Ultraviolet multiphoton ionization

Figure 2 Incident Beam Ultraviolet Multiphoton Ionization Spectrum...

Figure 4 Scattered beam ultraviolet multiphoton ionization spectrum.

Three-photon absorption has also been observed by multiphoton ionization, giving Rydberg states of atoms or molecules [36]. Such states usually require vacuum ultraviolet teclmiques for one-photon spectra, but can be done with a visible or near-ultraviolet laser by tluee-photon absorption. 

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,... 

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. 

J. Schwarz, P. Rambo, J. C. Diels, Measurements of multiphoton ionization coefficients with ultrashort ultraviolet pulses, Applied Physics B 72, 343-347 (2001)... 

Photoionization time-of-flight mass spectrometry is used almost exclusively in all experiments described in this review. The ionizing laser sources have included excimer lasers for photon energies up to 7.87 eV and tunable ultraviolet sources up to 6.5 eV. In some early studies, multiphoton ionization was used, but it has become quite clear that this usually results in dissociative ionization. Such effects have been observed in many systems, ranging from Si49,5o ( 51 jQ jjjg transition metals. Thus single-photon ionization has... 

Various forms of radiation have been used to produce ions in sufficient quantitites to yield neutral products for subsequent analysis. In principle, it should be possible to use intense beams of UV below ionization threshold for this purpose. To date, however, efforts to collect neutrals from resonant multiphoton ionization (REMPI) have not succeeded. In one experiment, 1 mbar of gaseous -propyl phenyl ether was irradiated at room temperature with a 0.1 W beam of 266 nm ultraviolet (from an 800 Hz laser that gives 8 n pulses) concurrent with a 0.5 W beam at 532 nm. The beams were intense enough not only to ionize the ether in the mass spectrometer, but also to excite it so that it expels propene. After several hours of irradiation < 10% of the starting material remained. Production of carbon monoxide and acetylene (decomposition products of the phenoxy group) could be detected by infrared absorption spectroscopy, but the yield of neutral propene (as measured by NMR spectroscopy) was infinitesimal. 

Figure 12 Raman-REMPI (resonantly enhanced multiphoton ionization) spectrum of the °Qi(AJ = 0, AK = -2,K- 1) transitions of the Vie e2g) mode of benzene in a molecular beam. An energy-level diagram is shown for the double-resonance experiment. The ultraviolet source was tuned to 36,474 cm and the Raman wave-number calibration is adjusted to match the / = 6 line reported in Ref. 109. The expansion was 13% benzene in argon at 80 kPa and the sampling was done at XfD = 175 (D = 0.20 mm nozzle diameter) using pump and Stokes laser energies of 2 and 0.5 mJ. (From Ref. 117, with permission.)...

In 440 nm photodissociation of glyoxal using vacuum ultraviolet laser-induced fluorescence and resonance enhanced multiphoton ionization, Li et al. (2006) have confirmed the occurrence of process (III) by observing directly the internal energy in the H2 product. The v = 1 vibration level of the H2 product was the highest populated with rotational states from /= 0 to 9. Of the energy available to the H2 + 2CO products, they found 3.1% appears as rotational energy (in v = 1 state), 17.8% appears as the H2 (v = 1) vibration, and 46.8% appears as H2 (v = 1) translation. 

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