Laser vaporisation

O Keefe, A. O., Ross, M. M. Baronavski, A. P. 1986 Production of large carbon cluster ions by laser vaporisation. Chem. Phys. Lett. 130, 18-19. 

Table 4.9 Characteristics of Au/oxide catalysts prepared by laser vaporisation.126...

Table 6.7 Activities of gold catalysts prepared by laser vaporisation.70...

Au/ZrC>2 catalysts are less active than Au/TiC>2 catalysts, whatever method of preparation is used deposition of colloidal gold,83,91 DP12 or laser vaporisation.70 Activity depends on the method used (Table 6.12), and appears to be due only to the presence of Au°. The reason for the difference between zirconia and titania is not understood Zr4+ is more difficult to reduce than Ti4+, so anion defects may be harder to form. The lattice structures also differ in monoclinic zirconia (baddleyite) the Zr4+ ion is unusually seven coordinate, and phase transitions into tetragonal and cubic structures occur at >1370 and >2570 K, respectively. However, the... 

Au/Ti02 formed by laser vaporisation of gold onto anatase suffered from low activity because the gold content was only 0.02%.13,28 The World Gold Council Au/TiC>2 catalyst performed better, showing 70% selectivity at 80% conversion (353 K). 

An interesting correlation is found between the kinetic stabilities of the fullerenes ] to C70. as determined by simple Hiickel theory HOMO-LUMO energy separations, and the intensities of photoionisation signals from carbon clusters produced by laser vaporisation of graphite. This correlation provides further circumstantial evidence that the observed C34 to C70 clusters are indeed fullerenes, closed carbon cages containing only five- and six-membered rings. 

Fig. 1. Photoionisation signals from carbon dusters produced by laser vaporisation of graphite. Selected experimental results from ref. [11.

Returning now to the graphite laser vaporisation results in fig. 1, we should first explain why we think it is appropriate to concentrate on kinetic, rather than thermodynamic, stability. In fact there are two main reasons for this, one of which is physical and the other theoretical. 

The mass spectrum of the smoke produced directly without further processing gives strong peaks at miz = 720 and 840. However, the material which is solvent extracted gives the spectrum shown in Fig. 1. This should be compared with the laser vaporisation results -- and shows that the fullerencs C (n = 62,64,66,68, etc.) are also present and apparently stable in air. 

Vibrational wavenumbers were predicted for NeHF and NeDF from ab initio calculations.685 ArnH3+ clusters produced by laser vaporisation showed a characteristic IR band near 350 cm-1.686 Ab initio calculations gave vibrational wavenumbers for HArF.CO and HKrF.CO,687 as well as for X. . . HKrCl, where X = N2, OC, HF. All of the latter are predicted to have higher vH-Kr wavenumbers than the parent monomer.688... 

Nowadays, carbon nanotubes can be produced by diverse techniques such as arc discharge [4-8], pyrolysis of hydrocarbons over catalysts [9- ll],laser vaporisation of graphite [12, 13], and by electrolysis of metal salts using graphite electrodes [14, 15]. The products exhibit various morphologies (e.g. straight, curled, hemitoroidal, branched, spiral, helix-shaped, etc.). 

If graphite/Co-Ni mixtures are used as a target during laser vaporisation at 1200°C, bundles or ropes of single-walled nanotubes are produced [13]. These usually appear to contain mainly (10,10) nanotubes (armchair tubes 13.8 A diameter see section 2.1.2) packed in a crystalline form. X-ray powder diffraction reveals that these ropes or bundles consist of 100-500 single-walled tubes... 

Laser vaporisation of h-BN under nitrogen at high pressure also leads to BN nanotubes, which apparently grow from disordered BN material [173]. In this case the tubes are poorly crystaUine, possibly due to dislocations and defects within the hexagonal framework. However, further high electron irradiation is capable of annealing such a defect [173]. 

Fig. 32. a Scanning force image of a bundle of tori (bundle of toroidal single walled nanotubes radius ca. 300 nm) generated by laser vaporisation of graphite [72]. The structure exhibits an apparent height of ca. 1.5 nm and width of 4-8 nm (Courtesy of C. Dekker). b Model of a toroidal 240-atom graphite structure (Courtesy of H. Terrones)... 

The presence of antimony trichloride in the products of laser vaporisation of PVC provide direct evidence for the production of volatile antimony trichloride. 

Laser vaporisation of chromium or molybdenum in a plasma containing ethyne results in preferential formation of MgC 2. Mass-selected photo-dissociation experiments show that these met-cars decompose by either metal atom or MC2 losses. 53 Chromium atoms react spontaneously with diazomethane in solid argon to yield Cr(CH2N2), Cr(CH2) and N2C1CH2. Addition of hydrogen to the matrix results in hydrogenolysis of the Cr-C bond.254... 

Gas phase spectroscopy of jet-cooled neutral molecules benefited from the development of laser-vaporisation and laser-desorption techniques as well as their coupling with a supersonic expansion. Improvements of spectroscopic procedures involving several lasers, e.g. the IRAJV double resonance spectroscopy, helped to collect information on weakly populated conformers. 

In laser vaporisation experiments, generating a plume , the laser s frequency may be synchronised with the resonance line of the element (analyte) to be analysed. The basic principles are (i) absorption of the radiation by the analyte (LAAS laser atomic absorption spectrometry) (ii) fluorescence (LIE, laser-induced fluorescence LEAFS) or (Hi) production of ionisation products (ions and electrons). LIF is an analytical method of high precision that is suitable for the measurement of diatomic species in the plume. Excitation spectroscopy or laser-excited fluorescence is not concerned with the spectral composition of the fluorescence but with how the overall intensity of emission varies with the wavelength of excitation. 

The structural unit [CAU] " is reported in the salt (Na )2[CAl4] . The product is made by laser vaporisation of an Al/C/Na composite target containing 75% Al, 5% C and 20% Nal by mass. A time-of-flight mass spectrometer was used to select specific ions and the product was analysed by photoelectron spectroscopy backed up by theoretical calculations. It contains a planar tetra-coordinate carbon atom. Such a pentaatomic unit is, of course, the smallest structural unit that may contain such an atom, making this product a compound... 

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