Laser-induced emission excitation

Fig. 10.16. Gradient electrochromatogram of derivatized neutral steroids. Macroporous monolithic column 350 (250) mm x 100 pm i.d. gradient mobile phase, acetonitrile-water-240 mM ammonium formate buffer, pH 3 (30 60 5-65 30 5 v/v/v) field strength, 600 V/cm injection, 100 V/cm for 10s detection, laser-induced fluorescence (excitation at 325 nm, emission at >495 nm). Reproduced with permission from Que et al. [76],...

Laser-induced emission spectroscopy analysis (LIESA) is a similar technology, in which a beam of laser light is directed onto the surface of the specimen. This induces a short-lived hot plasma that comprises free electrons, excited atoms, and ions of very high electric charge. 

Cossart-Magos C, Cossart D, Leach S, Maier JP, Misev L (1983) High-resolution gas phase emission and laser induced fluorescence excitation spectra of 1,3,5-C6fsh and l,3,5-c f3d critical bands in the Jahn-Teller effect analysis. J Chem Phys 78 3673... 

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

The inherent complexities of LIBS make it one of the most frustrating of all atomic reservoirs. The following experimental conditions and parameters are best suited to laser-induced emission spectral analysis (i) argon as a buffer gas at reduced pressure (140 hPa) (ii) reduced laser irradiance (Hi) long delay times (>30 /xs) between laser pulse and detector gate pulse and (iv) use of analyte and reference spectral lines with comparable excitation energies [127]. 

We consider a model for the pump-probe stimulated emission measurement in which a pumping laser pulse excites molecules in a ground vibronic manifold g to an excited vibronic manifold 11 and a probing pulse applied to the system after the excitation. The probing laser induces stimulated emission in which transitions from the manifold 11 to the ground-state manifold m take place. We assume that there is no overlap between the two optical processes and that they are separated by a time interval x. On the basis of the perturbative density operator method, we can derive an expression for the time-resolved profiles, which are associated with the imaginary part of the transient linear susceptibility, that is,... 

In contrast to spontaneous emission, induced emission (also called stimulated emission) is coherent, i.e. all emitted photons have the same physical characteristics - they have the same direction, the same phase and the same polarization. These properties are characteristic of laser emission (L.A.S.E.R. = Light Amplification by Stimulated Emission of Radiation). The term induced emission comes from the fact that de-excitation is triggered by the interaction of an incident photon with an excited atom or molecule, which induces emission of photons having the same characteristics as those of the incident photon. 

Another technique for measuring the lifetime of excited activator atoms in solid-state lasers has been published by Gilrs h If the pulsed laser is operated close above threshold, only a single spike (i.e. a short pulse of induced emission) appears, whereas many spikes are emitted when the laser ist running well above threshold. This... 

The observation of the induced emission, its time behavior and threshold conditions allow to study with new techniques details of chemical reactions which lead to specific states of the molecular or atomic reaction product A quantitative study of such laser systems will also yield information about collisional deactivation rates of the excited states (see also Section 111.4). 

Several other chemical lasers with similar excitation schemes soon followed vibrationally excited CO was formed during flash photolysis of a CSj-Oj mixture at 1 torr with 150 torr He buffer gas Investigations of the line spectrum (270 new laser lines have been found 00) gain and power output of induced emission under various helium pressures and with the addition of CO, established that CO was being selectively excited by different mechanisms into different vibrational levels. The experimental results enabled a few reaction schemes to be selected out of several other possibilities, which could be excluded. 

Fig. 6 Free-solution CE separation of PNA/DNA hybrid from excess PNA probe. M13 mpl8 ssDNA 4.2 X 10-8 M, and PNA probe 1.3 X 10-7 M. Detection LIF 488/520 nm. Buffer TBE, 7 M urea (pH 8.0). CE conditions 50-mm-i.d. polyacrylamide-coated capillary (27 cm in length and 20 cm to detector), 10 s gravity injection, separation voltage — 10 kV. Laser-induced fluorescence detection with excitation at 488 nm and emission at 520 nm. The buffer contained Triszborate (pH 8.0) with 7 M urea buffer. (From Ref. 37.)...

Under lamp excitation tourmaline is practically non-luminescent, while under X-ray excitation it exhibits impurity luminescence from Fe centered at 700-750nm and Mn + centered at 560-570nm (Kusnetsov and Tarashchan 1988). The natural tourmaline in our study consisted of four samples. The laser-induced time-resolved technique enables us to detect Cr + emission centers (Fig. 4.58). 

The monazite structure consists of distorted PO4 tetrahedra with each metal atom roughly equidistant from nine oxygen atoms. Minor amounts of other rare-earth elements may occur. Steady-state liuninescence under X-ray excitation of monazite revealed emission of Gd, Tb, Dy and Sm (Gorobets and Rogojine 2001). Laser-induced time-resolved liuninescence enables us to detect Sm +, Eu and Nd emission centers (Fig. 4.70). 

Thorite and orangite (orange thorite) have a tetragonal structure and are isostructural with zircon. Steady-state spectra under X-ray and laser (337 nm) excitations are connected with REE " ", namely Sm " ", Tb ", Dy " " and Eu ". Reabsorption lines of Nd " have been also detected (Gorobets and Rogojine 2001). Laser-induced time-resolved luminescence enables us to detect Eu " and uranyl emission centers (Eig. 4.70). 

Fig. 5.71. a-f Unidentified emission center in apatite laser-induced time-resolved luminescence spectra of apatite, a Steady-state luminescence spectrum b Time-resolved spectrmn with narrow gate where yellow band with short decay time dominates c-d Time-resolved spectra after heating at 800 °C e-f Excitation bands of Mn and short-lived yellow band, correspondingly... 

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