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Reactions of niobium clusters and photodissociation of van der Waals complexes between NO and pDFB

Authors Wu, Qifei
Issue Date 1998
Summary Niobium cluster ions, Nbx+ (x = 2-15) and Nbx- (x = 5-l6), are produced in a laser vaporization/supersonic expansion cluster ion source. Their reactions with NO and NO2 are studied in a fast flow reactor. The difference mass spectra for the reactions demonstrate that dissociative chemisorption and molecular adsorption take place in different cluster size regimes. Detailed analysis of the product formation mechanism suggests that the cluster cations are fragmented into two parts in the small cluster range: the nitrogen retained cluster cations, NbxN+, and the neutral niobium oxide block, NbO. The relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size. Reactions of the anionic clusters with NO and NO2 are discussed in terms of electron transfer. Fragmentation of the clusters is extensively observed. The reactions of Nbxplus or minus with n-butane and isobutane are also studied. The relative reactivity of the cluster cations shows a decreasing trend with a zigzag shape as a function of cluster size. Reaction products are observed within the small cluster size range (x = l-6), and dehydrogenation from the butane molecules is found to be the dominant channel. The degree of dehydrogenation increases with cluster size (x = l-6), but after x ≥ 7, dehydrogenation reactions suddenly stop, and are replaced by molecular adsorption on large cluster cations. The degree of dehydrogenation for isobutane is found to be larger than that for n-butane and the product distribution for isobutane is found to be more diverse than that for n-butane. The results are explained by a prompt insertion mechanism. The reactivity of niobium cluster anions towards butanes is found to be lower than that of cluster cations. A reasonable structure and formation mechanism for an unusual product anion, NbC4H5 - is proposed. Photodissociation and photodetachment of van der Waals complex ions, [(NO)x(N2O3)y (pDFB)zplus or minus (pDFB = p-difluoro-benzene), are studied by the combination of supersonic expansion/electron impact ionization and time-of-flight mass spectrometry. For the positive cluster ions, [(NO)x(N2O3)y(pDFB)z +, the photodissociation fragment patterns at λ = 456 nm and λ = 700 nm are compared and discussed. At 456 nm, photodissociation of the cluster cations forms abundant fragments, whereas at 700 nm, photodissociation reactions are much suppressed. The result is explained in terms of accessing different excited states. The dependence of the photofragment ion intensities on cluster size is also discussed. Photodissociation and photodetachment of van der Waals complex anions at λ = 456 nm are compared, showing the competition of the two channels. For (NO2...pDFB)-, only photodissociation channel is observed. A resonance structure or a chemical bond is suggested to be responsible for the relative high electron affinity for this complex.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 1998
Language English
Format Thesis
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