Flash lamp, excitation source

ISA s (Spex s) Fluorolog Tau-3 lifetime system and Spectronic Instruments SLM-AMINCO 48000 DSCF spectrofluorimeter both use xenon flash lamp excitation (t5 ically 150-450 W) and have modulation frequencies of up to 310 MHz. (Most systems can also be operated as steady-state fluorimeters). SLM also manufactures a multi-harmonic s3rstem based on a pulse- modulated continuous wave light source. 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

The limitations on the total pressure in the FP-RF cell are far less severe than those for FFDS. The lower end of the pressure range that can be used is determined by the need to minimize diffusion of the reactants out of the viewing zone. The upper end is determined primarily by the need to minimize both the absorption of the flash lamp radiation by the carrier gas and the quenching of the excited species being monitored by RF. In practice, pressures of 5 Torr up to several atmospheres are used. The kinetic analysis is again typically pseudo-first-order with the stable reactant molecule B in great excess over the reactive species as outlined earlier. Table 5.5 gives some typical sources of reactive species used in FP-RF systems. 

Figure 12 illustrates another simple apparatus, in this case all electronic. The source of excitation being an EG G FX-12 flash lamp with a pulse duration of about 1 / sec. Simple filters or monochromators are used to isolate the exciting radiation from the fluorescent radiation. The fluorescence is detected by a photomultiplier tube, a 1P21 being a typical example. 

Bhaumik et al. (63) have reported the design of a stroboscopic instrument that is a substantial improvement upon the basic design of Peterson and Bridenbaugh. This apparatus makes use of a PEK-XE9-2 100-nsec xenon flash lamp as the excitation source. The lamp is fired on the order of 50 times a second with a peak input power of 4 Mwatts. The average power is reasonably low, being about 20 watts. 

The study of short lived excited states is limited by the low concentra- lions in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates and laser sources have been helpful in this respect. Triplet-triplet absorption, absorption by excited singlet state to higher singlet state and Absorption by exciplexes (Section 6.6.1) can be effectively observed by sequential biphotonic processes. 

By using high intensity flash lamps and laser sources, photophysical and photochemical properties of the triplet states can be studied. These sources also help to study emission from upper excited state. 

Decay. The decay time requirements must be adhered to very precisely for cathode-ray tube phosphors. The measuring devices consist of fast excitation sources (flash lamps, lasers), photomultipliers with very low time constants, and an oscilloscope [5.440]. 

By using either a continuous or pulsed source of radiation and by measuring the amount of radiation absorbed by the reaction products, it is possible to determine product state distributions. The source of radiation can either be monochromatic (resonance lamp or laser) or broad-band (flash lamp or arc lamp) used in conjunction with a form of monochromator at the detector. The amount of absorption is monitored by an appropriate photosensitive or energy-sensitive detector. Particular care must be taken in the case of resonance lamps to avoid self-reversal of the output of the source, as this will complicate the quantitative analysis of product densities [17]. Similarly, laser sources must not be operated at such high output powers that the transitions involved become saturated, as this also complicates the analysis. Absorption measurements can be used for a wide range of reaction products, both ground and excited states of atoms, radicals and molecules [9,17, 22]. 

The mechanism consists of using a conventional energy source (flash-lamp or other) to excite atoms or molecules from the ground state to some excited state, so that an inversion of population occurs in the Arrhenius55 sense this is usually best understood in a three-level laser, although two-level lasers are also discussed (see Problem 10.10.1), and many are four-level lasers (see Fig. 10.14). 

Typical labels used for HTS assays are chelates and cryptates based on lantha-rtide ions such as europium (Eu ) [113-120] and terbium (Tb ) [119, 121, 122]. These ions show long excited-state hfetimes of several 100-1000 ps, which aUow for time delays of > 100 ps and the use of a flash lamp as excitation source. The lantharride-based labels are commercially available from CisBio International (Eu cryptates), Perkin Ehner life Sciences (Eu chelates), Amersham (Eu chelates), and Panvera (Tb chelates). Pig. 15 shows examples of a cryptate from CisBio International and a chelate from Perkin Ehner Life Sciences. 

Lasers consist of a gain medium surrounded by a resonant optical cavity. The medium is pumped by an external energy source, usually by light from a flash lamp or from another laser or by an electric discharge. Laser emission occurs only when the number of particles in an excited state exceeds that in some lower energy state (population inversion, Figure 2.4) so that light amplification takes place in the cavity. 

The basic principle is to observe the change in absorbance after an intense radiation pulse has created a significant population of short-lived reactive intermediates. Early experiments used xenon flash lamps as excitation sources and were able to detect intermediates with lifetimes >10 second. Modern experiments use pulsed lasers as excitation sources. The monochromatic output of a laser allows selective excitation the narrow pulse width allows detection of species with lifetimes as low as 10 second. A complementary experiment uses a pulse of electrons from a linear accelerator to generate the reactive species. More experimental detail is available in many reviews [139]. 

Norrish and Porter developed the technique of flash photolysis in 1949 (Norrish and Porter, 1949 Porter, 1950). This technique, in which a flash lamp was used as an excitation source, had limitations in intensity and duration of the flash. Short pulses and high intensities of flash lamps are mutually exclusive. However, with the advent of pulsed lasers, flash photolysis equipment operating even in the femtosecond time scale is available. Table 12.3 lists some commonly used pulsed lasers. 

Figure 8. Excitation energy dependence of the S, decay rate (reciprocal of the measured fluorescence lifetime) in vapor-phase naphthalenes. Excitation source was a deuterium flash lamp. (From ref. [4] with permission.)...

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