosphorus powders. The magic numbers of these cluster ions appearing in the recorded spectrum were 4, 7, 11, 14, 18, 21 and 25. On a home-made tandem time-of-flight mass spectrometer, P n + ( n = 4–25) were studied via collision-induced dissociation (CID) by crossing the mass-selected phosphorus cluster ion beam with a supersonic nitrogen beam. After collision, P n + has the tendency to lose P 4 units and the number of tetramer fragments ejected upon dissociation increases with the parent phosphorus cluster size. The CID results show that the tetramer is the structural unit of the phosphorus clusters and larger cluster ions are generally less stable. Among the daughter ions surviving from the collisions, P 4 + is most stable, and P 7 + and P 8 + come next.

Rearrangements of Ammonium Nitrate Cluster Ions with High Internal Energy

Sputtering of condensed-phase ammonium nitrate yields an extensive distribution of negative cluster ions of the form [(NH 4 NO 3 ) n NO 3 ] - , n ≥ 3. Collision-induced dissociation of mass-selected cluster ions suggests that the first two members of the series, n = 1 and n = 2, are missing because the ions rearrange to lose one or more ammonia molecules. Gradient-corrected density-functional calculations show that NH 4 NO 3 is strongly hydrogen bonded and that [(NH 4 NO 3 )NO 3 ) - has no hydrogen bonds, consistent with this ion rearranging to lose NH 3 to form the strongly hydrogen-bonded ion [H(NO 3 ) 2 ] - .

Thermodynamical properties of ionized lithium oxide clusters, Li2n+pO+n

Lithium oxide clusters Li2n+pOn+ are generated by combining reactive nucleation in a gas aggregation source and photoionization. Unimolecular dissociation of mass selected cluster ions provides evidence that the excess of metal atoms evaporates first leading to the most stable species Li+(Li2O)n, which then evaporate Li2O molecules. The evaporation rate behavior as a function of cluster size demonstrates that Li+(Li2O)n can be prepared with different temperatures. It is discussed how metal evaporation from metal-rich oxide clusters leads to oxygen saturated clusters with a lower temperature. An estimate of the dissociation energies of Li+(Li2O)n are given for small sizes n≤10 from photoevaporation experiment.

Fluorescence of Ar(2p)-excited argon clusters

Fluorescence of argon and argon clusters is investigated in the Ar(2p)-excitation regime (240–310 eV). Fluorescence excitation spectroscopy shows a different evolution as a function of the average neutral cluster size compared with total electron and ion yields as well as partial ion yields of mass-selected cluster ions. These differences are rationalized in terms of various multiple ionization processes occurring in argon clusters. The dispersed fluorescence shows a broad continuum between 165–290 nm which is interpreted as emission of the “third continuum” of argon. The results suggest that the process leading to fluorescence of clusters in the Ar(2p) regime is due to the radiative decay of doubly charged clusters.

Size-dependent photodissociation cross sections for Sr+(NH3)n, n=3–6: Rydberg state formation and electron transfer

We report photodissociation cross sections for the mass-selected cluster ions Sr+(NH3)n, with n in the range from 3 to 6. The cross sections exhibit large redshifts that increase monotonically with increasing cluster size. For the n=3 cluster, the absorption cross section peaks near 700 nm, while the n=6 cluster spectrum appears to reach a maximum at 1.5 μm, in the near infrared region of the spectrum. A spectral moment analysis of the cross sections shows that <r2≳ for the ground state electronic radial distribution increases by an order of magnitude over the cluster size range from 1 to 6 solvent molecules. In conjunction with path integral Monte Carlo calculations carried out by Martyna and Klein, we argue that the increase in <r2≳ is diagnostic of the increasing Rydberg character of cluster states that correlate with 5s and 5p atomic orbitals on Sr+. An analysis of the charge transfer to solvent process as described by a thermodynamic Born cycle suggests that the rapid stabilization of solvated ion-pair states with increasing cluster size provides a plausible mechanism for introducing Rydberg character into cluster valence states based on 5s/5p atomic ion orbitals.

Oxidation of small carbon cluster ions by O2: Effects of structure on the reaction mechanism

We report cross sections for reaction of mass-selected carbon cluster ions (Cn+, n=2–14) with O2 over a collision energy range of 0.1–10 eV. The results give insight into the oxidation mechanism and how it is affected by the isomeric structure of the reactant cluster ion. Small clusters, which are linear, react with no activation barrier, producing primarily CnO+ at low energies. Larger clusters, which are monocyclic, have activation barriers for reaction, and the product distribution is quite different. Reactions were also studied under multiple collision conditions, and addition of up to two O atoms is observed. Possible reaction mechanisms are discussed and comparison is made with the thermal kinetics results of McElvany and co-workers [J. Chem Phys. 86, 715 (1987)].

