Laser-induced fluorescence measurements

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A22 state. The v" = 0 to r = 0 transition is around 308 nm and the v" = 0 to v = 1 at 282 nm. Two schemes have been used excitation using 282 nm into v = 1 of the upper electronic state, or excitation using 308 nm into v = 0 of the upper state. Collisional quenching deactivates some of the v = 1 into u = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v = 1 of the upper state into v" =1 of the lower state). Collisional deactivation of v = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm... 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

Figure 18. Experimental arrangement used in the author s laboratory to measure laser-induced fluorescence signals from flame species...

The most straightforward fluorescent technique is simply the measurement of the fluorescence intensity. Laser-induced fluorescence (LIP) is one of the most sensitive detection methods known. In LIP, the fluorescence intensity excited hy a laser beam is used to quantify the amount of fluorophore present. When combined with a separation technique such as capillary electrophoresis, LIP is a powerful bioanalytic method. Detection limits can reach less than 100 femtomolar and potentially the single-molecule level (reviewed in Reference 17). 

Very high sensitivity can be obtained by measuring laser induced fluorescence (LIF). In most analyte mixtures, not all the components are naturally fluorescent. Thus, derivatisation of the analytes with a fluorescent marker is necessary for detection. It is essential, that all sample components are homogeneously derivatised. Sometimes derivatives are not very stable, so care has to be taken. 

FIGURE 9.6 Overview of the experimental apparatus for measuring laser-induced fluorescence and photodissociation action spectra of gas phase ions stored in a 3-D quadrupole ion trap. Optical component abbreviations Lenses (L1-L4) P-BaBj04 crystal (BBO) Mirrors (M1-M8) Shutter (Sh) Irises (Irl-lr2) Pinholes (PH1-PH2) Brewster-angle windows (BWl-BW2) Neutral density filter (ND) Long-pass filter (LP). L3 is inside the ring electrode (see Figure 9.7). 

This was demonstrated by Schade [1482], who measured laser-induced fluorescence spectra of diesel oil and gasoline, excited by a nitrogen laser at X = 337.1 nm. Time-resolved spectroscopy exhibits two different lifetimes that are determined by measuring the intensity ratio of the LIF in two different time windows (Fig. 10.25). This time-resolved spectroscopy increases the detection sensitivity for field measurements. Mineral-oil pollutions of 0.5 mg/1 in water or 0.5 mg/kg in polluted soil can be detected [1482]. Using photoacoustic spectroscopy with a dye laser, concentrations down to 10 -10 mol/1 of transuranium elements could be detected in... 

Since in densely populated industrial areas air and water pollution has become a serious problem, the study of pollutants and their reactions with natural components of our environment is urgently needed [15.75]. Various techniques of laser spectroscopy have been successfully employed in atmospheric and environmental research direct absorption measurements, laser-induced fluorescence techniques, photoacoustic detection, spontaneous Raman scattering and CARS (Chap. 8), resonant two-photon ionization, and many more of the sensitive detection techniques discussed in Chap. 6 can be applied to various environmental problems. This section illustrates the potential of laser spectroscopy in this field by some examples. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

In laser vaporisation experiments, generating a plume , the laser s frequency may be synchronised with the resonance line of the element (analyte) to be analysed. The basic principles are (i) absorption of the radiation by the analyte (LAAS laser atomic absorption spectrometry) (ii) fluorescence (LIE, laser-induced fluorescence LEAFS) or (Hi) production of ionisation products (ions and electrons). LIF is an analytical method of high precision that is suitable for the measurement of diatomic species in the plume. Excitation spectroscopy or laser-excited fluorescence is not concerned with the spectral composition of the fluorescence but with how the overall intensity of emission varies with the wavelength of excitation. 

This has been demonstrated by Schade [15.79], who measured laser-induced fluorescence spectra of diesel oil and gasoline, excited by a nitrogen-laser at A = 337.1 nm. Time-resolved spectroscopy exhibits two different lifetimes that are determined by measuring the intensity ratio of the LIF in... 

Altkorn R and Zare R N 1984 Effects of saturation on laser-induced fluorescence measurements of population and polarization Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

Wilkerson C W Jr, Goodwin P M, Ambrose W P, Martin J C and Keller R A 1993 Detection and lifetime measurement of single molecules in flowing sample streams by laser-induced fluorescence Appl. Phys. Lett. 62 2030-2... 

Direct Measurement of HO, in the Troposphere. Techniques to measure tropospheric concentrations of HO have been reviewed (O Brien Hard, submitted to Advances in Chemistry, 1991) so only a summary will be given here. The most extensively researched technique for [HO ] measurement in the troposphere is based on laser-induced fluorescence (LIF) of HO. This approach has been developed in many configurations directing the laser into the free atmosphere and collecting fluorescence back scatter (LIDAR) (105,106,107) LIF of air sampled at atmospheric pressure... 

Hard et al (reference 110, 125, and submitted to/. Geophys, Res. 1991) have developed a system for the chemical conversion of HO2 to HO via the reaction HO2 + NO —> HO -I- N02. The hydroxyl radical is then measured by their low-pressure laser-induced-fluorescence instrument. Their multi-sample-channel LIF PAGE system is thus capable of simultaneous measurements of [HO ] (directly) and [H02 ] (by conversion to HO ). 

George, L.A. Development of a Direct, Low Pressure, Laser-induced Fluorescence Technique for NO2, Ambient Measurements and Urban NO, Ph.D. Thesis, Portland State University Portland, OR, 1991,1-135. 

Laboratory work involved making calibration curves which show the response of the system for various concentrations of pollutant, e.g., phenol. Typically, remote laser-induced fluorescence measurements from both the laboratory apparatus and the mobile unit are made on... 

In studies of molecular dynamics, lasers of very short pulse lengths allow investigation by laser-induced fluorescence of chemical processes that occur in a picosecond time frame. This time period is much less than the lifetimes of any transient species that could last long enough to yield a measurable vibrational spectrum. Such measurements go beyond simple detection and characterization of transient species. They yield details never before available of the time behavior of species in fast reactions, such as temporal and spatial redistribution of initially localized energy in excited molecules. Laser-induced fluorescence characterizes the molecular species that have formed, their internal energy distributions, and their lifetimes. 

There are many nonintrusive experimental tools available that can help scientists to develop a good picture of fluid dynamics and transport in chemical reactors. Laser Doppler velocimetry (LDV), particle image velocimetry (PIV) and sonar Doppler for velocity measurement, planar laser induced fluorescence (PLIF) for mixing studies, and high-speed cameras and tomography are very useful for multiphase studies. These experimental methods combined with computational fluid dynamics (CFDs) provide very good tools to understand what is happening in chemical reactors. 

Figure 4. Experimental laser-induced fluorescence, upper plot, and calculated spectra, lower plot, of the linear He P Cl feature in the ICl B—X, 3-0 region. An P Cl(X,v" = 0) rotational temperature of 0.19 K was measured for the experimental spectrum, and a temperature of 0.20 K was used in the calculations. Adapted from Ref. [51].

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

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