Laser-induced fluorescence , absorption

Important plasma diagnostics include Langmuir probes, optical emission spectroscopy, laser induced fluorescence, absorption spectroscopy, mass spectrometry, ion flux and energy analysis, and plasma impedance analysis. A plasma reactor equipped with several of these diagnostics is shown in Fig. 51 [35, 160]. A capacitively coupled plasma is sustained between the parallel plates of the upper (etching) chamber. The lower (analysis) chamber is differentially pumped and communicates with the etching chamber through a pinhole on the lower electrode. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

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... 

Solinova V, Kasicka V, Koval D et al (2004) Analysis of synthetic derivatives of peptide hormones by capillary zone electrophoresis and micellar electrokinetic chromatography with ultraviolet-absorption and laser-induced fluorescence detection. J Chromatogr B 808 75-82... 

The principal analytical methods for complex samples are those that separate the mixture by differential migration and then detect the separated components. The separation methods are chromatography, electrophoresis, and field flow fractionation the detection methods—which need not be selective but must be sensitive—include absorption, laser-induced fluorescence, electrochemistry, and mass... 

Clearly, direct techniques for measuring OH are needed that provide concentrations either at a point or over relatively restricted spatial scales. Two (absorption and laser-induced fluorescence) are direct, spectroscopic methods and two others (mass spectrometry and a radiocarbon method) rely on conversion of OH to another species that is measured. Each of these approaches and some of the intercomparisons that have been carried out are discussed briefly in the following sections. A good overview of these methods is found in a review by Eisele and Bradshaw (1993) and articles by Crosley (1994, 1995a, 1995b) and papers in a special issue of the Journal of the Atmospheric Sciences [52 (19), October 1, 1995]. 

Brauers, T., U. Aschmutat, U. Brandenburger, H.-P. Dorn, M. Hausmann, M. BeBling, A. Hofzumahaus, F. Holland, C. Plass-Dulmer, and D. H. Ehhalt, Intercomparison of Tropospheric OH Radical Measurements by Multiple Folded Long-Path Laser Absorption and Laser Induced Fluorescence, Geophys. Res. Lett., 23, 2545-2548 (1996). 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

The pump and probe pulses employed may be subjected to a variety of nonlinear optical mixing processes they may be prepared and characterized by intensity, duration, spectral band width, and polarization. They may arrive in the reaction chamber at a desired time difference, or none. The probe pulse may lead to ionizations followed by detections of ions by mass spectrometry, but many alternatives for probing and detection have been used, such as laser-induced fluorescence, photoelectron spectroscopic detection, absorption spectroscopy, and the like. 

Silylenes are short-lived intermediates, and their detection requires fast methods such as ultraviolet (UV)24 or laser-induced fluorescence spectroscopy.25 The characteristic absorption maxima in the UV-visible spectra of these species, which are assigned to n - p transitions of electrons at the silicon atom, were used as a fingerprint to prove the occurrence of silylenes in matrices or solution. In addition, these transient species, which under normal conditions are too short lived to be observed by a slow detection method such as infrared (IR) spectroscopy, can be isolated in inert hydrocarbon or noble gas matrices, thus allowing the accurate measurement of their IR spectra. 

The photolytic and probe pulses are colinear when they reach the sample. The photolytic pulse produces excited states and photofragments, and the probe pulse which follows closely behind must be used to analyse the concentration and/or the chemical nature of the transients. The major detection processes are known as laser-induced fluorescence (LIF) and multiphoton ionization (MPI). Transient absorptions can also be used in some cases, and this is similar to ps spectroscopy. 

Figure 8.12 Laser-induced fluorescence kinetics of dissociation reactions. I, reaction of Nal followed at the absorption wavelength of the excited molecule II, same reaction monitored at the absorption wavelength of the free Na atom III, reaction of NaBr, followed at the absorption wavelength of the excited molecule...

FIGURE 1. Polarization of laser induced fluorescence of 0H(X n) photodissociated from H2O in the 145-185 nm region. The absorption transition moment is perpendicular to the molecular plane corresponding to the transition A P-j-XlA]. The dissociated 0H(x2n) is also in the molecular plane, since the induced fluorescence intensity of OH is preferentially polarized along the Z axis (25) perpendicular to the molecular plane. The OH radical rotates on the H2O plane XY plane) after dissociation. The unpaired p-orbitals of excited H2O and of dissociated OH are perpendicular to the molecular plane. 

Some of the most powerful tools for in situ discharge diagnostics are optical (62). Plasma-induced emission spectroscopy, laser-induced fluorescence, laser absorption, and laser optogalvanic spectroscopy have all been... 

Laser-induced fluorescence (LIF) has also been utilized as a highly sensitive detection principle for CE [48-51]. However, while the LIF detector is now able to achieve zeptomole (10 21) detection limits, conventional derivatization techniques are inefficient at these exceptional levels [52]. Also, CE has successfully been coupled with mass spectrometry (MS) [53], nuclear magnetic resonance (NMR) [54, 55], near-infrared fluorescence (NIRF) [56, 57], radiometric [58], flame photometric [59], absorption imaging [60], and electrochemical (conductivity, amperometric, and potentiometry) [61-63] detectors. A general overview of the main detection methods is shown is Table 1 [64]. 

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... 

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