Ultraviolet laser ionization spectroscopy

Figure 5 Vibrational and rotational temperatures of hydrogen desorbing from Pd(l 0 0) as a function of the surface temperature. The experimental results have been determined by tunable vacuum ultraviolet laser ionization spectroscopy for D2 while the theoretical results have been derived from six-dimensional quantum calculations for H2 (after [50]).

Excitation of very high-lying states close below the ionization limit, e.g., by ultraviolet lasers or by two-photon absorption, enables the detection of absorbed laser photons by monitoring the ions. Because of the high collection efficiency of these ions, ionization spectroscopy represents the most sensitive detection method, superior to all other techniques in all cases where it can be applied. Its experimental drawback is the need for two lasers, including at least one tunable laser. 

In atomic spectroscopy, a substance is decomposed into atoms in a flame, furnace, or plasma. (A plasma is a gas that is hot enough to contain ions and free electrons.) Each element is measured by absorption or emission of ultraviolet or visible radiation by the gaseous atoms. To measure trace elements in a tooth, tiny portions of the tooth are vaporized (ablated) by a laser pulse1 and swept into a plasma. The plasma ionizes some of the atoms, which pass into a mass spectrometer that separates ions by their mass and measures their quantity. 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

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. 

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. 

Xu, H., Que, G., Yu, D., Lu, J.R., (2005),Characterization of petroporphyrins using ultraviolet-visible spectroscopy and laser desorption ionization time of flight mass spectrometry. Energy and Fuel, 19,517-524. 

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