Chemical Reactions in Molecular Cluster Ions

The promotion and observation of chemical reactions in molecular cluster ions is discussed in terms of results from three recent experimental studies. The presence of “magic numbers” in cluster ions of the type Arn · X+, where X is either a reactant or fragment ion, provides evidence of molecules occupying sites on or close to the surface of a cluster. Results from experiments on excitation transfer are used to discuss the possibility that a single chemical reaction in an ion Y+, could be promoted through the selective excitation of a host cluster of the form XnY+. Finally, results are present from a new experiment designed to study laser-induced chemical reactions in mass-selected cluster ions. Preliminary data are given for photoinduced reactions in cluster ions of 1,3-butadiene.

A double/triple time-of-flight mass spectrometer for the study of photoprocesses in clusters, or how to produce cluster ions with different temperatures

An apparatus is described for the measurement of photo processes in mass-selected cluster ions. A gridless reflectron type of time-of-flight (TOF) mass spectrometer is used to select an ion of known mass. The selected ion is photoexcited with a pulsed dye laser. The charged fragments are mass separated in a second linear TOF. A new combination of reflectron and linear TOF is described, which allows one to measure the complete photofragment mass distribution over an arbitrary large mass range for each laser shot, which was not possible earlier. It is discussed how clusters with different temperatures can be prepared. The first measurement of a temperature-dependent effect for mass-selected free cluster ions is presented.

Thermal bimolecular reactions of size selected transition metal cluster ions: Nb +n + O 2, n = 1–6

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, 5D) and 20% excited state (4d 3 5s 1, 5F). Collisional deactivation by He occurs at a rate constant ⪡ 1 × 10 −15 cm 3s −1.

Collision-induced fragmentation of small (CO 2) +n clusters

An apparatus for investigating the energy dependence of collision-induced fragmentation of internally cold, mass-selected molecular cluster ions is described. First results on the collisional energy dependence of the cross sections for fragmentation of small (CO) + n clusters in collisions with Ar atoms are presented. Interesting structure is observed in the cross sections and the results indicate that the collision occurs between the Ar atom and one CO 2 molecule within the cluster. This leads to either vibrational excitation of a loosely bound CO 2 monomer which then leaves the cluster or excitation of the entire cluster to a dissociative state.

Neutral mass-selected lead cluster beams

Neutral lead cluster beams with ultra-narrow size distribution were produced by neutralization of mass-selected lead cluster ions, Pb n + withn≤12, in sodium vapor under near-resonant conditions. Absolute charge exchange cross sections were measured as a function of cluster size and are on the order of 40 A2 forn≥4. Possible fragmentation of the clusters associated with charge transfer was examined by translational spectroscopy. No indication of fragmentation was found.

Study of clusters by mass spectrometry

A review of mass spectrometric methods used to study different kinds of clusters is presented. These methods have proved to be indispensable in the cluster field owing to their great flexibility, sensitivity and wide dynamic range. This reviews presents new mass spectrometric methods and techniques such as quadrupole analysers and RF lenses, different ion-trap geometries, time-of-flight (TOF) methods including the reflectron geometry). A variety of ionization methods, both for the production of positive and negative ions, with different amounts of released excess energy, are discussed in detail. Special attention is given to different degree of post-ionization fragmentaqtion and metastability of ions produced by the respective ionization methods. The ability to follow the metastable decay processes in rather different time-windows is also one of the salient features of the mass spectrometric methods reviewed here, involving both classical and new geometries. Detection of very large cluster ions by conventional methods is not always effective, and alternative methods like post-acceleration and using a conversion dynode in front of the multiplier will be included here. Moreover, mass spectrometric methods offer the possibility of producing beams of mass-selected cluster ions necessary for the study of a whole line of process like optical spectrometry, photodetachment, photofragmentation, photoionization, electron beam ionization, and charge-exchange processes. Examples of these processes are also discussed in this review.

Ultraviolet photoelectron spectra of gallium arsenide clusters

The ultraviolet photoelectron spectra (UPS) of mass-selected negative gallium arsenide cluster ions in the (2–50) atom size range was measured with a photon energy of 7.9 eV. The measured photodetachment thresholds displayed a strong even/odd oscillation through the largest clusters in this range, suggesting the presence of a substantial HOMO—LUMO gap in the corresponding neutral clusters which evolves to the band gap of bulk GaAs crystals.

