Monochromatic light source

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

The first Raman and infrared studies on orthorhombic sulfur date back to the 1930s. The older literature has been reviewed before [78, 92-94]. Only after the normal coordinate treatment of the Sg molecule by Scott et al. [78] was it possible to improve the earlier assignments, especially of the lattice vibrations and crystal components of the intramolecular vibrations. In addition, two technical achievements stimulated the efforts in vibrational spectroscopy since late 1960s the invention of the laser as an intense monochromatic light source for Raman spectroscopy and the development of Fourier transform interferometry in infrared spectroscopy. Both techniques allowed to record vibrational spectra of higher resolution and to detect bands of lower intensity. 

Four different laboratories have built IR kinetic spectrometers for use with organometallic compounds. A fundamental feature of all these spectrometers is that the detector is AC coupled. This means that the spectrometers only measure changes in IR absorption. Thus, in the time-resolved IR spectrum, bands due to parent compounds destroyed by the flash appear as negative absorptions, bands due to photoproducts appear as positive absorptions, and static IR absorptions, due to solvents, for example, do not register at all. The important features of these spectrometers are listed in Fig. 2. Since three spectrometers have a line-tunable CO laser as the monochromatic light source, we begin with the CO laser. Then we look in more detail at spectrometers designed for gas phase and solution experiments. 

When such non-monochromatic light sources are used, it is necessary to average the above formula for ICf1 (T)I2 over the g(co) dm probability function in computing the probability of finding the molecule in state after time T, given that it was in < >i up until t = 0, when the light source was turned on. In particular, the proper expression becomes ... 

Excimer lamps are quasi-monochromatic light sources available in UV wavelengths. The light is produced by silent electrical discharge through gas in the gap between two concentric quartz tubes. Electronically activated molecules are produced in the gas phase and decompose within nanoseconds to produce photons of high selectivity. This process is similar to the process in excimer lasers. 

Technically speaking, the sample is irradiated for a few microseconds with an intense pulse containing all frequencies included in the domain to be sampled. This can be compared to a polychromatic radiation source (like comparing a polychromatic to a monochromatic light source). For example, when working at 300 MHz, the frequency range has to be at least 6000 Hz in order to irradiate all the protons irrespective of their environment. Under these conditions, a small fraction of each type of proton (but not all the protons) will absorb the resonance frequency. 

Less heavily substituted benzenes also undergo photochemical scrambling of the type observed in the vapor phase for the xylenes. Thus 1,4-difluoro benzene irradiated at 2690 A produces solely quantities of the 1,3- and 1,2-isomers.58 Upon irradiation with a less monochromatic light source, the 1,3-isomer gives 1,2 and 1,4, and the 1,2-isomer gives the 1,3 and 1,4, although in these cases there is some polymer formation.58... 

Finally, enrichment of isotopic species has been achieved for a number of atoms and molecules using an appropriate monochromatic light source that preferentially excites an isotopic species of interest in mixtures of other isotopic species. The photochemistry associated with isotopic enrichment is briefly described in Chapter VIII. Great efforts have been made recently to obtain information on the detailed photochemical processes involving smog formation, stratospheric pollution, and atmospheres of other planets, and brief discussions of these subjects are also presented in the chapter. 

Fluorescence detectors are commonly used in many HPLC analyses. Compounds absorb light from a monochromatic light source and release it as fluorescence emission. The detector equipped with filters responds only to the fluorescent energy. Presence of trace impurities that fluoresce can cause interference in the test. 

Until recently optical communications were restricted by the lack of fast monochromatic light sources and sensitive photodetectors. Prospects for optical communications improved considerably about two decades ago when a powerful light source became available with the invention of the laser. After that, the transmission medium was the bottleneck of an optical communication system. At that time an intensive search for a new transmission medium was started, particulary because free space propagation could be ruled out for civil use as a consequence of the relative frequent occurrence of atmospheric disturbances. 

This is called the interferogram. It is a function with light intensity as ordinate, and mirror displacement (or time) as abcis. This interferogram of a monochromatic light source can be mathematically formulated as a cosine function. 

Resonance Raman (RR) spectroscopy is a powerfiil and versatile technique for the study of both vibrational and electronic structures of chromophoric molecular systems. RR spectra are obtained by irradiation of the sample with a monochromatic light source whose energy is close to that of an electric-dipole-allowed electronic absorption band. Most of the Raman bands are attenuated by the absorption, but some bands may be greatly enhanced. This effect arises from a coupling of the electronic and vibrational transitions, and the vibrational modes that do show enhancement are localized on the chromophore, that is, on the group of atoms that give rise to the electronic transition. 

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