Benzene single vibronic level

Also worthy of comment is the survey of the single-vibronic-level fluorescence spectra of benzene published recently.13 The study confirms that the dominant fluorescence structure from every emitting level is that arising from transitions in which the e2g mode ve changes quantum number by 1, while = 0 for all other modes except vt. All other transitions are at least an order of magnitude... 

The work that has so far come forward on single vibronic level relaxation shows that these experiments are explicitly dependent on facts about relaxation from a Boltzmann distribution of levels for their interpretation. Without this, it is impossible to use the single-level data in support of theory. The new experiments thus place the Boltzmann studies in an extremely crucial role. For that reason, the derivation of a well supported and elementary set of facts about decay from the thermal vibrational levels of benzene vapor is given in the first sections dealing with excited state relaxation. 

Interpretation of experiments using optical selection of single vibronic levels in the state of benzene is dependent on correct assignment of the absorption bands used for selection. " Close examination of existing absorption analyses shows that much structure is still unassigned and that the assignment of other structure is uncertain. For this reason the Mjj absorption spectrum has been reexamined. ... 

Fig. 13. Qualitalive fluorescence yields of Parmenter and Schuyler from single vibronic levels of benzene vapor. The solid lines indicate the levels from which moderate to strong fluorescence has been observed when they are individually excited with pressures low enough (ca. 0.2 torr) to preclude significant collisional deactivation prior to electronic decay. The dashed lines indicate levels from which emission was too weak to be observed. The notation on the right identifies the vibrational level. For example, (2 X 6) + 1 indicates emission from the vibrational level (2v + of the state.

Such data involving single vibronic levels do not presently exist for any polyatomic molecule except benzene. The first results from benzene experiments of this type were published in 1969. Since then further and more quantitative results have come forth and they are beginning to build a detailed picture of radiative and nonradiative decay in a large molecule. The published segments of those experiments will be discussed subsequently. However, all of these experiments were preceded by low-pressure studies using the 2536 A Hg line for excitation of benzene, and they will be discussed first. 

In 1932 Kistiakowsky and Nelles showed that intense resonance fluorescence could be excited in low pressures of benzene with the 2536 A Hg line. This revealed an opportunity to study isolated molecule relaxation, which was not used until 1964. Since that time, Kistiakowsky and co-workers as well as others have completed extensive studies of isolated molecule relaxation from the several levels populated by absorption of 2536 A radiation. Although the experiments are now being superseded by those which explore relaxation from single vibronic levels, the 2536 A experiments play an important role in answering fundamental questions about isolated molecule properties. They formed the first extensive body of kinetic data on large-molecule intramolecular electronic relaxation, and they still remain one of the most secure experimental descriptions of the intramolecular nature of intersystem crossing. 

Picosecond studies of the dimethyltetrazine dimer, and the benzene/phenol dimeric have also appeared. The dimethyltetrazine dimer study simply reports an IVR time of 35 ps for the 6a vibronic state. This result is in reasonable agreement with the linewidth estimated from spectroscopic data. The benzene/phenol study reports the rate of complex disappearance following single vibronic level excitation. This rate is found to increase with excitation energy above 1275 cm". ... 

Sebnger, B.K. and Ware, W. (1970) Fluorescence lifetime of benzene and benzene-dg vapor excited to single vibronic levels./. Chem. Phys., 53, 3160. 

Parmenter, C.S. and Schuyler, M.W (1970) Single vibronic level fluorescence. III. Fluorescence yields from three vibronic levels in the Bj state of benzene. Chem. Phys. Lett., 6, 339. 

The lowest vibronic levels may be treated in terms of the weak-coupling case, as sparsely spaced, quasi-stationary v states nearly identical (except for an accidental degeneracy) with zero-order n states. In view of the low vibronic-level density, the conditions for a single-resonance excitation are easily fulfilled so that the excited, radiant v> ( s state decays by emission of the resonance fluorescence. This is the case for benzene, aniline, etc. 

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