Ionization resonant multiphoton schemes

A When I was a graduate student, lasers were just being developed and physicists were calling them a solution in search of a problem. 1 had a clear idea what they might be good for. First 1 used them to take the first fluorescence spectra of molecules. Later 1 pioneered the use of laser-induced fluorescence and resonance-enhanced multiphoton ionization as detection schemes... 

Hgure 2 Survey of laser ionization schemes for gas-phase analytes single photon ionization (SPI), multiphoton ionization (MPI) with distinction between resonant ionization (Rl), resonance enhanced multiphoton ionization (REMPI), and nonreso-nant multiphoton ionization (NRMPI). Real and virtual states are denoted by R and S, respectively, while C refers to the ionization continuum. 

The most widely used of these tecluiiques is resonance-enlianced multiphoton ionization (REMPI) [ ]. A schematic energy-level diagram of the most conunonly employed variant (2 + 1) of this detection scheme is illustrated in the... 

Figure B2.5.16. Different multiphoton ionization schemes. Each scheme is classified according to the number of photons that lead to resonant intennediate levels and to the ionization continuum (liatched area). Adapted from [110].

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 photolysis of aromatic species with tetranitromethane in perfluoro alcohol solvent has been studied, in which the radical cations were observed by EPR spectroscopy.284 Photo-stimulated reaction of 1- and 2-haloadamantanes and 1,2- and 1,3-dihaloadamantanes with various carbanionic nucleophiles afforded products rationalized through an SrnI mechanism.285 286 Photolysis of the cycloadduct formed between a functionalized derivative of C6o and diazomethane has been shown to afford a pah of ling-opened structures (125) and (126) via a proposed biradical intermediate (127) (Scheme 19). The UV-photolytic fragments of /-butyl iodide (T and /-Bu ) have been ionized by resonance-enhanced multiphoton ionization for TOF mass spectro-metric analysis.287 A two-dimensional position-sensitive detector provided angular distribution and translational energy data. 

Since the second laser ionizes the excited atom in DLI, this step may be accomplished by an off-resonant photon (see Fig. 1). If a nitrogen laser-pumped dye laser provides the resonant photon, a fraction of the nitrogen laser beam can conveniently ionize the atom from the laser-excited state 14,15). It is theoretically possible to photoionize every atom in the periodic table except helium and neon using five RIS ionization schemes involving stepwise and/or multiphoton excitation4). Presumably these... 

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

We have used the laser resonance-enhanced multiphoton ionization (REMPI) method to probe the final state distributions of S atoms formed in the 193-nm photodissociation of several organosulfur molecules and radicals [58-60], Using the rate-equation scheme and calibrating with the known photodissociation cross sections for the formation of S( P2,i,o, >2) in the 193-nm photodissociation of CS2, we have estimated the absolute cross sections for the photodissociation of organosulfur radicals HS [59,60] and CH3S [58,60] leading to the formation of S( P2,i,o o)-... 

Figure 5.16 Scheme for state selection of ions by resonance enhanced multiphoton ionization. The Rydberg R and ion (X) states have the same geometry so that Av = 0 transitions dominate in the ionization step. Taken with permission from Baer (1986). 

The conformer selectivity in IR-UV double resonance methods relies on the excitation bands of different conformers being resolvable in the UV spectrum. For tmresolved UV spectra, indicating either short excited state lifetimes (<1 ns) or strong conformational heterogeneity, the IR-UV ion dip scheme can be modified by introducing either a femtosecond ionization laser (IR femtosecond multiphoton ionization) [80,81] or an additional IR step (IR-IR-UV spectroscopy) [21,82-84]. 

Figure 8 Multiphoton excitation and ion spectroscopy Bottom spectrum cold, mass selected UVspectmm (S, <- So transition) of fluorobenzene, providing wavelengths for efficient and selective ionization. Middle spectra photoelectron spectra induced by UV-resonance enhanced two-photon absorption. Choosing different intermediate states [S,(0,0) or S,(6b )] results in different populations of the final fluorobenzene radical cations. Top spectrum spectroscopy of the excited ionic state of the fluorobenzene radical cation measured by muKiphoton dissociation spectroscopy. The ions have been prepared via the neutral 0°o transition. A special excitation scheme has been used to optimize cation spectroscopy (for further details see text).

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