Radiative Lifetime Increase

It may be generally said that each technique for detector performance enhancement by way of the increase of optical path through the active region simultaneously increases radiative lifetime and thus increases the BLIP detectivity limit. The benefit for detector performance, however, can be seen only in those cases when radiative lifetime is the dominant one. This case is analyzed in this section. 

Even the presence of a conventional high-reflection layer (metal, DBR or hybrid mirror) at the photodetector backside influences the reabsorption radiative lifetime 

As an illustration, we consider the calculation of radiative lifetime of bulk Hgi jjCd cTe detector strucmres surrounded by Ge/PbF2 photonic crystal. HgCdTe bandgap was determined according to [25], Fermi level according to [294], and HgCdTe refractive index according to [295]. 

Radiative lifetime increases with a larger number of periods of photonic crystals and decreases with operating temperamre. From the point of view of radiative 

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At higher pressures, where the number of collisions within the radiative lifetime increases, relaxation processes are important. 

For example, a 450-A radius CdS cluster should have a l-ps radiative lifetime observable below 7 K. As the temperature increases, higher states are populated and the superradiant effect disappears. As a result, the radiative lifetime increases with increasing temperature in this size regime. 

Any strict division between the methods for radiative lifetime increase and those for the optical path increase is impossible. 

Since we have inhibited one pathway leading to triplet depopulation by deuteration, it is clear that it will take longer for the triplet to decay by the radiative pathway and the lifetime of the triplet is increased. If phosphorescence were the sole pathway leading to triplet decay, the measured triplet lifetime would correspond to the radiative lifetime and would be equal to... 

Chemiluminescence is believed to arise from the 2Bj and the 2B2 electronic states, as discussed above for the reaction of NO with ozone [17]. The primary emission is in a continuum in the range =400-1400 nm, with a maximum at =615 nm at 1 torr. This emission is significantly blue-shifted with respect to chemiluminescence in the NO + 03 reaction (Xmax = 1200 nm), as shown in Figure 2, owing to the greater exothermicity available to excite the N02 product [52], At pressures above approximately 1 torr of 02, the chemiluminescence reaction becomes independent of pressure with a second-order rate coefficient of 6.4 X 10 17 cm3 molec-1 s-1. At lower pressures, however, this rate constant decreases and then levels off at a minimum of 4.2 X 1(T18 cm3 molec-1 s-1 near 1 mtorr, and the emission maximum blue shifts to =560 nm [52], These results are consistent with the above mechanism in which the fractional contribution of (N02 ) to the emission spectrum increases as the pressure is decreased, therefore decreasing the rate at which (N02 ) is deactivated to form N02. Additionally, the radiative lifetime and emission spectrum of excited-state N02 vary with pressure, as discussed above for the NO + 03 reaction [19-22],... 

An approximation of the lifetime in PS at RT using an electron-hole pair density equal to one pair per crystallite and the radiative recombination parameter of bulk silicon give values in the order of 10 ms [Ho3]. The estimated radiative lifetime of excitons is strongly size dependent [Sa4, Hi4, Hi8] and increases from fractions of microseconds to milliseconds, corresponding to an increase in diameter from 1 to 3 nm [Hy2, Ta3], as shown in Fig. 7.18. For larger crystallites a recombination via non-radiative channels is expected to dominate. The experimentally observed stretched exponential decay characteristic of the PL is interpreted as a consequence of the randomness of the porous skeleton structure [Sa5]. 

In the previous sections we have shown that all existing theories qualitatively indicate that the probability for internal conversion decreases as the energy separation increases. Because of this, and the fact that excited singlet states have rather short radiative lifetimes we feel that internal conversion from the first excited singlet to the ground state, as discussed in these theories, would not be competitive with the radiative process. 

Additional details on some of these methods are described in other sections of this review. Attempts have also been made to determine excited-state populations in single-source mass-spectrometric experiments from an analysis of ionization efficiency curves.38ad There are several difficulties in applying such methods. For instance, it is now known from photoionization studies that ionization processes may be dominated by autoionization. Therefore, the onset of a new excited state is not necessarily characterized by an increased slope in the electron-impact ionization-efficiency curve, which is proportional to the probability of producing that state, as had been assumed earlier. Another problem arises because of the different radiative lifetimes that are characteristic of various excited ionic states (see Section I.A.4). 

