Conventional source of radiation

Laser radiation is emitted entirely by the process of stimulated emission, unlike the more conventional sources of radiation discussed in Chapter 3, which emit through a spontaneous process. 

Conventional monochromatic radiation has a span of frequencies, and will thus excite simultaneously all chemical species which absorb within that narrow range. Lasers have a precisely defined frequency. This allows species with absorptions close to each other to be identified and monitored by separate lasers, or by a tunable laser, in contrast to the indiscriminate absorption which would occur with conventional sources of radiation. 

Photochemical reactions occur under the influence of radiation. Conventional sources of radiation, and modem flash and laser photolysis techniques, are both extensively used. 

With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides. 

Conventional sources of electromagnetic radiation are incoherent, which means that the waves associated with any two photons of the same wavelength are, in general, out-of-phase and have a random phase relation with each other. Laser radiation, however, has both spatial and temporal coherence, which gives it special importance for many applications. 

The suggestion that the molecular building blocks of life could be formed in space is intriguing since such regions would seem to be rather unlikely places for the development of chemistry. The ISM is cold (temperatures of 10-30 K) and "empty" with pressures of less than 10 2 torr such that the probability for a collision between two compounds is low and, at such low temperatures, the "reaction rate" would be expected to be very low (hence in most industrial chemistry the reactants are heated to increase their reactivity). Nevertheless the detection of such molecules within the ISM makes it clear that these are chemically active zones. The solution to this apparent paradox is that the chemistry in the ISM is somewhat different from the conventional chemistry we observe on Earth, much of it being induced by radiation. The ISM contains several different sources of radiation, namely ... 

The excitation rate of free carriers in a semiconductor depends upon the rate of absorption of photons, which is a measure of the intensity of the absorbed radiation. Because the intensity is proportional to the square of the electric field vector, a photoconductor or a photovoltaic detector is a square law detector. Therefore, an alternative to the conventional way of viewing photoexcitation is that the semiconductor acts as a mixer element, beating the electric field vector against itself in a homodyne manner. Thus if two coherent sources of radiation having different frequencies (wavelengths) are superimposed upon a semiconductor, mixing action will occur. The resultant intensity will contain the four terms shown below ... 

One of the principal difficulties in these time-resolved experiments is the low intensity of the conventional sources of pulsed resonance radiation, as the seven-hour running time of Dodd et at. (1967) emphasizes. Excitation by means of pulsed tunable dye lasers seems likely to overcome this difficulty and we now briefly refer to some recent experiments using this technique. 

The emitted beam of coherent radiation is narrow and can be focused into a very small area. This means that the density of radiation that can be delivered for any one pulse over a small area is very high, much higher than can be delivered by conventional light sources operating with similar power inputs. 

Work on EXAFS then progressed very little until the advent of the synchrotron radiation source (storage ring), described in Section 8.1.1.1. This type of source produces X-ray radiation of the order of 10 to 10 times as intense as that of a conventional source and is continuously tunable. These properties led to the establishment of EXAFS as an important structural tool for solid materials. 

A third source of initiator for emulsion polymerisation is hydroxyl radicals created by y-radiation of water. A review of radiation-induced emulsion polymerisation detailed efforts to use y-radiation to produce styrene, acrylonitrile, methyl methacrylate, and other similar polymers (60). The economics of y-radiation processes are claimed to compare favorably with conventional techniques although worldwide iadustrial appHcation of y-radiation processes has yet to occur. Use of y-radiation has been made for laboratory study because radical generation can be turned on and off quickly and at various rates (61). 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

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