Radiative lifetimes measurement

The radiative lifetime rr is l/kT. It is the real emission lifetime of a photon that should be measured independently of the other processes that deactivate the molecule. However, since these processes occur in parallel to the radiative process, it appears impossible to eliminate them during radiative lifetime measurements. Therefore, we will measure a time characteristic of all deexcitation processes. This time is called the fluorescence lifetime and is lower than the radiative lifetime. A fluorophore can have one or several fluorescence lifetimes in this case, we can determine the fractional contribution of each lifetime and calculate the mean fluorescence lifetime r0 or (r) ... 

Ruby sample holder and experimental arrangement for radiative-lifetime measurements. 

Atomic Radiative Lifetimes Measured by Pulsed Laser Spectroscopy in the UV/VUV Spectral Region... 

Ber Berg, L.-E., Ekvall, K., Hishikawa, A., Kelly, S. Radiative Lifetime Measurements of the B State of BaH by Laser Spectroscopy, Phys. Seripta 55 (1997) 269-272. 

U. Berzinsh, S. Svanberg Atomic radiative lifetimes measured by pulsed laser spectroscopy in the UV/VUV spectral region. Adv. Quantum Chem. 30, 283 (1998)... 

P. Quinet, P. Pahneri, E. Biemont, Z.S. Li, Z.G. Zhang, S. Svanberg Radiative lifetime measurements and transition probability calculations in lanthanide ions. J. of Alloys and Compounds 344, 255 (2002)... 

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... 

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Measurement of the aggregate radiative lifetime and that of its absorption spectrum half-width are the two experimental ways to estimate the value of the delocalization length. 

In addition, the excitation delocalization length of an aggregate could be estimated on the basis of the measurement of the aggregate radiative lifetime and its absorption spectrum half-width as described above [10, 11]. Such estimation, being interesting as itself, could also be regarded as the lower limit of physical dimension of the aggregate. 

There are many molecular interactions which influence the fluorescence decay times. The measured fluorescence lifetime r is usually shorter than the radiative lifetime tr because of presence of other decay rates which can be dependent on intramolecular processes and intermolecular interactions (Figure 10.3). The measured fluorescence lifetime (r) is given by the inverse of the total rate of dynamic processes that cause deactivation from the excited (mostly singlet Si) state... 

The previous formula indicates that the radiative lifetime tq (and hence the radiative rate A) can be determined from luminescence decay-time measurements if the quantum efficiency rj is measured by an independent experiment. Methods devoted to the measurement of quantum efficiencies are given in Section 5.7. 

EXAMPLE 1.7 The fluorescence lifetime measured from the metastable state Ej/2 ofNd + ions in the laser crystal yttrium aluminum borate (YAl3(B03)4) is 56 lus. If the quantum efficiency from this state is 0.26, determine the radiative lifetime and the radiative and nonradiative rates. 

Recall that the radiative lifetime, tq = 1 /A, can be determined from Equation (1.20) by measuring the fluorescence lifetime t from a luminescence decaytime experiment, and provided that the nom-adiative rate Am is known. Eor pro-ces ses where the nom-adiative rate is negligible (Am 0), t = tq and so we will measure lifetimes in the range of nanoseconds for electric dipole transitions and lifetimes in the range of microseconds for magnetic dipole transitions. 

Cr + ions in aluminum oxide (the ruby laser) show a sharp emission (the so-called Ri emission line) at 694.3 nm. To a good approximation, the shape of this emission is Lorentzian, with Av = 330 GHz at room temperature, (a) Provided that the measured peak transition cross section is c = 2.5 x 10 ° cm and the refractive index is = 1.76, use the formula demonstrated in the previous exercise to estimate the radiative lifetime, (b) Since the measured room temperature fluorescence lifetime is 3 ms, determine the quantum efficiency for this laser material. 

In Table E7.5, the fluorescence lifetimes and quantum efficiencies measured from different excited states of the Pr + ( Po and D2) and Nd + (" Fs ji) ions in a LiNbOs crystal are listed, (a) Determine the multiphonon nonradiative rate from the 19/2 and In/2 states of the Er + ion in LiNbOs. (b) If a fluorescence lifetime of 535 /us is measured from the excited state Fs/2 of the Yb + ion in this crystal, estimate the radiative lifetime from this state. 

The underlying reason for all these observations is that the growth of any product of reaction always reflects the lifetime of the precursor species, ferf-butoxyl radical in our examples. Interestingly, this is the same concept that applies when one measures properties, such as fluorescence that is, the observable fluorescence lifetime reflects the lifetime of the singlet state and not its radiative lifetime. 

There are established techniques for the determination of and np (Section 10.2). In this expression, kf and kp are reciprocals of the radiative lifetimes of flucrescene and phosphorescence states, respectively, kf can be obtained experimentally from the integrated area under the absorption curve and kp is obtained from the measured decay rates for phosphorescence at 77K in EPA. In Table 5.3 the observed quantities, their symbols, relation to rate constants and sources of studies are summarized. 

Emission characteristics of a molecular system can be expressed by three types of measurements (1) observation of emission and excitation spectra, (2) measurement of quantum efficiencies, and (3) determination of decay constants or radiative lifetimes. 

The lifetime of triplet acetone at 25° in the vapor phase, as measured from the rate of decay of phosphorescence, is 0.0002 sec,318 so that the rate of decay is 5 x 103 sec-1. This figure represents the sum of the rates of all decay processes. Since the data at 40° 308 indicate that decomposition and internal conversion of triplet acetone occur approximately 40 times as fast as emission, the radiative lifetime must be on the order of 0.01 sec. Measurements of the rate of phosphorescence decay from solid acetone at 77°K, where all activated fragmentation and most radiationless decay normally disappear, have actually yielded values approximately one-tenth as large as that obtained in the gas phase at room temperature.319 The most recent measurements of the lifetime of triplet acetone at 77°K in frozen glasses does indeed yield an estimate of 0.01 sec for the radiative lifetime of triplet acetone.318... 

The radiative lifetime for the 52IIr state has been measured by Jeunehomme and Duncan,222 who produced the state by electron impact. For the radiative step... 

The measured lifetime t can be expressed by the pure radiative lifetime and the rate of predissociation kp... 

Since the radiative lifetime is nearly independent of v (852), it can be seen that the measured decay rate 1/t is proportional to kp, which in turn is proportional to the quantum yield of I atom production. Therefore, the wavelength dependence of decay rate follows approximately the quantum yield curve shown in Fig. V-22, that is, the decay rate is faster when the quantum yield of atom production is larger. However, the exact correspondence may not be expected, since both the B3n and ln states contribute to the 1 atom production, while only the B3n state gives rise to fluorescence. Then the percent absorption due to a transition to the B3fl state must be known at each wavelength. 

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