Nonradiative transition channels

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 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 nonradiative rate constants in Table VII present another magnitude of difficulty. We must first discover what channel or channels these rate constants describe. The data are useless until this question is answered, but the answer is hard to formulate. By definition, nonradiative transitions are hard to see, and those in benzene are no exception. There are no data that directly comment on this problem. The final state of the nonradiative channel has not been seen under the conditions used for the data in Table VII. The closest experiments are the 2536 A isolated molecule exercises described in Section VII A, and they show only that at least part of the relaxation from at least one state near 2000 cm" leads to triplet formation. 

The nonradiative transitions in pyridine vapor have been studied using femtosecond, time-resolved mass spectroscopy. In these experiments, pyridine vapor was excited with a femtosecond light pulse of 277 nm, which is below (-300 cm ) the channel three threshold. Time-resolved mass spectroscopy revealed a fast decay component of 400 fs, which describes the initial motion on the pyridine potential surface and shows components of 3.5 and 15 ps, which were assigned to the formation of Dewar-pyridine 112 and azabenzvalene 111, respectively (Scheme 28). ... 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

In the treatment of radiationless transitions presented above, we have mainly considered the case of a closed channel decaying into a single open channel, which latter consists of the dense vibronic manifold of some one electronic state (statistical limit). That description is obviously incomplete, since both radiative and nonradiative decay processes occur simultaneously. Clearly, a complete theoretical description of the radiationless transition... 

The imbedded nature of the potential curves in Figure 6 for electron transfer in the inverted region is a feature shared with the nonradiative decay of molecular excited states. In fact, in the inverted region another channel for the transition between states is by emission, D,A -> D+,A + hv, which can be observed, for example, from organic exciplexes,74 chemiluminescent reactions,75 or from intramolecular charge transfer excited states, e.g. (bipy)2Rum(bipyT)2+ - (bipy)2Run(bipy)2+ + hv. 

Polymer photophysics is determined by a series of alternating odd (B ) and even (Ag) parity excited states that correspond to one-photon and two-photon allowed transitions, respectively [23]. Optical excitation into either of these states is followed by subpicosecond nonradiative relaxation to the lowest excited state [90]. This relaxation is due to either vibrational cooling within vibronic sidebands of the same electronic state, or phonon-assisted transitions between two different electronic states. In molecular spectroscopy [146], the latter process is termed internal conversion. Internal conversion is usually the fastest relaxation channel that provides efficient nonradiative transfer from a higher excited state into the lowest excited state of the same spin multiplicity. As a result, the vast majority of molecular systems follow Vavilov-Kasha s rule, stating that FT typically occurs from the lowest excited electronic state and its quantum yield is independent of the excitation wavelength [91]. 

Photoluminescence (PL) is the emission of light from a material following its illumination. The frequencies of both absorption and emission in luminescence are determined by the transitions between the electronic states, i.e., they correspond to the visible, or close to it, region of the spectrum. The rapidly decaying luminescence typical for atoms and molecules is usually called fluorescence. The lifetime, t, of an excited electronic state can be represented as originating from two competitive channels, radiative versus nonradiative, as... 

For each photoelectron that leaves the surface, an atom with a core hole is left behind in a highly excited state, which relaxes both by radiative and nonradiative processes. In a radiative recombination process, the core hole is filled in an electronic transition from a core level of lower binding energy or a valence level. The surplus energy is released by the emission of an X-ray photon, in a so-caUed X-ray fluorescence process. In this process, the emitted photon has a lower energy than the exciting photon and dipole selection rules apply for both, excitation and de-excitation. Conversely, Auger processes are nonradiative de-excitation channels... 

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