Nonradiative decay channels

The interpretation of our CPG data is complicated by the presence of comparatively fast radiative and nonradiative decay channels for the singlet exciton, which compete with the field-induced dissociation. In order to provide a clear picture of the observed mechanism and disentangle it from the singlet exciton decay dynamics, we define the following phenomenological time-dependent parameter ... 

Owing to the electron-vibrational interaction in molecules, there is one more possible decay channel for SES. This is the nonradiative relaxation (internal conversion), in which the electron energy is transferred into vibrational energy of molecules (in the condensed phase, into thermal energy of the medium). If the molecule fluoresces, there may also occur fluorescence from the lowest excited state. (According to the empirical rule of Kasha,64 the molecular fluorescence occurs from the lowest excitation level irrespective of the wavelength of the exciting radiation.)... 

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

Let us first consider a simple Auger process with only a single decay channel [Nc = 1, see Eq. (4)]. Although not characteristic of K-shell Auger, such processes take place upon (n - l)p ionization of alkaline earth atoms, for example, in (2p 1) Mg+, where the only nonradiative decay pathway involves the two 3s electrons (2p-1) Mg+ (3s-2) Mg2+ + e. Assume that... 

Irrespective of the exact nature of the biexponential fluorescence decay of PdG (emission from two different conformers or bifurcation of the initial irir -state population to two nonradiative decay channels), it is important to note that the subpicosecond excited-state decay, characteristic of guanine or guanosine, is clearly absent in PdG. Thus, the presence of the exocyclic ring, which hinders the out-of-plane deformation of the six-membered ring (C2 in particular), leads to a dramatically reduced internal conversion rate. 

Among the best well-known examples of photostability after UV radiation, the ultrafast nonradiative decay observed in DNA/RNA nucleobases, has attracted most of the attention both from experimental and theoretical viewpoints [30], Since the quenched DNA fluorescence in nucleobase monomers at the room temperature was first reported [31] new advances have improved our knowledge on the dynamics of photoexcited DNA. Femtosecond pump-probe experiments in molecular beams have detected multi-exponential decay channels in the femtosecond (fs) and picosecond (ps) timescales for the isolated nucleobases [30, 32-34], The lack of strong solvent effects and similar ultrafast decays obtained for nucleosides and nucleotides suggest that ultrashort lifetimes of nucleobases are intrinsic molecular properties, intimately... 

Thayer et al. (240-242) on propynal are the only reasonably complete nonradiative rate calculations done on a carbonyl. Their values for the intersystem crossing and internal conversion rates are low by factors of 10 and 80, respectively, for the vibrationless excited state when compared to the experimental values. They correctly predict the energy dependence of the decay channels, although they fail to predict the large enhancement of the intersystem crossing rate for three vibronic levels. Also, the energy dependence of the collision-induced nonradiative transitions seems to be well reproduced. 

The presence of interfaces within a polymer LED can also introduce additional nonradiative decay channels. This is particularly important in proximity to a metal electrode. Excitons which are able to diffuse to the metal surface are liable to be quenched directly by interaction with the metal wave function. This mechanism is therefore active only within a few nanometers of the interface. At larger distances (up to about 100 nm), excited molecules can couple to the surface plasmon excitations in the metal, thus providing a further nonradiative decay channel. The combined effects of changes in the radiative and nonradiative rates in two-layer LED structures have been modelled by Becker et al.,83 who have been able to model the variation in EL efficiency with layer thickness due to changes in the efficiency of exciton decay. 

The main conceptual advance made in the last few years is the acceptance that electron-transfer process at dye-sensitised systems under barrierless conditions can be purely electronic. A measurement of the nonradiative decay channel due to electron transfer under these conditions gives a direct determination of the electronic coupling. Subsequent to the initial work pointing this out, there have been a number of determinations of extremely fast electron-transfer times at dye-sensitised surfaces. For dye-derivatised TiOi electron-transfer times from 10 fs to 100 fs have been reported by a number of groups (Rehm et al, 1996 Tachibana et al, 1996 Hannappel et al,... 

