Excess vibrational energy dependence of quantum beats and fluorescence decay in molecules with intermediate case level structure: 1B1 pyrimidine with 0-1300 cm - 1 excess energies
We have measured the biexponential decay and quantum beats from jet-cooled pyrimidine excited to 1B1(nπ*). Single vibronic level excitation of the Q branch of ten vibronic levels produces observable quantum beats in the long component decay of almost all of these levels. Although the ratio of the preexponential factors Af/As becomes progressively larger in a theoretically well-understood manner, the quantum beat pattern remains deceptively simple for all the vibronic levels probed 0-1300 cm-1 above the S1 band origin. We attribute this anomaly to a few levels strongly coupled to the optically prepared singlet resulting in the observed beat pattern, while a dense manifold of less strongly coupled levels dominates the characteristic intensities of the biexponential decay.