Supersonic laser induced fluorescence

New to the fourth edition are the topics of laser detection and ranging (LIDAR), cavity ring-down spectroscopy, femtosecond lasers and femtosecond spectroscopy, and the use of laser-induced fluorescence excitation for stmctural investigations of much larger molecules than had been possible previously. This latter technique takes advantage of two experimental quantum leaps the development of very high resolution lasers in the visible and ultraviolet regions and of the supersonic molecular beam. 

Other techniques that have been used to determine polycyclic aromatic hydrocarbons in soil extracts include ELISA field screening [86], micellar elec-tr okinetic capillary chromatography [ 87], supersonic jet laser-induced fluorescence [88,89], fluorescence quenching [90], thermal desorption gas chromatography-mass spectrometry [81,90,100], microwave-assisted extraction [91], thermal desorption [92], immunochemical methods [93,94], electrophoresis [96], thin layer chromatography [95], and pyrolysis gas chromatography [35]. 

New fusion applications include the concept of production of intense negative ion beams ( ) (for neutral beam injection for heating and diagnostics in tokamaks or other magnetically confined plasmas (26, 28)) by using photodissociation to ion pairs (e.g. NaLi + hVyy Na + Li") in supersonic molecular beams. Another promising concept is the use of laser induced fluorescence to monitor very low tritium concentrations (as little as 10 Tj/cm ) under fusion reactor conditions (29). 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Figure 7. Schematic diagram of the supersonic beam apparatus which combines laser-induced fluorescence spectroscopy with time-of-flight mass spectrometry. Reproduced with permission from Ref [92a].

Interactions between aniline and neutral molecules (N2, H2, CH4 and the rare gas atoms, He, Ne, Ar and Kr) produce van der Waals complexes which are formed in a supersonic jet and are studied by laser-induced fluorescence spectroscopy177,178. 

The Sq->S2 absorption spectrum of nitrosobenzene in a supersonic jet has been measured indirectly by monitoring yields of photoeliminated NO." Photolysis of the furoxan (63) (Scheme 5) generates two molecules of NO and the acetylene (64)." At 17%, the yield is low, but this is the first example of the photoelimination of NO from a furoxan to have been reported. A method for detecting nitrobenzene has been developed in which laser photolysis is combined with laser-induced fluorescence of the resulting NO fragments." A similar approach to detecting 2,4,6-trinitrotoluene in soil and groundwater has also been described." ... 

A number of less commonly used analytical techniques are available for determining PAHs. These include synchronous luminescence spectroscopy (SLS), resonant (R)/nonresonant (NR)-synchronous scan luminescence (SSL) spectrometry, room temperature phosphorescence (RTP), ultraviolet-resonance Raman spectroscopy (UV-RRS), x-ray excited optical luminescence spectroscopy (XEOL), laser-induced molecular fluorescence (LIMP), supersonic jet/laser induced fluorescence (SSJ/LIF), low- temperature fluorescence spectroscopy (LTFS), high-resolution low-temperature spectrofluorometry, low-temperature molecular luminescence spectrometry (LT-MLS), and supersonic jet spectroscopy/capillary supercritical fluid chromatography (SJS/SFC) Asher 1984 Garrigues and Ewald 1987 Goates et al. 1989 Jones et al. 1988 Lai et al. 1990 Lamotte et al. 1985 Lin et al. 1991 Popl et al. 1975 Richardson and Ando 1977 Saber et al. 1991 Vo-Dinh et al. 1984 Vo- Dinh and Abbott 1984 Vo-Dinh 1981 Woo et al. 1980). More recent methods for the determination of PAHs in environmental samples include GC-MS with stable isotope dilution calibration (Bushby et al. 1993), capillary electrophoresis with UV-laser excited fluorescence detection (Nie et al. 1993), and laser desorption laser photoionization time-of-flight mass spectrometry of direct determination of PAH in solid waste matrices (Dale et al. 1993). 

Later workers accepted Porter s assignment without reservation [88], The gas phase spectrum was reproduced by Berry using phenyl azide as the precursor [25]. Laser-induced fluorescence attributed to triplet phenyl nitrene was observed upon pumping the 368 nm transition in ordinary gas phase experiments and in supersonic jet expansions where very highly resolved spectra could be generated [89a]. 

In the experimental studies of state specific NO2 unimolecular dissociation (Miy-awaki et al., 1993 Hunter et al., 1993 Reid et al., 1994, 1993), NO2 is first vibra-tionally/rotationally cooled to 1 K by supersonic jet expansion. Ultraviolet excitation is then used to excite a NO2 resonance state which is an admixture of the optically active and the ground electronic states. [It should be noted that in the subpicosecond experiments by Ionov et al. (1993a) discussed in section 6.2.3.1, a superposition of resonance states is prepared instead of a single resonance state.] The NO product states are detected by laser-induced fluorescence. Both lifetime and product energy distributions for individual resonances are measured in these experiments. A stepwise increase in the unimolecular rate constant is observed when a new product channel opens. Fluctuations in the product state distributions, depending on the resonance state excited, are observed. The origin of the dynamical results is not clearly understood, but it apparently does not arise from mode specificity, since analyses of... 

Ste2 Steimle, T.C., Gengler, J., Chen, J. A study of the A H / B —X lfi band systems of ealeium monohydride (CaH) using a supersonic molecular beam source and laser-induced fluorescence detection. Can. J. Chem. 82 (2004) 779—790. 

E.J. Breford, F. Engelke, Laser induced fluorescence in supersonic nozzle beams predissociation in the Rb2 C /7 and D /7 states. Chem. Phys. Lett. 75, 132 (1980)... 

Laser induced fluorescence (LIF) techniques in supersonic free jets can also yield useful information on the potential energy curves of open shell atomic systems. This type of studies has provided high quality data on tbe ground and excited states of NaAr type of molecules. The LIF technique was also successfully applied for probing the potential surfaces of XeF. The B<-X fluorescence excitation spectrum of XeF in a supersonic free jet is sufficiently simplified that rotational analysis and accurate vibrational spacing are readily obtained, overcoming the complexity of gas phase emission spectroscopy, mainly due in this case to isotopic richness of natural Xe. 

Laser-induced fluorescence also offers the possibility of looking at the solvation structure in its isolated form, that is, short-range order without the continuum contribution. This has attracted keen interest in supersonic molecular beam studies of van der Waals complexes (27-28). The successive additions of n solvating molecules B form a solvated species AB. Spectral shifts and lineshape changes in the absorption of A as it becomes AB, or in the laser-induced fluorescence decay of those consecutively... 

Figure 7.3 Apparatus for flash studies of fast reactions in molecular beams (1). Schematic drawing of a molecular-beam flash apparatus. The pump and probe pulses (see text. Section 4.2.4.3) are produced by a tunable dye laser, a beam-splitter, and a delay line, not shown in the figure (see Figure 7.4). The two pulses are recombined by the beam-splitter BS and sent coaxially into the molecular-beam apparatus. A supersonic jet of (e.g.) argon gas is generated by expanding the gas through a nozzle into a vacuum chamber (not shown), for time-of-flight measurements. This apparams was used in smdies of the dissociation of iodine molecules and subsequent recombination (see below. Section 7.3.4.1 and Ref. [17]). Monitoring was by laser-induced fluorescence (LIF).

The following radiative lifetimes i of single A Ai(0,V2,0)1 q(J = 3/2) levels of NH2 in the supersonic free jet were determined from the decay curve of laser-induced fluorescence (X is the excitation line) [2] ... 

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