Higher level emission

Fig. 12. X-ray excited emission spectrum of GdFa at 300 K. Note higher-level emissions.

The emission of Tb is due to transitions D4- Fi udiich ate mainly in the green. Often there is a considerable contribution to the emission from the higher-level emission I>3- Fj, mainly in the blue. Figure 3.11 gives an example of a Tb emission spectrum. Since the J values, involved in the transitions, are high, the crystal field splits the levels in many sublevels which gives the speebtim its complicated appearance. 

The Tb ion may not only emit from 04 (green), but also from 03 (blue). AE is about 5000 cm", much larger than in the case of Eu . Diluted Tb + systems show, therefore, always some blue Tb " emission, unless Vmm is very high. Please note that in these examples we consider only ions interacting with their immediate surroundings and not with other luminescent ions. In Chapter 5 the quenching of higher level emission due to interaction with another centre of the same kind will be discussed (cross relaxation). 

It is important to realize that we have met now two processes which will suppress higher-level emission, viz. multiphonon emission (Sect. 4.2.1) which is only of importance if the eneigy difference between the levels involved is less than about 5 times the highest vibrational frequency of the host lattice and which is independent of the concentration of the luminescent centres, and cross relaxation which will occur only above a ceitain concentration of luminescent centers since this process depends on the interaction between two centers. 

Fig. 5J. Quenching of higher-level emission by cross relaxation. Left-hand side Eu the D emission on ion I is quenched by transferring the energy difference D - Do to ion 2 which is promoted to the F3 level. Right-hand side the D3 emission on ion I is quenched by transferring the energy difference D3- D4 to ion 2 which is promoted 10 the Fo level...

Tb -" Tb -" is usually used as the activator of green-emitting phosphors due to but it also has blue emission from the higher-level emission... 

In spectroscopy it is common for transitions to be observed as absorptive lines because the Boltzmaim distribution, at equilibrium, ensures a higher population of the lower state than the upper state. Examples where emission is observed, which are by definition non-equilibrium situations, are usually cases where excess population is created in the higher level by infiising energy into the system from an external source. 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

Sulfur in gasoline contributes to the SO air quality problem and deactivates the catalyst in the catalytic converter. Emissions from a poisoned converter contain higher levels of VOC, NO, and CO. As stated earlier, VCR2 and NOj are catalyzed by sunlight to form smog. 

An additional limit to the size of a passive array relates to the current which flows in an OLED when it is under reverse bias [189]. When a given pixel is turned on in the array, there are many possible parallel paths for the current, each involving two diodes in reverse bias and one forward. Hence, as the number of rows and columns increases, there is a higher level of background emission from non-selected pixels that limits the contrast ratio of the array. As a result, the contrast degrades as N increases. 

Radiative Saturation. Higher levels of radiation create a larger population in the excited state, allowing stimulated emission to become a competing process. In this process, atoms in the excited state absorb photons, which re-emit coherently that is with the same frequency, phase and direction as the incident photon. Thus stimulated emission does not produce backscat-tered photons. As the incident energy increases, a greater proportion of the excited atoms absorb a photon and produce stimulated emission before they decay naturally. The net result is that the population of atoms available to produce backscatter decreases, i.e., the medium saturates. 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

The vacancy left in the low-energy K-shell will be filled by an electron from a higher level with the resultant emission of radiations of extra nuclear origin, X-rays, which are distinguished from the accompanying nuclear y radiation. It should be noted that the n p criterion is the same for both positron and electron capture processes and it is not unusual to find both occurring with different atoms of the same nuclide. 

The most commonly observed line emission arises from excitation of atomic electrons to higher level by an energy source. The energy emitted by excited atoms of this kind occurs at wavelengths corresponding to the energy level difference. Since these levels are characteristic for the element involved, the emission wavelength can be characteristic for the element involved. Sodium and potassium produce different line spectra. 

Low-level exposures to -hexane can possibly occur for much of the United States population, especially those that live in urban areas or those that commute in areas with heavy traffic, due to emissions of -hexane associated with motor fuel use. As such, the general population will be exposed to very low levels at all times, while those living in urban centers may be exposed to slightly higher levels. 

The x-ray emissions are categorized as K, L, M, etc., emissions and as alpha (a) and beta (/3) emissions. It is a K emission if the electron drops from any higher level to the K shell. It is an L emission if it drops from any higher level to the L shell, etc. The a emissions are those that involve electrons that drop just one principle level, such as from the L shell to the K shell (Ka emissions) or from the M shell to the L shell (La emissions). The /3 emissions are those in which electrons drop two levels, such as from the... 

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