Photodissociation and photoionization of carbon and carbide clusters
Ticknor, Brian Wesley
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Clusters of carbon, noble metal (Cu, Au) carbide, and silicon carbide are produced in the gas phase as cations in a molecular beam using laser vaporization in a pulsed nozzle cluster source and are detected with time-of-flight mass spectrometry. Neutral clusters of carbon are produced in a similar source and are studied by vacuum ultraviolet photoionization mass spectrometry. Cationic carbon clusters are mass selected and photodissociated using the third (355 nm) harmonic of a Nd:YAG laser or the tunable output of a dye laser. Dissociation proceeds by elimination of C3, and resonance enhanced photodissociation (REPD) is used to collect spectra for C6 and C8 from 620 - 660 nm. Some transitions are detected which may aid in the future gas phase spectroscopy of these species. Mass selected carbides are studied by photodissociation at 355 nm. Small copper and gold carbide clusters with an odd number of carbons fragment by losing the entire carbon cluster, while species with an even number of carbons show significant loss of neutral C3. This even-odd CuC30. In the silicon carbide studies, Si3C is especially abundant in the mass spectrum, and is also a common fragment from larger clusters, suggesting that it has high relative stability. The Si7-10C clusters lose neutral Si2C and Si3C, which are concluded to be stable neutral leaving groups. Neutral carbon clusters (Cn, n = 2 - 15) produced in a laser vaporization molecular beam source are studied by vacuum ultraviolet (VUV) photoionization mass spectrometry using tunable VUV light from the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. The mass spectra, recorded by single photon ionization at different energies, reveal the distribution of neutral carbon clusters grown from a laser vaporization source. Ionization threshold spectra are recorded from 8 - 12 eV for C4-15, and comparison to theory allows the assignment of isomeric structures for some cluster sizes. The data suggests the presence primarily of a linear structure for C7, C9, C11, and C13, and of a cyclic structure for C10.