Nonradiative excited state decay vibrations

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

The electronic relaxation of the excited singlet states of benzene vapor is rather typical of polyatomic molecules. Relaxation is predominantly nonradiative, and by paths that are sensitive to the degree of vibrational excitation in the decaying electronic state. Indeed, vibrational excitation appears as perhaps the most crucial parameter in setting the course of excited state decay. 

An attempt will be made to separate primary facts about relaxation parameters from the less secure (and sometimes transitory) inferences derived by mechanistic interpretation of the data. Accordingly, for each vibrational domain, attention will be directed first to the elementary separation of excited state decay into the channels of radiative and nonradiative relaxation without effort to further identify the nature of the nonradiative decay. This question will then be discussed separately, for it is an involved and incompletely resolved issue. 

Electronic relaxation in different excited vibronic levels corresponding to the same electronic configuration can be experimentally studied, provided that, as mentioned above, (1) single vibrational levels within the initial electronic state are populated and (2) the excited molecule decays nonradiatively on a timescale much shorter than the mean time between deactivating collisions or by other means such as infrared fluorescence [115]. For typical polyatomic molecules in the gas phase, a narrow-band optical excitation pulse (as small as 1 and shorter relative to the genuine decay times wiH result in the selection of a single vibronic state. U nder these conditions,... 

Finally, nonradiative decay can occur. This name is given to the process by which the energy of the excited state is transferred to the surrounding molecules as vibrational (thermal) energy without light emission. The proeesses that can occur after photochemical excitation are summarized in Fig. 13.1. 

The vibrational overlap is both important and difficult to guess. In polyatomic molecules the large number of excited states near resonance will tend to increase the rates of all nonradiative processes, including decay to both A and B. However, intuition tells us that in a very large molecule not all vibrational levels will be important. For example, fluorescence lifetimes of alkyl derivatives of aromatic hydrocarbons are essentially independent of the length of the attached alkyl chain.26... 

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.)... 

Figure 2a illustrates the concepts of radiative and nonradiative decay, fluorescence quantum yield, and fluorescence decay. A molecule in an excited electronic state can relax by several channels. Molecules excited to a vibrational level in the excited state undergo vibrational relaxation (cooling, yellow arrows in Fig. 2a) to the lowest vibrational levels of the excited state in a... 

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