Radiationless transitions benzene

Calculation of radiationless transition probabilities in benzene / for benzene at 77°C = 0.2... 

Prepared State. Here the Hamiltonian H is the time-independent molecular Hamiltonian. Both H0 and T are time independent. The initial prepared state is an eigenket to H0 and thus is nonstationary with respect to H = H0 + T. One example is provided by considering H0 as the spin-free Hamiltonian 77sp and the perturbation T as a spin interaction. A second example is provided by considering H0 as the spin-free Born-Oppenheimer Hamiltonian and T as a spin-free nonadiabatic perturbation. In the first example spin-free symmetry is not conserved but double-point group symmetry may be. In the second example point-group symmetry is not conserved, but spin-free symmetry is. The initial prepared state arises from some other time-dependent process as, for example, radiative absorption which occurs at a rate very much faster than the rate at which our prepared state evolves. Mechanisms for radiationless transitions in excited benzene may involve such prepared states, as is discussed in Section XI. 

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. 

Figure 5.12. Qualitative state diagram for the fluorescence quenching of benzene by radiationless transition into one of the higher vibrational levels of the isomeric benzvalene. The back reaction is a hot ground-state reaction.

A recent tunneling effect model for radiationless transitions (198) has been applied to benzene and other aromatic hydrocarbons. The CH stretching vibrations are considered as dominant for the non-radiatlve process. Rate constants for the radiationless process Sg calculated by theory are of the same order of magni-... 

The Influence of Final-State Spectrum in Radiationless Transitions. Part II. Application to Benzene. 

The photochemistry of unsubstituted benzene has recently been summarized by Bryce-Smith S9> and will not be repeated here. Several very recent reports should be mentioned, however. Ward and Wishnok 6°) studied the liquid phase vacuum ultraviolet photolysis of benzene and were ible to identify Dewar benzene 78, benzvalene 79 and fulvene 80 in the relative amounts of 1 5 2, respectively. The quantum yields for the formation of 78—80 were estimated to be 0.006, 0.03 and 0.012, respectively. The low quantum yields suggest that radiationless transitions to... 

Much work, both experimental and theoretical, has been carried out on the radiationless transitions of collision-free Si benzene. At vibrational energies greater than about 3000cm"1, both the quantum yield [58] and lifetime [59] of fluorescence and the quantum yield of S, - TISC [60-62] decrease dramatically relative to those at lower excess energies. Concomitantly, the Al9(S0) absorption system exhibits a sudden... 

For larger molecules, rotationally resolved absorption spectra could, for the first time, be measured, as has been demonstrated for the UV spectra of benzene Celle Spectral features, which had been regarded as true continua in former times, could now be completely resolved (Fig. 7.33) and turned out to be dense but discrete rotational-line spectra [7.52,7.53]. The lifetimes of these upper levels could be determined from the natural linewidths of these transitions [7.54]. It was proven that these lifetimes strongly decrease with increasing vibrational-rotational energy in the excited electronic state because of the increasing rate of radiationless transitions [7.55]. 

Braitbart O, Castellucci E, Dujardin G, Leach S (1983) Radiationless transitions in excited electronic states of the benzene cation in the gas phase. J Phys Chem 87 4799... 

Calculated radiationless transition rates for benzene and deuterobenzene. /. Chem. Phys., 51, 4548. 

Figure 7.14 Possible radiationless processes following creation of 6 benzene, one of the vibronic levels which is El-accessible from ground-state benzene (cf. Fig. 7.13). This level may undergo internal conversion (IC) to an isoenergetic, vibration-ally hot So molecule, or it may undergo intersystem crossing (ISC) to an isoenergetic level in triplet state T.,. The T - So phosphorescence transition can be monitored for experimental evidence of ISC. Time-dependent S — Sq fluorescence decay furnishes a probe for depopulation of S., through radiative (fluorescence) and nonradiative (IC, ISC) decay.

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