Spectroscopy resonance-enhanced multi-photon ionization

REMPI resonance-enhanced multi photon ionization spectroscopy... 

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. 

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). 

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. 

As we have seen, collisions are important for the signal generation in LEI. In low-pressure experiments photoionization instead is the principal origin of the signal. The term Resonance Ionization Spectroscopy (RIS) is then frequently used. Several examples of opto-galvanic detection schemes for different atoms are shown in Fig.9.11. If multi-photon excitation of the atoms to be studied is used the technique is referred to as REMPI (REsonance Multi-Photon Ionization) spectroscopy. The selectivity of RIS and REMPI can be further enhanced by using a mass spectrometer to ana-... 

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