Strong-field photoionization of sputtered neutral molecules for chemical imaging and depth profiling

Photoionization of molecules sputtered from molecular thin films and biological cells has been achieved using laser radiation in the short-wavelength/low-intensity and the long-wavelength/high-intensity regimes. Data presented in this thesis suggest that molecules subjected to cluster ion beam bombardment are desorbed with lower internal energies and high translational kinetic energies than those subjected to atomic ion bombardment. In addition, data presented in this thesis shows that it is possible to significantly reduce molecular fragmentation induced by the laser field by increasing the photoionization wavelength. The combination of C60 primary ion sources with strong-field, long-wavelength photoionization is observed to increase the efficacy of chemical imaging and molecular depth profiling time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments.;Mass spectrometric examination of data taken from coronene thin films prepared by physical vapor deposition (PVD) comparing a 40 keV C60 + ion source with a 20 keV Au+ ion source reveals that photoionization of molecules sputtered by cluster ion beams result in more hydrogen-loss (Hn-loss) and less hydrocarbon-loss (CnHn-loss) molecular fragments than molecules sputtered by atomic ion beams. Due to the fact that Hn-loss fragments originate from a lower energy excited state than CnHn-loss fragments, these data suggest that the molecules sputtered by the 40 keV C60 + ion source contain less internal energy than those desorbed by the 20 keV Au+ ion source.;Photoionization of molecules sputtered from several model systems has been achieved using high field 125 fs pulses in the mid-infrared spectral range. By examining the photoionization mass spectra as a function of wavelength, it is apparent that the photoionization mechanism is changing from a non-adiabatic multi-electron excitation process to a process that involves tunnel ionization. Comparison of the photoionization mass spectra of coronene, guanine, adenine, dopamine, serotonin and histamine molecules all show decreased photofragmentation as the photoionizing wavelength is increased from 800 nm to 1450 nm. In addition, photodissociation is seemingly decoupled from laser power density at the long-wavelength limit around 1450 nm suggesting that increased power densities at longer wavelengths may provide 2 to 3 orders of magnitude increase in the sensitivity for detecting intact neutral molecules sputtered by 40 keV C60+ ions.;Increased sensitivity for detecting intact molecular ions by strong-field photoionization of cluster ion sputtered neutral molecules is discussed in terms of its significance to a variety of other methods, including freeze-etching and ion beam sputter removal of sample overlayers, aimed at increasing the efficacy of bioimaging with focused ion beams and mass spectrometry. The data presented in this thesis demonstrate that the removal of deposited water by sublimation does not adversely affect the morphology or the distribution of the molecules in the topmost surface layers of the patterned cholesterol thin films.;Additionally, the ability of C60 cluster ion beams to sputter clean biological surfaces to reveal obscured localized chemical information has been shown using similar patterned cholesterol PVD thin films covered by a layer of amorphous water ice. The cholesterol thin films are analyzed using ToF-SIMS chemical imaging prior to and following the removal of nanometers of amorphous water ice. In these experiments, the chemical information is maintained after sputtering with C60 as a result of minimal damage caused by the ion bombardment. Moreover, the surface damage produced by the preparation of the frozen cholesterol films was found to have a greater effect on the loss of cholesterol signal than the ion beam induced chemical damage resulting from the C60 cluster beam.;Lastly, molecular depth profiles of an organic thin film of guanine vapor deposited on a silver coated silicon substrate are obtained using a 40 keV C60+ cluster ion beam in conjunction with a reflectron type, ToF-SIMS instrument. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. (Abstract shortened by UMI.)...