Mass-selected clusters

Viggiano A A, Arnold S T and Morris R A 1998 Reactions of mass selected cluster ions in a thermal bath gas Int. Rev. Phys. Chem. 17 147-84... 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

The deposition of mass and charge selected ions onto surfaces is underway but is in its infancy. How do the ions survive the collision with a surface This question has a myriad of answers depending on many variables and will have a future in investigative studies. A soft landing is now a possibility (280) and allows the potential spectroscopic investigation of trapped ions. So far no transition metal ions have been examined using this method but it is only a matter of time. Soft landings via inert gas matrices also have potential in the surface deposition of mass selected clusters. 

The kinetic method [42,43] is a relative method for thermochemical data determination which is based on measurement of the rates of competitive dissociations of mass-selected cluster ions. This method was introduced by Cooks [44] for proton affinity determination. Later, an extension of this method was proposed by Fenselau [45]. 

S. Fedrigo, T. L. Haslett, M. Moskovits, Direct Synthesis of Metal Cluster Complexes by Deposition of Mass-Selected Clusters with Ligand Iron with CO. J. Am. Chem. Soc. 1996, 118, 5083-5085. 

Figure 3. A schematic of the apparatus used to produce mass selected clusters.

We have recently developed a gas-phase ion chromatography technique and applied it to carbon cluster cations " " and anions""". A pulse of mass-selected cluster ions is injected into a high-pressure drift cell filled with 2-5 torr of helium. The ionic mobilities of different isomeric structures depend on their different collision cross-sections with He, and the isomers are therefore separated while drifting through the cell, under the influence of a weak electric field. The absolute value of the ionic mobility for a given cluster together with computer simulations often allows unambiguous determination of the cluster... 

The Fourier transform ion cyclotron resonance (FT-ICR) trapping of mass-selected cluster ions is an important emerging technique for the study of ion cluster reactivity. " This technique offers very high resolution and sensitivity. An alternative approach has been used by Brucat et al. who demonstrated that the reactivity of cluster ions can be studied in an experimental configuration identical to that used for the study of neutrals, except that ions are detected directly by pulsed extraction in the time-of-flight mass spectrometer. Other experiments " are exploring the reactions of mass-selected cluster ions in beam-gas-cell or drift-tube type configurations. This approach avoids the problems of mass overlap and offers a direct probe of cluster and cluster-adduct stabilities. For further experimental details, the reader is referred to the references. 

In the remainder of this section, we will therefore focus on the interaction of mass-selected cluster beams with solid surfaces. 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

Fig. 1.48. Absorption spectra of small gold clusters Au2, Au4, Aug, and Au2o in comparison to the spectra of gold atoms (lower left frame) and nanometer size gold islands (upper right frame). AU spectra were obtained with the cavity ring-down method. The mass-selected clusters were soft-landed onto an a-Si02 substrate. The gold nanoparticles were grown on the same substrate by thermal evaporation...

The interaction of metallic clusters with MgO (001) has been advanced by the experiments performed by Heiz et a/.146,147 In this approach, a thin film of MgO is grown on a support by oxidation of Mg, and this is followed by deposition of mass selected clusters. Due to the procedure employed, the MgO film can be prepared with either large or small amounts of defects. The possibility of performing chemical reactions on top of mass selected cluster deposited on a given support creates an invaluable bridge between theory and experiment for model-supported catalysts. 

Early interest in the properties of clusters arose from the study of the absorption properties of interstellar matter. The development of cluster beams has been traced [654] to 1956, when experimenters attempted to produce intense molecular beams at low temperatures. This required the use of cooled nozzles, with the implication that numerous slow collisions would occur within them. Mass spectroscopy then revealed a far more complex distribution of masses than anticipated, and the study of free clusters was born. It is only when clusters could be formed as beams that they became suitable objects for spectroscopic study in the laboratory, and progress in the physics of mass-selected clusters was thereafter very rapid. 

In experiments on beams of mass-selected clusters, the difficulty is that beams already contain extremely low densities of clusters (typically 108 clusters per cm3). After mass selection, the number of clusters will have... 

Farrar J M 1993 Electronic photodissociation spectroscopy of mass-selected clusters solvation and the approach to the bulk Cluster Ions ed C Y Ng and I Provis (New York Wiley)... 

Fragmentation of core excited clusters becomes already evident from mass spectra, where the mass distribution is concentrated at much smaller masses than estimated from the average cluster-size N) (cf. Fig. 9). In addition, it is shown in Fig. 11 that photoion yields of mass-selected cluster fragments reflect changes in electronic structure as a function of (N). However, these results give no specific information on the mechanisms that lead to stable product ions. 

However, as a result of the loss of parent ions during the diffraction exposure by these inelastic processes, each diffraction exposure is limited to ca 10-20 s after which the trap is reloaded with ions. To summarize, each diffraction cycle is composed of ion loading, mass selection, vibrational relaxation, annealing, e"-beam exposure, ion ejection, and a second identical sequence to record a detector background. In a typical experimental run over ca 4-5 h, the diffraction pattern is collected for ca 300-400 cycles representing an average over ca 3-4 x 10 mass-selected clusters. 

Fig. 4.42 Setup for studying the decomposition of mass-selected cluster ions...

The kinetic method is an approximate scheme to determine relative thermochemical properties based on the rates of competitive dissociation of mass-selected cluster ions. As an example, consider the proton-bound dimer system ... 

Molecular clusters offer an environment in which the size of the solvent cage surrounding a chromophore can be controlled to study the effect of solvation on reaction dynamics. In addition, when charged species are employed, this type of investigation can be accomplished for mass-selected clusters by using mass spectrometric techniques. 

Although rather good overall understanding has been reached, there still exist some unsolved problems. Experimentally one would like to go to much lower temperatures than the present 35 K, but no idea is available today how to measure the optical response large, cold clusters. Electron diffraction experiments on mass-selected clusters will soon be available [74], and similar results for inner shell excitation will probably also be done in the near future, especially when the new generation of free-electron lasers becomes available. 

---