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RISS 인기검색어
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- 저자 : Gunaratne, K. Don Dasitha (검색결과 1 건)
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Photoelectron spectroscopic investigations of transition metal silicides and oxides
Gunaratne, K. Don Dasitha The Pennsylvania State University 2012 해외박사(DDOD)
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This dissertation presents the experimental progress in the use of photoelectron imaging spectroscopy to probe the electronic structure of negatively charged transition metal silicides and oxides. By measuring the electronic transitions that occur due to the use of an appropriate photon energy, the electron affinity of the neutral species can be measured and the ground and excited electronic states of the anion and neutral can be deduced. The introductory chapter explains the basics of photoelectron spectroscopy and how it is coupled with recently developed imaging techniques to simultaneously obtain energy and angular distribution of the photodetached electrons. The beta parameter, which is instrumental in quantifying the angular distribution, is discussed in detail. The motivation for pursuing the transition metal silicide and oxide studies and their increasing importance in technology is also emphasized. Photoelectron imaging spectroscopy employed in our laboratory involves custom-built ultra high-vacuum instrumentation which couples a time-of-flight (TOF) mass spectrometer with an electron imaging apparatus. Chapter 2 discloses the details of the experimental setup required to conduct these studies; from the laser ablation techniques that form the species of interest, to the mass selection of the negatively charged clusters and the subsequent photodetachment of electrons from the highest occupied molecular orbitals of the anion. The modifications made to the safety features of the instrument, in addition to the improvements to the high-vacuum configuration, are described in detail. The first known photoelectron spectroscopic investigation of the ZrSi - diatomic species has been conducted as part of this dissertation. Chapter 3 discusses the results of the 532 nm and 355 nm wavelength experiments on ZrSi-. Conflicts between the two previous theoretical studies which proposed the ground state of the neutral has been noted while new electronic state assignments have been assigned based on experimental evidence. This study supports the 2Sigma+: (1&sgr;) 2 (1pi)4 (1delta)0 (2&sgr;) 2 (3&sgr;)1 anion ground electronic state (and valence orbital electron configuration) which is in agreement with theoretical results. The assigned ground state of the neutral, however, differs from that based on current theoretical results. The ground state of ZrSi was assigned as the 3Sigma+: (1&sgr;)2 (1pi) 4 (1delta)0 (2&sgr;)1 (3&sgr;) 1 based on the angular distribution and the relative intensities of the photoelectron signal. A low-lying excited state of the neutral was assigned as 3pii with valence orbital configuration, (1&sgr;) 2 (1pi)3 (1delta)0 (2&sgr;) 2 (3&sgr;)1. The electron affinity of ZrSi is measured as 1.584 eV, while a low-lying excited state of the neutral is identified 0.238 eV above the ZrSi neutral ground state. Additionally, Franck-Condon simulations were performed to compare with the experimental photoelectron spectrum and estimate the vibrational temperature of the anions created in our cluster source. Extending the transition metal silicide studies to other metals, Chapter 4 presents results of NbSi, MoSi, PdSi and WSi diatomic anions which were photodetached by photons with a wavelength of 532 nm. Similar to ZrSi, two major transitions originating from the anion were observed and the transition energies are reported. A significant finding of this study is the change of photoelectron angular distribution among the 4d-row transition metal-silicon diatomics. The main feature (X) changes from having an anisotropic distribution for ZrSi- and NbSi- to an isotropic distribution for MoSi- and PdSi-. The basis of this observed change is explored further and connections are made to the increasing stability of the transition metal d-orbitals when moving from Zr to Pd. Correlated with the metal d-orbital stability is the increased donation of silicon 3p-electrons to the highest occupied molecular orbitals of the diatomic. The atomic electron negativity values also support these conclusions. To the best of our knowledge, this study represents the first use of anion photoelectron imaging spectroscopy to examine the evolution of bonding trends in transition metal silicides. Investigations regarding transition metal oxides are presented from Chapter 5 onward. The photoelectron spectroscopic features of ZnOH- are discussed in a combined experimental and theoretical study. The vertical detachment energy of ZnOH- was measured to be 1.78 eV, while that of ZnO- is 0.3 eV higher. The curious broadening of the photoelectron spectra of ZnOH- when compared with ZnO - is investigated. The effects of the vibrational states and even rotational states on the photoelectron spectrum is considered in order to explain the experimental photoelectron signal. The vibrational spectrum could explain the broadening if the source temperature is much higher than the range we estimated in Chapter 3 (150-300 K). Also, as expected, the rotational states were not significantly effecting the broadening of the photoelectron spectrum. Due to the importance of ZnO-based material for dilute-magnetic semiconductors, ZnO-cluster formation, growth and stability has been investigated in Chapter 6. Large ZnO-based clusters have been observed with stoichiometries, Zn nOn, ZnnOn+1 and ZnnO n+2 (n=3-6). The ZnnOn+1 unit was concluded to be the relatively stable species and it has been proposed as a suitable building block for doping with magnetic atoms. The electron affinity of Zn3O 3 (1.14 eV) is lower than Zn3O4 (2.02 eV), supporting the argument that the Zn3O4 unit is more stable. Chapter 7 provides a summary of the research presented in this dissertation while sharing insights regarding possible avenues of extending this research.
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