Collisional excitation and fragmentation of alkali-metal halide cluster ions

Caesium iodide cluster ions have been formed by bombardment of the solid salt in vacuo with xenon atoms possessing kiloelectron volt (keV) kinetic energies, and the ions have been mass-selected on the basis of magnetic deflection. The mass-selected cluster ions with translational energies of ca. 10 keV have been allowed to collide with thermal inert and other gases, and the translational energy spectra of surviving cluster ions and of fragment ions have been measured using a cylindrical electric sector analyser. Cluster ions suffered large translational energy losses ΔE(e.g. 100 eV) in those collisions leading to fragmentation. Different fragment ions were associated with translational energy losses of different magnitudes. For a given fragment ion, the magnitude of the translational energy loss tended to be related to the mass of the collision gas, typically being larger for lighter gases. Translational energy losses tended to be greater in the case of larger incident cluster ions. The translational energy spectra are discussed in terms of direct momentum transfer in an impulsive collision.

Cluster/Surface Interactions and Cluster/Matrix Deposition

ABSTRACTThis paper reports the most recent results from the cluster chemistry program at the Naval Research Laboratory, in which our efforts in the characterization of gas-phase cluster properties have been extended to studies of condensed-phase species. First, in an attempt to investigate the fundamental interactions between mass-selected cluster ions and surfaces, two tandem mass spectrometers were constructed or modified, and the results of the initial experiments will be discussed. The emphasis will be on illustrating the general utility of ion/surface collisions to study fragmentations, reactions, and deposition. Second, clusters were deposited into a matrix in order to perform spectroscopic analyses. The initial experiments have been directed at optimization of the techniques. Finally, recent measurement of the ionization potentials of large carbon clusters will be reported. These results are especially significant because of the new developments in large-scale production, isolation, and characterization of these species.

Alkali-metal clusters as prototypes of metal clusters

This paper presents three experiments on mass-selected alkali-metal cluster ions: the unimolecular dissociation after photoexcitation in order to determine the binding energies, the photoabsorption cross-section in an extended energy range 0.6–4 eV, and the ionization potentials of doubly and triply charged clusters. These results are well understood assuming that the valence electrons of alkali-metal clusters constitute a Fermi electron gas in a potential well.

Neutral Mass-Selected Lead Cluster Beams

ABSTRACTExperiments are described which illustrate that resonant neutralization of mass-selected cluster ions provides a universal method to produce neutral mass-selected metal cluster beams. Specifically, lead cluster ions, Pbn+ with n d 12, were neutralized by charge exchange reactions in atomic sodium and other target materials. Absolute charge exchange cross sections are on the order of 40 Å2 and up to 30% of the incident ions were neutralized. Possible fragmentation of the clusters associated with neutralization was investigated by translational spectroscopy. No indication of fragmentation was found for resonant electron transfer with atomic Na as target gas.. Thus, neutral metal cluster beams with ultra-narrow size distribution are produced. Deposition of such size-selected metal clusters on substrates opens new perspectives in materials science.

Time-resolved measurements of the photodissociation and recombination dynamics of I−2 in mass selected cluster ions

We present picosecond time-resolved pump-probe measurements of the photodissociation and recombination dynamics of I−2 surrounded by a specific number of CO2 molecules in mass selected I−2 (CO2)n clusters. The transient bleaching data can be fit to an exponential absorption recovery of 30±10 ps in I−2 (CO2)9 clusters and 10±5 ps in I−2 (CO2)16 clusters. The data demonstrate the feasibility of measurements of real-time reaction dynamics in microsolvent environments.

Collective excitation in closed-shell potassium cluster ions

Photoabsorption spectra of mass-selected potassium cluster ions K 9 + and K 21 + were obtained by photoevaporation spectroscopy in the range 0.6 to 4.7 eV. A giant resonance due to the collective dipole oscillation of the valence electrons dominates the photoabsorption spectra. For these spherical clusters, experimental evidence for the Lorentzian shape of the resonance is obtained. A blue-shift and line narrowing are observed for K 21 +. The dependence of damping on cluster size is discussed.

Magnetic time-of-flight photoelectron spectrometer for mass-selected negative cluster ions

Design considerations and initial results are presented for a new type of time-of-flight photoelectron spectrometer which is particularly suited to the study of cold metal and semiconductor cluster anions prepared in a supersonic molecular beam. The desired cluster is extracted from the molecular beam, mass-selected after an initial time-of-flight, and decelerated as it enters the photoelectron spectrometer. Photoelectrons ejected from the cluster by an ArF excimer laser are collected with >98% efficiency in an intense pulsed magnetic field of carefully controlled divergence. This divergent field parallelizes the photoelectron trajectories and maps smoothly onto a low, uniform magnetic field which guides the electrons along a 234-cm flight tube leading to a microchannel-plate detector. The strong magnetic fields and simple, open design provide excellent rejection of stray photoelectrons in a clean, ultrahigh-vacuum environment. The UPS spectrum of Si20− is given as an example.