Laser-assisted secondary ion mass spectroscopy and its applications in practical surface analysis
Karahan, Mehmet Cem.
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This thesis presents the development and the implementation of a combined practical surface analysis platform. The combined system incorporates a Laser Desorption (and Ablation) Mass Spectroscopy (LDMS) system into a Time-of- Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) instrument under a single virtual control platform. This is accomplished via customized instrumentation control as well as data acquisition and analysis routines all implemented on the LabVIEW programming environment. The system is named Laser-Assisted Secondary Ion Mass Spectroscopy (LASIMS). The system has the flexibility of carrying out surface characterization and chemical imaging experiments utilizing both the ToF-SIMS and LDMS techniques, generating complementary information from the same area of the sample at (nearly) the same time. A subclass of LDMS is the powerful Matrix-Assisted Laser Desorption/Ionization (MALDI) spectroscopy technique, which now is used routinely in this system. The combined system offers many other practical uses and applications such as laser cleaning of thin metallic films coating an insulating sample to carry out rapid ToFSIMS analysis of such samples, as well as laser-induced enhancement of secondary ion detection of radionuclides, such as 137Cs and U, and the study of isotopic Oxygen exchange on reactive oxide mineral surfaces, which are all detailed in the thesis. We have also tested the fundamental question of whether or not laser and particle beam excitations couple during the desorption and ionization process. The test basically utilizes a short (few ns) laser pulse to bring the surface of the sample to just under laser desorption/ionization threshold by means of local heating and/or photochemical processes, while a focused Ga+ beam that is well synchronized (temporally and spatially) with the laser pulse is used to further energize the sample surface (LASIM excitation) to yield additional molecular fragments from that surface. We observed no additional fragments as a result of LASIM excitation, suggesting no coupling between particle and laser desorption/ionization mechanisms.