Resonance-enhanced Multiphoton Ionization REMPI

For ion TOF measurement a probe laser was used to ionize reaction products in the reaction zone. The (1 + F) resonance-enhanced multiphoton ionization (REMPI) method was adapted for H-atom detection. The necessary vacuum ultraviolet (VUV) radiation near 121.6 nm (for Lyman-a transition) can readily be generated by a frequency-tripling technique in a Kr cell.37 The sensitivity of this (1 +1 ) REMPI detection scheme is extremely high owing to the large absorption cross-section of Lyman-a transition,... 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

In resonance-enhanced multiphoton ionization (REMPI, also commonly referred to as resonance ionization—RI) near-UV photons can be used for ionization [60]. When... 

Resonance splitting, 38 447 Resonantly enhanced multiphoton ionization (REMPI),46 147... 

The vibration spectrum of the first excited state of guanine was measured using laser desorption jet-cooled resonance-enhanced multiphoton ionization (REMPI) spectrometry <1999JA4896>. The millimeter wave spectrum of purine was collected using a free jet spectrometer, and the observed rotational spectrum was assigned to the N(9)-H tautomer <1996CPL189>. 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the... 

Photolysis of the dimer, reaction (44), proceeds primarily via generation of Cl + ClOO (Cox and Hayrnan, 1988 Molina et al., 1990). For example, Molina et al. (1990) reported the quantum yield for this channel at 308 nm to be unity, with an uncertainty of 30%. Okumura and co-workers (Moore et al., 1999) and Schindler and co-workers (Schmidt et al., 1998) have reported that the quantum yield is less than 1.0. For example, Schmidt et al. (1998) used resonance-enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry to follow the production of oxygen and chlorine atoms as well as CIO in vibrational levels up to v" = 5 in the photolysis of the dimer. At a photolysis wavelength of 250 nm, the quantum yield for chlorine atom production was measured to be 0.65 + 0.15, but CIO was not observed. Assuming that all of the excited dimer dissociates, this suggests that the production of CIO in vibrational... 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Recently, new 2D-methods for the analysis of complex mixtures have been developed for time-of-flight mass spectrometry (22), which could also be utilized in external ionization FTMS. Specifically, the combination of IR-laser desorption of nonvolatile neutrals, followed by adiabatic cooling to 2°K in a supersonic jet, and subsequent compound-selective Resonance-Enhanced Multiphoton Ionization (REMPI) could increase the role of FTMS in the analysis of biological mixtures. The coupling of supersonic jets to the external ion source would also be of interest in ion- and neutral cluster experiments. 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

The isomeric o-, m- andp-chloroaniline, which under El give identical spectra, have been characterized by means of resonance-enhanced multiphoton ionization (REMPI) combined with a linear TOF mass spectrometer151. The method is based on the difference the three isomers display in the shapes of their metastable parent ion decays. 

This section will focus on the structure and energetics of chiral molecular complexes studied with Fourier-transform IR (FT-IR), microwave, LIF, hole burning (HB), IR fluorescence dip spectroscopy, resonance-enhanced multiphoton ionization (REMPI Fig. 5), and RET spectroscopy. 

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