Radiative processes of excited states

Emission from the upper electronic excited states of polyatomic molecules, in violation of Kasha s rule which allows emission only from the lowest excited states Q), have been observed in a reasonable variety of molecules (2 11). With notable exceptions of azulene and thioketone, however, such emission is usually very weak, because the rates of nonradiative decay processes greatly exceed the rates of radiative processes when excited states other than the lowest excited states are involved. 

In radiolysis, a significant proportion of excited states is produced by ion neutralization. Generally speaking, much more is known about the kinetics of the process than about the nature of the excited states produced. In inert gases at pressures of a few torr or more, the positive ion X+ converts to the diatomic ion X2+ very rapidly. On neutralization, dissociation occurs with production of X. Apparently there is no repulsive He2 state crossing the He2+ potential curve near the minimum. Thus, without He2+ in a vibrationally excited state, dissociative neutralization does not occur instead, neutralization is accompanied by a col-lisional radiative process. Luminescences from both He and He2 are known to occur via such a mechanism (Brocklehurst, 1968). 

The lifetime of the excited state will be influenced by the relative magnitudes of these non-radiative processes and thus time-resolved spectroscopy can provide information on the dynamics of excited state depletion... 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

It would be elegant to finish the part on photophysics and photochemistry of liquid alkanes by giving a picture that unifies the temperature- and energy-dependence results obtained in fluorescence and photodecomposition studies. However, the spectroscopic information available for alkane molecules is not sufficient to identify the exact excited states involved in the radiative and nonradiative processes [55]. Because of the lack of information, there are different views on the positions and identities of excited states involved [52,55,83,121,122]. 

Photophysical processes, that is, ones not involving any change in composition of an A, have become of much interest to the inorganic photochemist, particularly in terms of excited state kinetic schemes. A brief discussion of the phenomenology and theory of radiative and nonradiative deactivations follows. 

In order to avoid such ambiguities, the definition of chemical species will depend on the simple concept of stability. In the absence of chemical reactions, a chemical species will last indefinitely. Thus an ion is a distinct chemical species, and an electron transfer reaction must be seen as a chemical change. However, an electronic excited state of an atom or molecule must inevitably decay back to the ground state, so the processes of excitation, emission and non-radiative deactivation are photophysical processes. 

The quantum yield of a primary photochemical process is related to the actual rate constant for the reaction and to the rate constant(s) for all other deactivation processes of the excited state, as well as to the population of the reactive state (from the state reached directly by the absorption of the photon). If the reactive state is not populated directly by the absorption process it must be populated through the non-radiative decay of higher state(s). If these higher state(s) can also decay without going to the reactive state, the population yield of this reactive state will be less than 1, and the quantum yield of the primary photochemical process cannot be greater than this yield. 

Fig 1. Sample from the manifold of excited states of a representative organic molecule. Straight lines represent radiative and absorptive processes wavy lines show nonradiative processes chemical transformations are not shown in this figure. 

This type of laser produces output pulses which are typically between 1 and 10 ns duration and are well suited to provide initial excitation in the study of the decay of excited states and other transient effects in small molecules. Many fundamental processes, however, occur on a time scale much shorter than the 1—10 ns resolution available with dye lasers of the type discussed above. These processes, such as the relaxation of large biological molecules and dyes in solution, exciton decay and migration in solids, charge-transfer and other non-radiative transfer processes between molecules, and many more, take place on a picosecond time scale. 

The electronic structure of Ni(CO)4 is not as well defined as those of either Cr(CO)6 or Fe(CO)5. This makes the assignment of processes in the early development of the excited-state dynamics somewhat speculative. However there are a number of unique features to the photophysics of CO-loss from Ni(CO)4. Firstly, the CO loss is very slow compared to the other two systems outlined herein taking approximately 600 fs. In addition the Ni(CO)3 fragment is produced in its St state and this state persists because there is no facile deactivation process available based on molecular motions. Deactivation can be achieved only by further CO loss or by radiative processes of either fluorescence or phosphorescence. The overall scheme of potential energy curves and pathways for photoinduced loss of CO from Ni(CO)4 is represented in Fig. 29. 

Finally, we discuss briefly the emission decay behavior. At T = 1.3 K, the emission decay time is mainly determined by radiative and non-radiative processes of state I. For Pt(2-thpy-hg)(2-thpy-dg) one finds a value of (120 3) ps, which thus lies between (110 3) ps and (140 3) ps of the perprotonated and per-deuterated compounds, respectively (see also Fig. 26). Apart from the effects of spin-lattice relaxation occurring in the first microseconds, the decay is strictly monoexponential, at least over five lifetimes. It is important that the decay is exactly equal, when measured on a vibrational satellite, which is related to the protonated part of the molecule (e.g. 713 cm satellite) and to the deuterated part (e. g. 685 cm satellite), respectively. (Fig. 27b) This result also strongly supports the assignment to a delocalized excited state. A similar behavior has also been observed for [Oslbpylj], for which the lowest triplet states are also delocalized [47]. 

Excitation and the relaxation (radiative and non radiative) processes of the Tryptophan solution and the colloids are represented in figure 18.7. The new relaxation pathways introduced by the metal nanoparticles (nonradiative decay rate, K p) are shown in figure 18.7(b). Although, one photon at 270nm and two-photons at 532nm are resonant with the excited states of the molecule, these wavelengths are not in resonance with the Plasmon energy level. 

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