ICE | The Israel Chemist and Chemical Engineer | Issue 6

8 Scientific Article The Israel Chemist and Chemical Engineer Issue 6 · July 2020 · Av 5780 Dr. Valery Bulatov was born in1945 in USSR. He finished his PhD in 1979 in chemical physics at the Institute of Chemical Physics, Russian Academy of Science, Moscow. He immigrated to Israel in 1990 and joined the Technion. He serves as a senior researcher in the Faculty of Chemistry, Technion. He has published numerous papers in physical chemistry and analytical chemistry. In 2015, he was awarded the Hershel Rich Technion Innovation Award. Currently, he is interested in development of spectroscopy-based methods for analytical chemistry and environmental chemistry, including laser-induced breakdown spectroscopy (LIBS), contaminated water analysis, soil contamination analysis, aerosols and hydrosols analysis, multiphoton electron extraction spectroscopy (MEES), historical artifacts, analysis and identification and investigation of hazardous and highly explosive materials. A new analytical tool: Multiphoton electron extraction spectroscopy (MEES) Danny Fisher, Shisong Tang, Vladimir V. Gridin, Valery Bulatov and Israel Schechter* Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel *Email: [email protected] 1. Historical background The ionization potential of most atoms and molecules is above 10 eV, so that their ionization, using UV light (3-5 eV per photon), should not be possible. Nevertheless, when using short laser pulses of high photon flux, nonlinear processes can take place, such that several photons can be simultaneously absorbed, resulting in ionization. Nonlinear laser multi- photon ionization (MPI) processes have been investigated for many years is physics and chemistry. In parallel to basic understanding of MPI processes and the associated effects, studies of their potential utilization for chemical analysis took place. Since almost all investigations of MPI processes were carried out under ultra-high vacuum conditions, the first analytical applications were in mass spectrometry. Later on, potential analytical applications of MPI under moderate pressure were suggested and, only recently, MPI at ambient conditions was introduced. The MPI method is regarded as a well-established ionization technique in mass spectrometry, which leads to little molecular fragmentation [1]. This is an advantage for molecular identification. Therefore, it was used for ionization of large molecules [2] and was shown to be a very effective and sensitive technique [3]. Besides mass information, additional selectivity was achieved by tuning the excitation laser to the wavelength of resonant transitions [4-6]. Concerning analytical applications, the main drawback of this technology is that the signals are generated by accelerated masses under ultra-high vacuum conditions. It means that test samples must be introduced into the vacuum, where they are diluted. Laser ionization in mass spectrometry requires focusing the light beam to a small volume. Therefore, the number of generated ions is small, originating from only a small interaction volume. This is a considerable analytical drawback. Moreover, the setup, which included a mass spectrometer and a tunable laser, was very expensive; therefore, the method was never popular. Another approach to utilizing MPI in analytical chemistry was based on measurements under moderate pressure (in the mTorr range). This improved the detection limits of vapors, simply because the number of analyte molecules in the interaction volume was much higher than in high-vacuum. A setup was developed in which not only was the pressure higher but also the interaction volume was increased by several orders of magnitude [7-9]. Obviously, because of the much shorter mean free path at these pressures, the detection could not be mass spectrometric but was based on the time- dependent voltages induced during the short flight of the ions. This method was tested for direct monitoring of ambient gases. It was sensitive, but not as good as mass spectrometry for molecular identification [10]. The third category of MPI analytical applications, introduced in the last decade, was based on detection of photoelectrons under ambient conditions. This method turned out to be a highly sensitive analytical tool for various substances in various forms and matrixes. It was applied to solids [11- 16], liquids [17-18] and aerosols [19-23]. A pulsed UV laser