Fluorescence natural lifetime

From spectroscopic measurements, we can estimate the fluorescence lifetime, t [. = (PpTi. where the natural lifetime, rN, can be calculated from the Strickler-Berg equation in CGS units [60] ... 

If the only way of de-excitation from Sj to S0 was fluorescence emission, the lifetime would be l/krs this is called the radiative lifetime (in preference to natural lifetime) and denoted by rf. The radiative lifetime can be theoretically calculated from the absorption and fluorescence spectra using the Strickler-Berg relation6 . 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

Shelving spectroscopy thus involves many decisions whether the antihydrogen atom has been excited to the metastable 2 2S /2 state or not. These decisions have to be made somewhat quicker than the natural lifetime of the metastable state and are based on the observation or the non-observation of fluorescent light at Lyman-a. The detection efficiency for fluorescent light from an antihydrogen sample in a magnetic trap with superconducting coils is probably rather... 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

From the oscillator strength / = 1.89 of the absorption band of anthracene (1) at 39,700 cm the natural lifetime of the excited state can be estimated from Equation (5.3) as 0.5 x 10 s. Since no fluorescence from this higher excited state S is observed, the actual lifetime must be smaller at least by a factor of I0 so one has... 

The natural lifetime of the triplet state = Mkf may be estimated from the observed lifetime and the quantum yields of fluorescence and phosphorescence. According to Equation (5.11)... 

Cline-Love and Upton found microsecond natural lifetimes for 5,5-disubstituted barbiturates.69 This suggests that the fluorescence transition is of the n - n type, although the absorption should be of the n - n type. Moreover, natural lifetimes of the excited states for barbiturates with unsaturated substituents are longer than those for barbiturates with alkyl groups. A small mixing of electronic levels of the fluorophore with the levels of the electronic environment provided by the unsaturated substituents have been proposed to explain this phenomenon.69... 

In contrast to kc, relatively little data exist for ks. This dearth can be attributed directly to the absence of prompt fluorescence when is above E, the most common disposition of the energy levels in complexes (22). However, in those few systems where is below E, intense fluorescence with tf 10 sec (23, 24, 25, 26) is observed at low temperatures. Since this value is close to the natural lifetime of A2, 3° and consequently 3° is not much larger than 10 sec" in these cases. 

Time-of-Flight. The term is used in experiments of high-energy physics, mass spectroscopy, and diffuse optical tomography (DOT). The TOF-distribution in DOT is the distribution of the photons versus time after propagation through a turbid medium. Fluorescence lifetime spectroscopy r is used for the fluorescence lifetime or the lifetime components in multiexponential decay functions. rn=natural lifetime in the (hypothetical) absence of all nonradiative decay processes, To = observed lifetime in the absence of external quenching processes, Tmt rotational depolarisation time. 

The excited-state lifetime of the molecule in absence of any radiationless deeay processes is the natural fluorescence lifetime", r . The natural lifetime is a constant for a given molecule and given refraction index of the solvent. Because the absorbed energy can also be dissipated by internal conversion, the effective fluorescence lifetime, is shorter than the natural lifetime, The fluorescence quantum efficiency", i.e. the ratio of the number of emitted photons to absorbed photons, reflects the ratio of the radiative decay rate to the total decay rate. Most dyes of high quantum efficiency, such as laser dyes and fluorescence markers for biological samples, have natural fluorescence decay times of the order of 1 to 10 ns. There are a few exceptions, such as pyrene or coronene, with lifetimes of 400 ns and 200 ns, and rare-earth chelates with lifetimes in the ps range. 

It is important to notice that the fluorescent lifetime is what is experimentally obtained, and the natural lifetime can be calculated. 

Natural fluorescence of protons, 480-481 Natural lifetime, 10,453 NAiyrA (Af-ace lrL-tyioainamids), 447,448 intensity decqr of, 488,491-492 structure, 489... 

Phosphorescence emission is the result of a transition between states of different multiplicity (typically Tj to Sq) that has a much smaller rate constant than that for fluorescence. Consequently, the natural lifetime, Xq, of the triplet state is long, varying between 10 s and seconds. The natural lifetime can be formulated according to Equation 4.4. 

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