Radiative decay lifetime

The absence of an enormous enhancement in radiative decay rates in the nanocrystals can also be verified by electronic absorption spectroscopy. The original claim stated that the Mn2+ 47) —> 6A1 radiative decay lifetime dropped from xrad = 1.8 ms in bulk Mn2+ ZnS to xrad = 3.7 ns in 0.3% Mn2+ ZnS QDs ( 3.0 nm diameter) (33). This enhancement was attributed to relaxation of Mn2+ spin selection rules due to large sp-d exchange interactions between the dopant ion and the quantum-confined semiconductor electronic levels (33, 124— 127). Since the Mn2+ 47 > 6Ai radiative transition probability is determined... 

Carrier and exciton dynamics in InGaN/GaN MQWs have also been studied at a high optical pumping power [34], At 7 K, a radiative decay lifetime of 250 ps was observed for the dominant transition at a generated carrier density of 1012/cm2. The time-resolved measurement showed that the decay of PL has a bimolecular recombination characteristic. At room temperature, the carrier recombination was found to be dominated by non-radiative processes with a measured lifetime of 130 ps. Well width dependence of carrier and exciton dynamics in InGaN/GaN MQWs has also been measured [35]. The dominant radiative recombination at room temperature was attributed to the band-to-band transition. Combined with an absolute internal quantum efficiency measurement, a lower limit of 4 x 10 9 cm3/s on the bimolecular radiative recombination coefficient B was obtained. At low temperatures, the carrier... 

The signal has phase 4> as well as amplitude q, the phase being related to the time required for the heat to reach the surface and the sound to reach the microphone. The phase will depend on the absorptivity e(cm-l), the thermal diffusivity ag (cm s- ) and the non-radiative decay lifetime (x). 

The intensity of fluorescence induced by the desorbing flux is determined by the quantity A/g/r with Ng the total number of desorbed atoms in the excited state and t their radiative decay lifetime. In a steady-state excitation... 

We now discuss the lifetime of an excited electronic state of a molecule. To simplify the discussion we will consider a molecule in a high-pressure gas or in solution where vibrational relaxation occurs rapidly, we will assume that the molecule is in the lowest vibrational level of the upper electronic state, level uO, and we will fiirther assume that we need only consider the zero-order tenn of equation (BE 1.7). A number of radiative transitions are possible, ending on the various vibrational levels a of the lower state, usually the ground state. The total rate constant for radiative decay, which we will call, is the sum of the rate constants,... 

If there are no competing processes the experimental lifetime x should equal Tq. Most connnonly, other processes such as non-radiative decay to lower electronic states, quenching, photochemical reactions or... 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

The three summands found in the right-hand side of expression (5.10) correspond to the three major channels (ways) of EEP losses the first summand characterizes the gaseous-phase de-excitation due to collisions, the second one stands for the gaseous-phase de-excitation on account of spontaneous radiation, and the third summand characterizes the heterogeneous decay of EEPs. A possible contribution of the radiative term to the value of ) D can be done a priori. With the radiative time of EEP lifetime r,ad known from the spectroscopy, one can easily estimate (by the formula of Einstein) the diffusion length over which the radiative decay of EEP will be perceptible ... 

In (8), the solvent-independent constants kr, kQnr, and Ax can be combined into a common dye-dependent constant C, which leads directly to (5). The radiative decay rate xr can be determined when rotational reorientation is almost completely inhibited, that is, by embedding the molecular rotor molecules in a glass-like polymer and performing time-resolved spectroscopy measurements at 77 K. In one study [33], the radiative decay rate was found to be kr = 2.78 x 108 s-1, which leads to the natural lifetime t0 = 3.6 ns. Two related studies where similar fluorophores were examined yielded values of t0 = 3.3 ns [25] and t0 = 3.6 ns [29]. It is likely that values between 3 and 4 ns for t0 are typical for molecular rotors. 

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

Lakowicz JR, Shen Y, D Auria S, Malicka J, Fang J, Gryczynski Z, Gryczynski I (2002) Radiative decay engineering 2. Effects of silver island films on fluorescence intensity, lifetimes, and resonance energy transfer. Anal Biochem 301 261-277... 

