Electron Attachment Reactions and Photochemistry of Transition Metal Carbonyls in the Gas Phase and on Surfaces

Citation

George, Patricia Margaret (1981) Electron Attachment Reactions and Photochemistry of Transition Metal Carbonyls in the Gas Phase and on Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yp54-sc75. https://resolver.caltech.edu/CaltechTHESIS:05072018-173514299

Abstract

Mechanistic studies of photoassisted and electron-assisted deposition of thin metal films, from organometallic compounds at low pressures have been carried out. Various types of spectroscopy and microscopy are used to characterize the complex processes occurring during film formation. Chapter I gives an introduction emphasizing the importance of such an approach.

Chapter II details some of the experimental methods used in this study. The three major techniques which were used to evaluate metal films deposited in this manner were Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy and energy dispersive X-ray microprobe analysis. A summary of the methodology associated with each is given and background information is supplied for necessary measurements and calculations.

Chapter III details the photoassisted mechanism which involves three stages: initiation, propagation and termination of film growth. Initiation is thought to occur primarily by the formation and subsequent reaction of photoelectrons and by photodecomposition. At low pressures formation and reaction of photoelectrons probably dominates while at higher pressures photodecomposition becomes increasingly important. The organometallic compounds are thought to react with electrons by dissociative electron capture, forming negative ions that decompose further to produce thin metal films. Propagation appears to involve a thermally active catalyst capable of effecting a number of turnovers in the dark before termination occurs. The limiting step appears to be loss of remaining CO ligands and the metal species is on the surface.

Chapter IV examines the dissociative electron attachment reactions of transition metal carbonyls. Dissociative electron attachment rates are measured for the transition metal carbonyls V(CO)₆, Cr(CO)₆, Fe(CO)₅, Ni(CO)₄, Mo(CO)₆ and W(CO)₆. Rates are measured as a function of the pressure of CO₂ added to relax epithermal electrons. Derived thermal rate constants for the process M(CO)_n → M(CO)^-_n-1 + CO are 0.6, 3.0, 2.0, 2.0, 1.3 and 1.2 x 10^-7 cm³ molecule^-1 s^-1, respectively. The differences in these rate constants may be attributed to the different stabilities of the molecular anion with regard to dissociation versus autodetachment. The measured rate of thermalization of electrons by CO₂ varies with the metal carbonyl used and depends on the variation of the dissociative electron capture cross section with electron energy. Each system is thus tightly coupled in that the electron energy distribution is determined not only by collisional processes involving CO₂ but varies as well with the energy dependent depletion of the distribution by reactant species.

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