Resonance-enhanced multi-photon ionization

REMPI Resonantly Enhanced Multi-Photon Ionization... 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

Therefore, heterogeneous catalysts present a greater potential for the application of HT and Combinatorial methods, because they involve diverse compositional phases that are usually formed by interfacial reactions during their synthesis, which in turn produce a variety of structural and textural properties, often too vast to prepare and test by traditional methods. In this respect the HT and Combinatorial methods extend the capabilities of the R D cycle, which comprises the synthesis, the characterization of physicochemical properties and the evaluation of catalytic properties. The primary screening HT method gives the possibility of performing a rapid test of hundreds or thousands of compounds using infrared detection methods [27-29]. Alternatively, a detection method called REMPI (Resonance Enhanced Multi Photon Ionization) has been used, which consists of the in situ ionization of reaction products by UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam [30, 31]. 

The recently introduced atmospheric-pressure laser ionization system (APLI) can be considered as a modification of APPI (Ch. 5.7.3). In APLI, the one-step photoionization of APPI is replaced by a two-photon process in resonantly-enhanced multi-photon ionization [148]. Enhanced response for polycyclic aromatic hydrocarbons (relative to APCI) was demonstrated. Molecular ions rather than protonated molecules are generated in APLI (cf. Ch. 6.5). 

Two-color, Resonance Enhanced Multi-Photon Ionization (REMPI) spectroscopy is similar to OODR, differing only in that the... 

Conventional photoelectron spectroscopy uses a rare-gas discharge lamp to produce radiation at the wavelength of the He 2p <— Is atomic transition (hu = 21.218 eV). Synchrotron radiation is now widely used for PES because its photon energy is widely tunable yet monochromatic. The initial state, in the first PES experiments, has been the molecular ground state but now, by exploiting Resonance Enhanced Multi-Photon Ionization (REMPI) excitar tion/detection schemes (see Section 1.2.2.3), any excited state of the molecule can be used as the initial state for PES (for a review, see Pratt, 1995). 

REMPI resonance-enhanced multi photon ionization spectroscopy... 

Koizumi, M., Sakamoto, T. (2008) Resonance enhanced multi-photon ionization of neutral atoms sputtered with Ga-FIB. Appl. Surf Sci., 255, 901-904. 

The two beams intersect at 90° inside the interaction region with t3rpical vacuum pressures of 10 Torr. The beams cross between plates that form a weak uniform electric field of 200V/cm. The crossing of the molecular and atomic beams is further intersected by an ionization laser. The ionization laser(s) ionizes the molecules using resonance enhanced multi-photon ionization, REMPI. Once ionized the molecules are extracted from the interaction region by the uniform electric field into the Velocity Mapped Ion-Imaging system. 

Matsumoto, J. Misawa, K. Idiiachi, S.I. Suzuki, T. Hayashi, S.I. Pujii, M. On-Site and Real-Time Mass Spectrometer Utilizing the Resonance-Enhanced Multi photon Ionization Technique. Shinku 2007, 50, 241-245. 

Thanner, R. Oser, H. Grotheer, H.-H. Time-Resolved Monitoring of Aromatic Compounds in an Experimental Incinerator Using an Improved Jet-Resonance-Enhanced Multi-Photon Ionization System Jet-REMPI. Eur. Mass Spectrom. 1998,4,215-222. 

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

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