Emission spectroscopy laser-induced fluorescence

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... 

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... 

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. 

Optical spectroscopies. These techniques are the least intrusive in situ plasma diagnostic methods. The most commonly used techniques are emission spectroscopy, absorption spectroscopy, laser-induced fluorescence. 

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. 

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. 

Graphite furnace AAS Atomic fluorescence spectroscopy Inductively-coupled-plasma optical-emission spectroscopy Glow-discharge optical-emission spectroscopy Laser-excited resonance ionization spectroscopy Laser-excited atomic-fluorescence spectroscopy Laser-induced-breakdown spectroscopy Laser-induced photocoustic spectroscopy Resonance-ionization spectroscopy... 

The alkaline earth hydroxide molecules were first made in flames and studied by emission spectroscopy [15]. Metal salt solutions are aspirated into atmospheric pressure flames or the salts are placed on a loop of wire in the flame. The MOH (M is an alkaline earth metal) molecules form through a complicated set of flame reactions [18]. More recent flame work has included laser-induced fluorescence studies of CaOH [29] and SrOH [30], Some of this work is motivated by the observation that soot formation is suppressed in flames when alkaline earths salts are added [31]. 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

Laser-induced fluorescence photofragment spectroscopy studies with state-selective interfragment recoil measurement have not been performed to any appreciable extent. A preliminary kind of experiment has recently been carried out in an MPD case by using the time-of-flight technique under bulk condition and with rather limited spectral resolution. A Doppler spectroscopy study has been carried out in a one-photon dissociation case where electronically excited fragments were produced under bulk condition, and linewidth measurement was made in emission interferomet-rically. A first experiment with tunable LIF detection has been made recently by McDonald et al., who dissociated HN3 at 266 nm and... 

At this point it is relevant to note the terminology employed in this chapter the expression laser-induced fluorescence (LIF) is used as a general term describing any fluorescence that is excited using a laser. A fluorescence excitation spectrum shows fluorescence emission yield as a function of excitation wavelength that is, it is similar to an absorption spectrum when that absorption results in radiative emission. It is noted that some authors reserve LIF as a synonym for fluorescence excitation spectroscopy. Dispersed fluorescence refers to dispersion of the emitted fluorescence light into its component wavelengths, that is, production of an emission spectrum. 

In the molecular beam, species selective absorption spectroscopy of a-Naphtol (NH3)n clusters was performed by one-colour resonant two-photon-lonlzatlon (R2P1). Based on the Identification of the spectral features of each complex, the laser Induced fluorescence (LIF) emission spectra were then measured as a function of the cluster size. The search for proton transfer in these clusters Involved the monitoring of fluorescence emission from the undissociated a-Naphtol and/or the Naphtolate anion. 

A combination of laser-induced fluorescence with classical absorption and emission spectroscopy has yielded a wealth of data on the excited electronic states of CeO. However, as noted by Linton et al., the spectroscopic data of upper states is less complete and are shown in table 54. Linton et al. (1983a, b) used empirical notation taken from atomic spectroscopy to designate these states. 

Lasers represent a special type of light source [16], [21], [60], [61]. They are used in trace analysis by fluorescence measurement or laser-induced fluorescence (LIF) (- Laser Analytical Spectroscopy) [62] - [64], in high-resolution spectroscopy, and in polarimetry for the detection of very small amounts of materials. Lasers can be of the gas. solid, or dye type [21]. In dye lasers, solutions of dyes are pumped optially by another laser or a flash lamp and then show induced emi.s-sion in some regions of their fluorescence bands. By tuning the resonator the decoupled dye laser line can be varied to a limited extent, so that what may be termed sequential laser spectrometers can be constructed [65]. In modern semiconductor lasers, pressure and temperature can also be used to detune the emission wavelength by 20-30nm [66], [67]. 

Maity techniques exist for examining the composition of the species (including radical species) generated in CVD reactors optical emission spectroscopy (OES) [80], FT-IR spectroscopy [81] laser induced fluorescence (LIE) spectroscopy [82], diode laser IR absorption spectroscopy [83], and MS [84-87]. Each has its own strengths and shortcomings. A major advantage of MS over other techniques is its... 

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. 

Using space-time-resolved laser-induced fluorescence and plasma-induced emission spectroscopy, the interaction between BCI3 and Ar gives metastable and dissociation product densities that vary nonlinearly as Ar is diluted by BCI3. A model is proposed in which argon metastable states indirectly enhance molecular dissociation [7]. 

Transient Infrared Absorption (TRISP) and laser-induced fluorescence. Because the CJ temperatures are only 2000-3000 K, most of the molecular products are in the ground electronic state. Emission spectroscopy looks selectively at only a few extraordinary molecules which are scarcely representative of most of the products. Infrared absorption, on the other hand is ideal for probing the vibrotational states of the ground state molecules, and the fast response time of TRISP makes it ideal for detonations. The technique has not been applied extensively and is difficult to implement, but our preliminary attempts have shown that we can do it with the proper laser apparatus. Broadband CARS is an alternative approach if the instrumental difficulties of TRISP cannot be overcome. 

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