|Ph.D Student||Vainrot Nataly|
|Subject||Multi-Photon Ionization Technique for Direct Analysis of|
|Department||Department of Chemistry||Supervisor||Professor Israel Schechter|
|Full Thesis text|
The goal of this thesis was developing a new kind of spectroscopic analysis that can be used for nondestructive chemical characterization of solids under ambient conditions. This method, named multiphoton electron extraction spectroscopy (MEES), is based on laser-induced multiphoton ionization. Multiphoton ionization is a nonlinear process that takes place at high irradiation intensities when the irradiated material simultaneously absorbs several photons and gets ionized. It can be readily monitored by detecting either the photoelectrons or the generated ions.
In our experimental setup, various organic solids were laser-irradiated and the photoelectrons produced were separated by means of high electric field and collected using a positive electrode. The laser wavelength was tuned in a wide UV range and the photocurrent was registered as a function of wavelength and integrated over time. The resulting spectra can be used for qualitative characterization and quantitative measurements of the tested compounds. The use of such multi-wavelength technique enabled us to significantly enhance the sensitivity and the selectivity of the method.
We mainly focused on polycyclic aromatic hydrocarbons (PAHs), which are well-known hazardous compounds. The method was also successfully tested for other organic materials such as explosives, narcotic drugs, and airborne aerosols. Moreover, the MEES technique was shown to work in some medical applications such as early diagnosis of dental caries. In addition, it was shown that the method can be easily employed in the scanning mode for mapping the chemical composition of solid surfaces.
A series of measurements were conducted to establish the optimal parameters of the experimental setup, namely, the applied voltage and power density as well as the thickness of the analyte layer. As a result, the sensitivity and the spectral characteristics were notably improved. MEES spectra, though being similar to absorption spectra, have much more features and can be used for compound identification. Due to the tunable laser employed, we could perform excitation at the optimal resonance wavelength of the tested compound, which led to significant signal enhancement, higher sensitivity, and lower LOD values. In conclusion, a new type of spectroscopic method has been developed for the identification and characterization of solids in a wide UV range under ambient conditions.