Cluster ions, mass-selected

In the example depicted in Fig. 1.32, Ni4 cluster ions are selected from the initial nickel cluster distribution. Treatment of this beam of tetranuclear clusters with a defined pressure of carbon monoxide and subsequent mass spec-trometric analysis of the products reveals the formation of a series Ni4(CO) cluster ions. The mass spectrum in Fig. 1.32 was obtained by increasing the CO pressure gradually until no change in the product spectrum was observed anymore, i.e., when saturation of the cluster with carbon monoxide has occurred [31]. In this case, the highest molecular weight ion has the formula... 

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... 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Cheshnovsky O, Yang S H, Pettiette C L, Craycraft M J and Smalley R E 1987 Magnetic time-of-flight photoeieotron spectrometer for mass-selected negative cluster ions Rev. Sci. Instnim. 58 2131... 

Our approach is similar to that employed in research of free cluster ions in the gas phase, where various measurements are conducted on the cluster which is mass selected out of the size-distributed clusters generated by laser sputtering. Based on the chemical compositions of the isolated MFCs, we discuss the determining factors of core size in connection with the formation processes. Some core-size dependent properties of the MFCs are also presented. 

Figure 9.7 Temperature-programmed reaction (TPR) spectra for CO oxidation at a series of model catalysts prepared by the soft landing of mass-selected Aun and AunSr cluster ions on MgO(lOO) thin films which are vacancy free (typically 1 % of a monolayer), (a) MgO (b) Au3Sr (c) Au4 (d) Au8. Also shown is the chemical reactivity R of pure Aun and AunSr clusters with 1 < n < 9. (Reproduced from Ref. 21).

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. 

IDSRS) and the complementary resonance ion-dip IR spectroscopy (RID1RS) are useful mass selective techniques for the probing of cluster ground state vibrations. 

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

McElvany, S., Nelson, H. H., Baronavski, A. P., Watson, C. H. Eyler, J. R. 1987 FTMS studies of mass-selected large cluster ions produced by direct laser vaporization. Chem. Phys. Lett. 134, 214 219. 

Mass spectrometry involves the detection of charged particles, and, in the present case, a portion of the neutral cluster beam is ionized. Ionization essentially involves electronic excitation and occurs on the time scale of the order of 10 16 s (Haberland 1985 Mark 1987). The mass spectrometric detection of the ions is usually achieved on a microsecond time scale after the ionization event. As a result, the ionization process is taken to be time zero in the discussion of the processes which occur following the actual ionization of the neutral clusters, yet before the mass selection of the cluster ions. That is, the resulting cluster ion will incubate in the ionizer for microseconds before being accelerated into the mass filter. On that time scale, the cluster ion may lose monomer units, and the cation within the cluster may fragment or react chemically with the adjacent molecules. 

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]. 

For proton affinity determination, the kinetic method involves the formation of the proton bound heterodimer between the two bases whose affinities are to be compared. By tandem mass spectrometry, the appropriate cluster ion [BiHB2]+ is selected and its spontaneous or collisional dissociation is observed. As shown in Figure 4.16, the competitive dissociation leading to the two protonated monomers is analysed and the relative abundances of the monomers [BiH]+ and [B2H]+ are measured. From these abundances, the relative proton affinities of the two bases Bi and B2 can be calculated and the proton affinity of one of the two bases can be determined, if the proton affinity of the other is known. 

FIb.1. Schematic diagram of the apparatus. A continuous supersonic Hg/Ar expansion produces the clusters. They can be ionised by electron or i oton impact. An electric pulse between the plates IH) and PI accelerates ions into the time-of-fiight mass spearometer. The first time focus is at the slit of the mass selector, the second is at the detector II. The mass selector can be used to select one mass only, say Hgjo, which can subsequently decay in the free-flight region into Hg, + Hg due to internal excitation. The reflector can separate parent (n = 20) and daughter (r — 19) cluster ions. 

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