Prompt-emission

These radionuclides are produced by irradiating targets with beams of hydrogen ions (protons), but frequently deuterium ions (deuterons) are used (see Table 21.10). Some products require beams of helium-4 and helium-3 ions. The typical process involves the capture of the proton with the prompt emission of a neutron. This is called a p,n reaction. However, in other cases there may be protons, alpha particles, or up to five neutrons emitted. The resulting products decay generally by positively charged electron (positron) emission, but also decay by capture of an orbital electron. 

In the presence of energy transfer (salts with non-vanishing spectral overlap between Elc emission and Ln3+-absorption30), see Table 11) the Elc emission has to be ascribed to the prompt emission from Eg93,95,156). In these cases the additional non-radiative deactivation of this level is already so effective that a further radiative deactivation channel, opened by the influence of a magnetic field, proves to be inefficient. 

Fluorescence is the prompt emission of light from molecules excited by radiation of higher frequency. Normally, absorption of ultraviolet radiation by a molecule leads to emission in the visible region higher frequency absorption leads to lower frequency emission. 

Figure 2. Product I prompt-emission spectra ---polystyrene film, 37 pE cm 2 absorbed in vacuum, EX 300 nm --------metha-...

Fluorescence spectra from a polystyrene film photolyzed in vacuum are shown in Figure 2. Similar but less intense spectra were observed in films irradiated in air. The Product I responsible for this spectrum was partially extractable from the film with methanol the fluorescence spectrum of the extract is shown also in Figure 2. Comparison of these spectra with those of a wide variety of reasonable model compounds suggests that Product I is related to 1,3-diphenyl-l,3-butadiene since the spectral match with 1,4-diphenyl-l,3-butadiene, shown in Figure 2, is quite close. Product I spectra were obtained also from the residual films after extraction, indicating that the diene moiety may form part of a photolyzed chain as well as exist as a short-chain fragment. Fluorescence spectra that could be related to higher polyenes were not detected in the vacuum exposures. In air exposures, however, the prompt emission spectra from films did exhibit a weak shoulder superimposed on the... 

Prompt emission spectra of films irradiated in either vacuum or air indicated the formation of at least two fluorescing species. These species are denoted as Products II and II. The most prominent emission, that of Product II, was highly structured with maxima at about 405, 435, and 455 nm. Product II can be associated with both the polymer chain and low molar mass fragments it was extractable from the photolyzed film with methanol but not with cyclohexane, and it was present in methylene chloride extracts of cross-linked residues of films that had been extracted previously with methanol. The fluorescence spectra are shown in Figure 6 excitation maxima correspond very well with the weak absorption bands noted in the spectra of the irradiated films. Plots of absorbance increases at 350 nm against fluorescence intensity at 435 nm were linear for both vacuum and air irradiations. Therefore, it is concluded that Product II is a major emissive contributor to the yellowing of poly(sty-rene-aft-methyl methacrylate) films subjected to 254-nm irradiation. 

Figure 7. Prompt-emission changes during irradiation of poly(styrene-a t-methyl methacrylate) films O, vacuum , air superscripts and subscripts refer to emission and excitation wavelengths (nm), respectively...

Figure 2. The prompt fluorescence (dashed line) and delayed emission (solid line) of P2VN-Py films at 77K. The mole fraction of pyrene for the corresponding delayed emission spectrum is indicated on the figure. Note that the delayed emission spectrum was obtained with a more red-sensitive photomultiplier than the prompt emission (see Experimental section) (1).

---