Extended light source

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

When extending the pump-probe scheme into the x-ray domain, we deal with two very different pump and probe light sources with respect to their pulse intensities. This poses stringent boundary conditions on the sample design in order to guarantee a feasible experiment [11]. To start with, we estimate the signal-to-noise ratio that one can anticipate with current technology. Hereby, we restrict the calculations to pump-probe experiments in... 

Low-pressure mercury and sodium arcs, as well as fluorescent tubes [2,3] of technical importance are necessarily extended light sources because their mode of operation is incompatible with compression of the source to a point without alteration of their emission spectra. 

Extended light sources may be installed around a tubular reactor or in the axis of an annular irradiated reaction volume. In the first case, an annular (or coaxial) radiation field focalized on the axis of the tubular reactor is created (Figure 10), and, in reaction mixtures of very low absorbance, irradiance as a function of the radius of the cylindrical reactor shows highest values in the axis of the reactor (positive geometry of irradiation, Figure 11 [2,3]). 

The extended light source may also be placed at the axis of a reactor composed of two coaxial cylindrical tubes (Figure 12). The emitted radiant power is absorbed by the reaction system contained in the annular reactor volume. Irradiance diminishes in a filled reactor with increasing radius (Eq. 36), this geometry is called the negative geometry of irradiation [2,3]. 

This geometry of irradiation makes the most efficient use of the light emitted by an extended light source. In fact, this geometry is used in all immersion-type photochemical reactors, and most industrial photochemical production units are based on this design. 

In actual practice, any tubular light source will have a finite diameter and will not behave as a true line source. Radiation from an extended light source will emanate from points displaced from the lamp s axis, causing the lamp to appear rather like a diffuse light source. In addition, imperfections in the... 

Further development of the emission models was made by Irazoqui et al. who introduced the three-dimensional nature of the extended light source [117]. Hence, the most significant feature of the extense source with volumetric emission (ESVE) model is the inclusion of a radiant energy source with finite spatial dimensions. In fact, the lamp is considered to be a perfect cylinder, the boundaries of which are represented by a mathematical surface of zero thickness (Figure 30). 

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