Excitation flame laser

LEI utilizes a pulsed dye laser to promote analyte atoms to a bound excited state. Laser excitation enhances the thermal (collisional) ionization rate of the analyte atom, producing a measurable current in the flame 12). The laser-related current is detected with electrodes and is a measure of the concentration of the absorbing species. LEI may proceed by photoexcitation (via one or more transitions) and thermal ionization or a combination of thermal excitation, photoexcitation, and thermal ionization. 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

The expln limits of mixts of gaseous Cl azide with Ar, N, and C dioxide are in Ref 3. The shock wave formed by the expansion of the gas into a vacuum is sufficient to cause de-compn (Ref 5) Qe -93.2 l.Skcal/mole, flame temp at 20mm 3380°K (Ref 6). Mixts of Cl azide.N trifluoride H 1 1 2 at 12—24 torr are initiated with a Xe flash lamp to produce explns which excite a H fluoride laser. Q azide, S hexafluoride, H mixts were similarly used (Ref 7)... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Figure 21-1 Absorption, emission, and fluorescence by atoms in a flame. In atomic absorption, atoms absorb part of the light from the source and the remainder of the light reaches the detector. Atomic emission comes from atoms that are in an excited stale because of the high thermal energy of the flame. To observe atomic fluorescence, atoms are excited by an external lamp or laser. An excited atom can fall to a lower slate and emit radiation.

Figure 21-1 also illustrates an atomic fluorescence experiment. Atoms in the flame are irradiated by a laser to promote them to an excited electronic state from which they can fluoresce to return to the ground state. Figure 21-4 shows atomic fluorescence from 2 ppb of lead in tap water. Atomic fluorescence is potentially a thousand times more sensitive than atomic absorption, but equipment for atomic fluorescence is not common. An important example of atomic fluorescence is in the analysis of mercury (Box 21-1). 

The laser atomic fluorescence excitation and emission spectra of sodium in an air-acetylene flame are shown below. In the excitation spectrum, the laser (bandwidth = 0.03 nm) was scanned through various wavelengths while the detector monochromator (bandwidth = 1.6 nm) was held fixed near 589 nm. In the emission spectrum, the laser was fixed at 589.0 nm, and the detector monochromator wavelength was varied. Explain why the emission spectrum gives one broad band, whereas the excitation spectrum gives two sharp lines. How can the excitation linewidths be much narrower than the detector monochromator bandwidth ... 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

The combination of theory with experiments dealing mainly with the excited state makes this volume invaluable for the research student as well as for the seasoned scientist, especially in such areas as laser development and laser chemistry, the chemical physics and kinetics of the atmosphere, studies of flames, and related topics. 

Non-equilibrium excitation in flames has been discussed from the point of view of possible inversions286. The possibility of laser action on several transitions of CN excited in active nitrogen has been discussed292 in terms of relevant rate equations and the threshold condition for oscillation. A chemical laser is of course a physical phenomenon, the performance of which depends critically on the rate... 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Walton et al. [269] separated organomanganese and organotin compounds by high performance liquid chromatography using laser excited atomic fluorescence in a flame as a high sensitivity detector. 

Barnes ct al. (61a) have detected ground state CH radicals in flame at atmospheric pressure by measuring the CH(/12A - A 2FI) fluorescence intensities excited by a tunable dye laser at 4315 A. 

The fluorescence technique combines the advantages of the large dynamic range of emission techniques with the simplicity and high selectivity of absorption techniques. Flame sources have been extensively used, however, for elements with refractory oxides, the ICP source has been found to be more satisfactory for AFS. A system for hollow cathode lamp excited ICP-AFS, as proposed by Demers and Allemand (1981), is commercially available as a modular simultaneous multielement ICP system. Although fluorescence techniques often offer two orders of magnitude sensitivity improvement over absorption, the multielement approach for AFS has not yet been commercially successful. Also promising for the future is the laser-excited furnace AFS where the detection limits for most elements are comparable to those of ICP-AES and for some elements, for eg, As, Cd, Pb, Tl, Lu, even lower (Omenetto and Human, 1984). The future for AFS techniques has been discussed by Stockwell and Corns (1992). 

Some of our recent studies of LIF on OH in flames demonstrate the close connection between current work in other areas of physical chemistry—in this case, state-to-state collisional energy transfer—and the development of diagnostic tools for combustion. In these experiments, measurements are made of the collisional redistribution of excited state population following laser excitation of OH to individual levels, in an atmospheric pressure flame. 

Figure 19. The laser-induced fluorescence excitation spectrum of the Ct swan band system in an acetylene-air flame (21)...

The recent availability of tunable dye lasers has markedly enhanced our ability to inquire into the chemistry and physics of combustion systems. The high sensitivity, spectral and spatial resolution, and non-perturbing nature of laser induced fluorescence makes this technique well suited to the study of trace chemistry in complex combustion media. A barrier to the quantitative application of fluorescence to species analysis in flames has been the need to take into account or bypass the effects of quenching. The use of saturated fluorescence eliminates quenching as a problem and has the further advantage that fluorescence intensity is insensitive to variations in laser power (1, 2 ). However, the generation of high concentrations of excited states under saturated excitation in an active flame environment opens up the possibilities for laser induced chemistry effects that also must be taken into account or avoided (3,4,5). 

Figure 4. Laser-excitation spectra for OH A2 - X2n in a ClHt-Oi-Nl (1.2 2.5 10) flame with 0% and 0.5% HaS. Fluorescence detected at 314.69 nm.

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