Collective emission

Fig. 24 Discovery of novel fluorophore, l,2-dihydropyrrolo[3,4-(3]indolizin-3-one, using a combinatorial approach (a) Synthetic schemes of fluorescent core skeleton (b) Collected emission spectra of selected compounds covering full-color emission wavelength (c) Table of photophysical properties of all fluorescent compounds. All the photophysical properties were measured in DCM (dichloromethane). Reproduced with permission from [92]...

Specificity in luminescence spectroscopy is achieved because each compound is characterized by an excitation and emission wavelength. The identification of individual compounds is made difficult in complex mixtures because of the lack of structure from conventional excitation or emission spectra. However, by collecting emission on excitation spectra for each... 

A larger-scale tyre combustion experiment was performed by Lemieux Ryan (1993) in order to collect emission data from a simulated open waste tyre fire. In addition to identifying a large number of organic compounds, including... 

The emission control residuals sometimes exceeded EP Toxicity hazardous waste limits for lead and cadmium because the melting point of grey iron is approximately 2,700°F, where the melting point for lead is only about 620°F. As the metal is melted, the lead and cadmium will tend to volatilize and be collected by either the baghouse or wet dust collection emission control system. 

Alternative Fuels Data Center (AFDC) A program sponsored by the Department of Energy to collect emissions, operational and maintenance data on all types of alternative fuel vehicles across the country. 

The efficacy of exciting and collecting emission data from PEG hydrogel sensors implanted in an in vivo rat model was also assessed experimentally.115... 

Schematically, two main systems can be used to collect 3D fluorescence data (time, wavelength, number of photons, see fig. 1). In a first type of system, light is directed into a monochromator connected to a photomultiplier tube and then to a fast oscilloscope (PM detection). The experimentalist thus collects luminescence decays at various wavelengths. This system is known to be very efficient for luminescence decay acquisition but is very time-consuming for the acquisition of emission spectra. In the second type of system, light is directed to a diode array detector (or CCD camera) and a subsequent electronic detection device (diode detection). The experimentalist collects emission spectra at various delay times (time zero for the pulse entering in the sample). This system is very efficient for emission data acquisition but, on the other hand, time-consuming for luminescence decay acquisitions. From this very schematic description, it appears that a system combining the two types of detections would be the optimum.

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

In table 1 we collect emission oscillator strengths for several transitions of H3, calculated with the initially employed QDO formalism (18) and with the modified one (22). As comparative data we include the HE f-values obtained by King and Morokuma (15) and by Martin (25). 

Collection of the dust takes place via extracting walls, roof domes, moveable hoods or extraction work benches. An effective aid when collecting emissions are hot-air curtains which direct additional air into the cabin. The additional air used may be cleaned recycled extraction air, in order to economise on heating energy. However, it should be noted, that a portion of fresh air will still always need to be provided from the outside. 

This emission exits the incident end of the fiber at angles that are equal or less than fi. Subsequently, the collected emission is collimated by the lens, reflected by the mirror (except for a few percent that is lost through the hole), and focused onto the entrance slit of an emission monochromator. Signals are processed with a photo-meter. 

The threshold condition for collective emission (superradiance) or maser action (gain in the medium exceeding the losses) is much lower for Rydberg atoms than it is for the same number density of atoms in low n states. To obtain an order of magnitude estimate for the point at which collective fects occur we estimate the amplitude of the electric dipole field E radiated by an atom at a distance corresponding to a neighbouring atom. That is, if we let L represent a linear dimension in the sample which contains N atoms,... 

With the use of a resonant cavity the threshold for collective emission is lower by the magnitude of the cavity finesse. Using a cavity finesse of a 100 or so Rydberg maser action on sodium has been observed. In this case the field ionisation signal was used to monitor the population as a function of time for the states 27S, 26P and 25P corresponding to transitions An = 1 at X = 1 470 mm and An = 2... 

Processes 1-3 and 5 occur without the necessity of optical feedback (or mirrors), and thus have been termed as mirrorless lasing [111]. Processes 2—5 are also coherent, whereas ASE is not. In addition, processes 2 and 3 are examples of cooperative emission, whereas ASE is a collective emission process. The superradiance process is very. similar to the SF laser action process, except that in superradiance the system is prepared coherently from t= 0, whereas in SF the system evolves in time to be coherent at t > 0 [111]. 

In the experiments described so far, the atoms are initially prepared in the upper state e> of the transition resonant with the cavity (collective emission process). We have also carried out collective absorption experiments in resonant cavities with the atoms starting from level f>. ... 

The driving cycles used for Japan and Europe are different from the 1975 FTP. A comparison of these cycles is shown in the literature (14). The European community has revised its test procedure to include a cold start portion. The new driving test starts collecting emission samples immediately after the vehicle key is turned on. This will increase the emphasis on cold start emissions. In addition, the Japanese driving cycle has been revised to reflect a more stringent test cycle. 

---