Herein, F is the radiative decay rate and km is the nonradiative decay rate, which comes from quenching. It has been demonstrated that silica nanomatrixes can change the fluorescence quantum yield and lifetime of fluorophores. Several groups have reported that both quantum yield and lifetime of fluorophores increased in DDSNs [27, 28, 52, 65-67]. However, the mechanisms regarding this enhancement were reported differently. 

The geometry of the nanoscaled metals has an effect on the fluorescence enhancement. Theoretically, when the metal is introduced to the nanostructure, the total radiative decay rate will be written as T + rm, where Tm corresponds to the radiative decay rate close to the metal surface. So, (1) and (2) should be modified and the quantum yield and lifetime are represented as ... 

Rh(bpyL3+ is an example of a complex that exhibits an almost pure n-n phosphorescence and demonstrates one of the limitations of nearly pure ligand localized emissions. At 77K, the complex is highly emissive with a beautifully structured blue ligand phosphorescence (Amax = 446 nmfor the first peak) having at in the tens of msec,(17) but it has no detectable room temperature emission. It is this very long radiative lifetime that causes the absence of room temperature emission. The radiative decay is so slow that it cannot compete effectively against inter- and intramolecular radiationless decay at room temperature. 

From the practical point of view, the radiative decay rate kr may be assumed to be independent of the external parameters surrounding the excited sensor molecule. Its value is determined by the intrinsic inability of the molecule to remain in the excited state. The radiative decay rate kr is a function of the unperturbed electronic configuration of the molecule. In summary, for a given luminescent molecule, its unperturbed fluorescent or phosphorescent decay rate (or lifetime) may be regarded to be only a function of the nature of the molecule. 

The term in Eq. (49) which describes radiative decay is relatively small under conditions where stimulated emission does not occur. Thus, only when all other contributions to the over-all decay are accurately determined will this term be accessible. At present only time-resolved atomic emission studies provide the required precision.11 The mean radiative lifetime has been obtained by observing the variation in the first-order decay coefficient for this emission as a function of inert gas pressure (Fig. 11). The slope of the graph so obtained yields the diffusion coefficient, while the intercept represents the sum of... 

In any low angular momentum state the radiative decay rate is usually dominated by the high frequency transitions to low lying states, and as a result it is impossible to control completely the decay rate using a millimeter wave cavity. In a circular i = m = n - 1 state the only decay is the far infrared transition to the n — 1 level, and Hulet et al. have observed the suppression of the decay of this level.26 They produced a beam of Cs atoms in the circular n = 22, = m = 21 state by pulsed laser excitation and an adiabatic rapid passage technique.27 The beam of circular state atoms then passed between a pair of plates 6.4 cm wide, 12.7 cm long, spaced by 230.1 jum, and held at 6 K. The 0 K radiative lifetime is 460ps, and... 

There are several considerations to bear in mind when using fluorescence detection. First, the approach is most useful when the photons to be detected have a vastly different wavelength than the exciting light and the most probable decay of the optically excited state, which need not be the same. Second, the branching ratio for the detected transition should be favorable. Third, the lifetimes of the initial and final state of the microwave transitions must be taken into account. If the microwaves are always on, at resonance, radiative decay occurs from the coupled pair of states. If the initial state of the microwave transition has a much... 

Two kinds of unimolecular decay lifetimes can be described. The first is the true radiative lifetime, i.e., the reciprocal of the rate constant for the disappearance of a species which decays only by fluorescence or phosphorescence. Since values of true fluorescence lifetimes may be calculated from the relationship between these quantities and the / numbers (vide supra) of the corresponding absorption bands, these values are (or at least approximations of them) are, in a sense, available. The second kind of lifetime is the reciprocal of an observed first order rate constant for decay of an excited state which may be destroyed by several competing first-order processes (some of which may be apparent first order) operating in parallel. We suggest that the two kinds of lifetime be distinguished by the systematic use of different symbols, as utilized by Pringsheim (4). 

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