Radiationless transitions lifetime

Sodium Acetate-Sodium Chloride Mixtures. Ramasamy and Hurtubise (12) obtained RTF and RTF quantum yields, triplet formation efficiency, and phosphorescence lifetime values for the anion of p-aminobenzoic acid adsorbed on sodium acetate and on several sodium acetate-sodium chloride mixtures. Rate constants were calculated for phosphorescence and for radiationless transition from the triplet state. The results showed that several factors were important for maximum RTF from the anion of p-aminobenzoic acid. One of the most important of these was how efficiently the matrix was packed with sodium acetate molecules. A similar conclusion was found for RTF however, the RTF quantum yield increased more dramatically than the RTF quantum yield. 

To get a rough idea of the rate of radiationless transition from higher excited states, consider the quantum yield of fluorescence from that state as the ratio of nonradiative (t ) to radiative (t ) lifetimes as follows ... 

The excited singlet-state radiative lifetime, o, of Si is the lifetime of Si in the absence of any radiationless transitions that is, the only... 

The lifetime of an excited state of a molecular entity (in the absence of any radiationless transition). Radiative... 

JABLONSKI DIAGRAM RADIATIONLESS TRANSITION FLUORESCENCE RADIATIVE LIFETIME RADICAL (or, FREE RADICAL)... 

The decay time of the Cr " band of approximately 150 ns is very short for such emission. Radiative energy transfer may not explain it because in such a case the decay curves of each of the ions are independent of the presence of the other. Thus non-radiative energy transfer may also take part, probably via multipolar or exchange interactions. In such cases the process of luminescence is of an additive nature and the lifetime of the sensitizer from which the energy is transferred is determined, apart from the probability of emission and radiationless transitions, by the probability of the energy transfer to the ion activator. 

The radiationless transitions are indicated by wavy lines with the corresponding rate constants or lifetimes. The rates of thermalization of a dye molecule after an absorptive or emissive transition, namely k in the excited singlet state Si and k" in the ground state So, axe so fast that they have not yet been reliably measured. From indirect evidence they are believed to be k k" rs 1012 — 1013 sec-1. The rate 21 °f radiationless internal conversion from S2 to Si has recently been measured for several dyes and was found to be in the range kzi = 1011 —1012 sec-1 5>6 While has not yet been measured directly, there is no reason to believe that it should be much different from A 21, and indirect evidence seems to... 

From the data of Hoogschagen and Gorter (104), the oscillator strength of the 5D4-+7F6 transition was obtained. By means of the Ladenburg formula, the spontaneous coefficient A46 was calculated. Using the relative-emission intensities, the rest of the A4J spontaneous-emission coefficients could be calculated. From these and a measured lifetime of 5.5 x 10 4 sec at 15°C, he calculated a quantum efficiency of 0.8 per cent. Kondrat eva concluded that the probability of radiationless transition for the trivalent terbium ion in aqueous solution is approximately two orders of magnitude greater than for the radiation transition. 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

In aromatic hydrocarbons, the radiationless transitions from the triplet to the ground state are dominated by CH stretching vibrational modes. In these hydrocarbons perdeuteration reduces kJsc and consequently enhances kp, the rate constant for phosphorescence emission. The lifetime rp, is considerably reduced. The reduction of kjsc (T, 5,) which... 

Fluorescence and phosphorescence are relatively rare. Molecules generally decay from the excited state by radiationless transitions. The lifetime of fluorescence is always very short (10-8 to 10-4 s). The lifetime of phosphorescence is much longer (10-4 to 102 s). Therefore, phosphorescence is even rarer than fluorescence, because a molecule in the T] state has a good chance of undergoing intersystem crossing to S0 before phosphorescence can occur. 

Besides a transition to a continuum level of an excited electronic state, dissociation can occur by another mechanism in electronic absorption spectroscopy. If the potential-energy curve of an excited electronic state A that has a minimum in UA(R) happens to be intersected by the U(R) curve of an unstable excited state B with no minimum in U, then a vibrational level of A whose energy lies near the point of intersection of UA and UB has a substantial probability to make a radiationless transition to state B, which then dissociates. This phenomenon is called predissociation. Predissociation shortens the lifetimes of those vibrational levels of A that are involved, and therefore by the uncertainty principle gives broad vibrational bands with rotational fine structure washed out. 

