Process REMPI spectroscopy

The use of resonance enhanced multiphoton ionisation (REMPI) spectroscopy Hnked to time of flight mass spectroscopy has been demonstrated in on-Hne monitoring of combustion by-products in industrial flue gases [14] and in a research project dedicated to the analysis of coffee roasting processes [15]. REMPI is a highly... 

Quite often, monochromatic laser light is applied under conditions of high photon flux, to excite the species of interest efficiently. Under such circumstances, a process that utilizes -photon resonance in the first excitation step and requires additional m photons for final ionization (most frequently m = 1 is encountered) occurs with measurable probability and is of analytical relevance. The technique is called REMPI and uses, as noted, stepwise resonant excitation of an atom or molecule via stable intermediate energy levels. It is usually described as (n- -m)-REMPI spectroscopy the most frequently used (2 -b 1)-REMPI principle is depicted in Figure 5.4. 

Resonance-enhanced multiphoton ionization (REMPI) has proved to be a versatile ionization technique for MS offering a number of advantageous features in the field of chemical analysis. Since it makes use of substance-specific excited states for the ionization process, it involves UV spectroscopy of the molecule to be ionized. Thus, it enables ionization of preselected compounds, control of the degree of fragmentation and, for a large number of substances, a high ionization efficiency. These features require that the excited molecular state(s) involved in the REMPI process not be significantly depleted... 

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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