FUNDAMENTAL STUDIES OF MOLECULAR SECONDARY ION MASS SPECTROMETRY IONIZATION PROBABILITY MEASURED WITH FEMTOSECOND, INFRARED LASER POST-IONIZATION
Open Access
- Author:
- Popczun, Nicholas James
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 01, 2017
- Committee Members:
- Nicholas Winograd, Dissertation Advisor/Co-Advisor
Nicholas Winograd, Committee Chair/Co-Chair
Barbara Jane Garrison, Committee Member
Philip C. Bevilacqua, Committee Member
Themis Matsoukas, Outside Member - Keywords:
- secondary ion mass spectrometry
secondary neutral mass spectrometry
mass spectrometry
time-of-flight
ionization
laser post-ionization
femtosecond
organic
surface science
ionization probability - Abstract:
- The work presented in this dissertation is focused on increasing the fundamental understanding of molecular secondary ion mass spectrometry (SIMS) ionization probability by measuring neutral molecule behavior with femtosecond, mid-infrared laser post-ionization (LPI). To accomplish this, a model system was designed with a homogeneous organic film comprised of coronene, a polycyclic hydrocarbon which provides substantial LPI signal. Careful consideration was given to signal lost to photofragmentation and undersampling of the sputtered plume that is contained within the extraction volume of the mass spectrometer. This study provided the first ionization probability for an organic compound measured directly by the relative secondary ions and sputtered neutral molecules using a strong-field ionization (SFI) ionization method. The measured value of ~10-3 is near the upper limit of previous estimations of ionization probability for organic molecules. The measurement method was refined, and then applied to a homogeneous guanine film, which produces protonated secondary ions. This measurement found the probability of protonation to occur to be on the order of 10-3, although with less uncertainty than that of the coronene. Finally, molecular depth profiles were obtained for SIMS and LPI signals as a function of primary ion fluence to determine the effect of ionization probability on the depth resolution of chemical interfaces. The interfaces chosen were organic/inorganic interfaces to limit chemical mixing. It is shown that approaching the inorganic chemical interface can enhance or suppress the ionization probability for the organic molecule, which can lead to artificially sharpened or broadened depths, respectively. Overall, the research described in this dissertation provides new methods for measuring ionization efficiency in SIMS in both absolute and relative terms, and will inform both innovation in the technique, as well as increase understanding of depth-dependent experiments.