A magnetic-field-induced change of dynamics from the small molecule behavior to the intermediate case in the S1 state of pyrimidine vapor

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Abstract
Fluorescence intensity and decay of pyrimidine vapor have been measured as a function of external magnetic field (H) with excitation at the individual rotational lines belonging to the 6a 2 0 and 122 0 bands of the S 0→S 1 transition. On excitation into very low rotational levels of 6a 2 or 122, dynamics at zero field is characterized by the small molecule limit, where fluorescence exhibits a nearly single exponential decay superimposed by the quantum beats, but the fluorescence decay at the initial stage of time becomes faster with increasing H and the decay profile becomes biexponential at high fields: A magnetic‐field‐induced change of dynamics from the small molecule behavior to the intermediate case occurs in the S 1 state. A field strength required for the change becomes smaller with increasing the excess vibrational energy above the S 1 origin (ΔE) and with increasing the rotational quantum number of the excited level (J’). On excitation into higher rotational levels, on the other hand, dynamics at zero field is characterized by the intermediate case, where fluorescence exhibits a biexponential decay, and only the slow component is efficiently quenched by H. Magnetic quenching of fluorescence is confirmed to become more efficient with increasing ΔE and with increasing J’. The efficiency both of the magnetic‐field‐induced change of dynamics and of the magnetic quenching of fluorescence is related to the level density of the triplet state coupled to S 1 at zero field. A field‐induced mixing between the triplet rovibrational levels which are coupled and uncoupled to S 1, respectively, seems to play a part in magnetic quenching, besides the field‐induced mixing among the spin sublevels belonging to the triplet levels coupled to S 1 at zero field. On the basis of the rotational state dependence both of the fluorescence decay at zero field and of the magnetic fieldeffects on intensity and decay profile of fluorescence, the relation between level structure and dynamics is discussed.

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