It is clear that a number of questions need to be answered. Why, in the condensed phase, is the intersystem crossing between two nn states so efficient What is the explanation of the conflict between the linewidth studies of Dym and Hochstrasser and the lifetime studies of Rentzepis and Busch, with respect to the vibrationally excited levels It was in an attempt to provide some answers to these questions that Hochstrasser, Lutz and Scott 43 carried out picosecond experiments on the dynamics of triplet state formation. In benzene solution the build up of the triplet state had a lifetime of 30 5 psec, but this could only be considered as a lower limit of the intersystem crossing rate since vibrational relaxation also contributed to the radiationless transition to the triplet state. The rate of triplet state build-up was found to be solvent-dependent. 

The well known anomalous fluorescence from S2 has been interpreted in terms of a much slower radiationless transition out of S2 than Si, such that for Si the fluorescence lifetime is severely shortened relative to the radiative lifetime. The anomaly is related to the unusual energy disposition of the two lowest excited singlet states. Hochstrasser and Li wished to ascertain whether the spectral linewidths were consistent with this interpretation and also whether the Si linewidths of azulene-ds were narrowed in comparison, as theoretically predicted. Their results are listed in Table 1. The spectral resolution was claimed to be <0.15 cm-1 as linewidths in the S2 system corresponding to the observed fluorescence lifetime are of the order of 10-4 cm-1, the linewidths of 0.50 cm-1 measured must be considered crystal-imposed. It is assumed that the maximum crystal inhomogeneity contribution to the Si linewidth is similarly 0.50 cm-1. This leads to a line broadening due to rapid nonradiative electronic relaxation of 1.61 (-hs) and 1.27 (-da) cm-1 as compared to 0.64 cm-1 (-hs) determined by Rentzepis 50> from lifetime studies of azulene in benzene solution at 300 K. 

Radiative lifetime (to) The lifetime of an excited molecular entity in the absence of radiationless transitions. It is the reciprocal of the first-order rate constant for the radiative step, or of the sum of these rate constants if there is more than one such step. The equivalent term, natural lifetime, is discouraged. Approximate expressions exist relating Tq to the oscillator strength of the emitting transition. 

For most carbonyl compounds, kp is approximately independent of vibrational excitation energy (Eypp,), whereas kfjR usually increases with Evpp,. Therefore, p becomes smaller and ip becomes shorter as Evpb increases. Typically, highly sensitive technique of fluorescence excitation spectroscopy permits measurement of Op over an extensive range of Evib- Hence the rates of radiative transitions as well as the rates of radiationless transitions of SVLs and SRVLs can be readily determined (135). For simple carbonyl compounds with small amounts of vibrational energy in the Sp state, collision-induced processes can become important at pressures above a few torr, since the lifetimes (tp) are comparable to the mean collision times (tu) at these pressures. Rate data reported for a number of aliphatic carbonyls are summarized in Table 2. 

Hie lifetimes for the various excitation and deexcitation processes hence vary over many orders of magnitude (Appendix III, Section A)—femtoseconds for light absorption, picoseconds for radiationless transitions between excited electronic states, nano- to microseconds for fluorescence, and milliseconds to seconds for phosphorescence. 

Assume that some excited singlet state can become deexcited by three competing processes (1) fluorescence (lifetime = 10-8 s), (2) a radiationless transition to an excited triplet state (5 x 10-9 s), and (3) a radiationless transition to the ground state (10-8 s). 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

The lifetime of the MLCT excited state of [Ru(bipy)3] has been measured in several solvents and at different temperatures. Among the- conclusions drawn are that k, is only slightly solvent-dependent, that k , agrees quantitatively with predictions of energy gap law for radiationless transitions, and that the solvent dependence of kinetic parameters for MLCT —> d, dctransitions can be considered in the context of electron-transfer theory. These results may have implications for the use of [Ru(bipy)3] as sensitizer. The solvent dependence of the MLCT transitions of [Ru(bipy)3], [Os(bipy)3], [Os(bipy)2(py)2p, and [Os(bipy)2 l,2-(Ph2P)2CsH4 f has been interpreted in... 

The details of the mechanism of decay of states in alkanes retain their interest. The effect of deuterium on fluorescence lifetimes has been discussed in terms of the theory of radiationless transitions. Analysis of fluorescence line shapes and Raman excitation profiles of tetradesmethyl-p-carotene in isopentane has been carried out at 190 and 230K . Solvation occurs over a time scale of about 100 fs whilst vibrational relaxation has a time scale of about 250 fs. The kinetics of the interaction of alcohols with the excited state of triethylamine shows involvement of a charge transfer exciplex . Ionizing radiation is a means of exciting saturated hydrocarbons and the complexity of three component systems containing saturated hydrocarbons, aromatic solvent, and fluorescent solute has been examined. ... 

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