A model for rotational-induced vibrational energy redistribution and K selectivity in pyrazine S 1( 1B 3u) state

Abstract

A model is presented invoking Z type Coriolis coupling between the singlet (S)-triplet (T) molecular eigenstates (MEs) in pyrazine and other background triplet T′ states that are usually considered to be unaccessible due to strict selection rules. As Z type Coriolis coupling vanishes for MEs with K″ = 0 (sub-rotational precessional quantum number) the main implication of this model is that the pyrazine molecules with MEs S-T( J″, K″ = 0) will behave in the limit of small molecule showing a sparse sharp ultra-high resolution MEs spectrum. On the other hand, pyrazine molecules with MEs S-T( J″, K″ ≠ 0) will decay in the statistical limit into T′ in a short time, i.e. 120 ps, resulting in the short component which is experimentally observed. Those MEs will be observed as a continuous low quantum yield and hence low amplitude background in the ultra-high resolution spectra. The main difference between this model and the intermediate level structure (ILS) theoretical model is that according to the presented model the dynamical behaviour of pyrazine is inhomogeneous. This model describes pyrazine excited state dynamics in close analogy to that found in benzene in the region near the onset of “channel three”. This Coriolis coupling can also be described as rotational-induced intramolecular vibrational energy redistribution (RIVR). Intrastate RIVR of the triplet component of the MEs in pyrazine is also associated with increased dilution and hence interstate electronic relaxation of the singlet component of the MEs. This paper reviews the wealth of experimental results available in the case of the S 1( 1B 3u) transition of pyrazine, and summarizes the difficulties encountered by the ILS theory confronting these results. After describing our K selective model, we show both a qualitative and quantitative fitting to our model of all the experimental results. Predictions to future experiments are included.