The model (9.73)—(9.75) was presented as an initial value problem We were interested in the rate at which a system in state 0) decays into the continua L and R and have used the steady-state analysis as a trick. The same approach can be more directly applied to genuine steady state processes such as energy resolved (also referred to as continuous wave ) absorption and scattering. Consider, for example, the absorption lineshape problem defined by Fig. 9.4. We may identify state 0) as the photon-dressed ground state, state 1) as a zero-photon excited state and the continua R and L with the radiative and nonradiative decay channels, respectively. The interactions Fyo and correspond to radiative (e.g. dipole) coupling elements between the zero photon excited state 11 and the ground state (or other lower molecular states) dressed by one photon. The radiative quantum yield is given by the flux ratio Yr = Jq r/(Jq r Jq l) = Tis/(Fijj -F F1/,). 

State. This state is broadened by its interaction with the radiative continua A, and the nonradiative manifold L. Spontaneous emission (i.e. fluorescence) is the process in which the state s) decays into the radiative continua R[, that is, to vibronic levels of the ground electronic states plus a photon. A competitive decay channel is the nonradiative decay of s> into the nonradiative manifold(s) L. Light scattering is a process that starts with a 1-photon level of the ground electronic state and ends in another such state, that is, 11, vi, k) 11, vj, k ). The elastic process vi = vj and I k I = I k I is called Rayleigh scattering. The inelastic process where these equalities are not satisfied is Raman scattering. 

When a mechanism for each of the relevant decay channels is assumed and a set of specific interaction matrix elements is calculated by ab initio or semi-empirical methods, multichannel quantum defect theory (MQDT) (see Section 8.9) provides a basis for a unified and global treatment of the competing nonradiative decay processes and how these competing processes are sampled by specific experiments. Giusti-Suzor and Jungen (1984) were the first to apply MQDT to... 

The rates k/ and k correspond to any nonradiative decay channel, which couple to the levels /) and /n). It should be kept in mind, however, that reverse collisionally induced electronic transitions may follow vibrational relaxation in the /) manifold, thus leading to further emission. This effect must be taken into account when the jj) level is not the lowest one in the j>) manifold (see Section III.B). 

The fluorescence quantum yields for all NABs are very low in aqueous solutions at room temperature, the excitation energy being dissipated through the nonradiative decay channels. On the other hand protonated purines show fluoresc ice at room temperature as well as after being absorbed on a chromatographic p >er. To obtain insight into the excited state properties of the neutral bases and nucleotides in polar solvents, different low temperature experiments were performed. The first low temperature work on nucleic acids was reported in I960, while the phosphorescence of nucleic acids was first reported for adenine derivatives in 1957. The first... 

The small-molecule differs from an intermediate-case system by the absence of nonradiative decay channels y, = y /. Its total emission yield Qs + Qi is always equal to one, while in the intermediate case, Qs + Qi< 1-Otherwise their behaviors are identical. 

In this kinetic scheme, k has been represented as a simple first-order constant. It may of course contain many constants, both first and second order, representing competing nonradiative decay channels. But a single pair of measurements, f and in a single experiment will describe only an apparent first-order constant, and this is the proper starting point for characterization of the data. Then subsequent examination of k r under a wide variety of experimental conditions can reveal the extent of its fine structure. 

Once again, the high-pressure data—particularly that of Noyes and co-workers—plays the key role in the single vibronic level experiments. The data discussed in Section V suggest that the only nonradiative decay channel from the lower levels in the state is intersystem crossing to the triplet state. They do not accurately define the domain of lower levels, but certainly the zero point and 6 level at 523 cm", and probably levels somewhat beyond the 6 1 level at 1446 cm" are included. Chemical relaxation cannot be excluded, but the sum of fluorescence and triplet yields from the thermal levels is near unity. Chemical relaxation must be small, and a plausible case can be built for its limitation to levels above 2500 cm" (Section VI). 

Schuyler. Lifetime measurements of levels above 3000 cm are being obtained by Callomon and co-workers which promise to display the sharp onset of a new nonradiative decay channel. Although all of the experiments are utterly dependent on the results of the more conventional high-pressure and condensed phase studies for interpretation, they extend into a new dimension of detail. The vibronic and even some rotational structure of relaxation channels is clearly visible. It is certain that these and similar studies in other systems will provide much stimulation for further theoretical developments in excited state relaxation. 

In the following discussion, we consider first the intramolecular nonradiative decay channels, which can occur for isolated oligomers, then intermolecular nonradiative decay channels, which may also operate in solid state thin films, where the oligomers or polymers are densely packed. We also consider the effects of interring torsion and coplanarity of the 7r-conjugated chains, which give rise to both intramolecular and intermolecular effects. 

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