After extraction, the fluorescent indicator was in the unbound state and gave input to the radiative relaxation. Therefore, the fluorescence lifetime increased and, consequently, the intensity as well. After MIP contacting with the analyte, the non-radiative processes were again efficient compared to the radiative processes and, subsequently, fluorescence was quenched. With steady-state fluorescence spectroscopy the cross-reactivity test towards structurally similar biomolecules was performed that yielded selectivity factors for guanosine, cAMP and cCMP of 1.5, 2.5 and 5.1, respectively. 

The possibility of deactivation of vibrationally excited molecules by spontaneous radiation is always present for infrared-active vibrational modes, but this is usually much slower than collisional deactivation and plays no significant role (this is obviously not the case for infrared gas lasers). CO is a particular exception in possessing an infrared-active vibration of high frequency (2144 cm-1). The probability of spontaneous emission depends on the cube of the frequency, so that the radiative life decreases as the third power of the frequency, and is, of course, independent of both pressure and temperature the collisional life, in contrast, increases exponentially with the frequency. Reference to the vibrational relaxation times given in Table 2, where CO has the highest vibrational frequency and shortest radiative lifetime of the polar molecules listed, shows that most vibrational relaxation times are much shorter than the 3 x 104 /isec radiative lifetime of CO. For CO itself radiative deactivation only becomes important at lower temperatures, where collisional deactivation is very slow indeed, and the specific heat contribution of vibrational energy is infinitesimal. Radiative processes do play an important role in reactions in the upper atmosphere, where collision rates are extremely slow. 

We have seen from the above work that the nonradiative rate constants dominate the luminescence behavior of ruthenium(II) complexes. If one can increase the value of the radiative rate constant, kr, without substantial increases in knr, then emission efficiency can be improved. The radiative rate constant is, in theory at least, related to the molar absorption coefficient, epsilon187. Demas and Crosby188, made a number of assumptions and calculated radiative lifetimes based on observed epsilon values, which were in good agreement with the experimental kr values. Watts and Crosby1895 went on to comment on the possible implications of the epsilon value. 

In a second step, Shen and Bray (1998a) studied the changes of the 5Do and 5Di lifetimes of SrFCl Sm2+ and CaFCl Sm2+ under pressure. In both systems they observed an exponential decrease as shown in fig. 13 for the case of the 5Do lifetime at room temperature. According to their analysis of the temperature effects, the measured lifetime of the 5Do 7Fo transition represents an almost pure radiative lifetime. A strong decrease under pressure therefore indicates an increase in the radiative rate Wj. This in turn was attributed to an increased elec-... 

Inspection of the data in Table II reveals a bathochromic shift of the of the triplet-triplet absorption with increasing solvent polarity, with a further bathochromic shift when the compound is adsorbed on microcrystalline cellulose. No phosphorescence emission in solution is seen from this compound at room temperature, indicating that the radiative lifetime of this triplet state is long. This is consistent with the lowest energy excited triplet state having predominantly (n, n ) character. The shape of the triplet-triplet absorption spectrum also suggests that this is the case(20). 

It is possible to calculate other spectroscopic details for the 4f" 15d and 4F configurations. For example, radiative lifetimes for Eu2+ typically increase with temperature, rather than... 

Luminescence lifetime depends upon radiative and nomadiative decay rates. In nanoscale systems, there are many factors that may affect the luminescence lifetime. Usually the luminescence lifetime of lanthanide ions in nanociystals is shortened because of the increase in nomadiative relaxation rate due to surface defects or quenching centers. On the other hand, a longer radiative lifetime of lanthanide states (such as 5Do of Eu3+) in nanocrystals can be observed due to (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus modifies the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002) (2) size-dependent spontaneous emission rate increases up to 3 folds (Schniepp and Sandoghdar, 2002) (3) an increased lattice constant which reduces the odd crystal field component (Schmechel et al., 2001). 

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