Roles of the Rydberg transitions in fast excitation transfer studied in cyclohexane andnhyphen;heptane using synchrotron radiation

摘要

Excitation spectra of PPO (2,5hyphen;diphenyl oxazole) in cyclohexane solution show peaks at 63thinsp;000 and 78thinsp;000 cmminus;1, while those innhyphen;heptane solution show a single peak near 66thinsp;000 cmminus;1. The 63thinsp;000 cmminus;1peak for cyclohexane and 66thinsp;000 cmminus;1peak fornheptane are ascribed to excitation transfer at respective valencehyphen;band absorption peaks. An extra peak at 78thinsp;000 cmminus;1for cyclohexane solution is tentatively ascribable to the Rydberg transitions and it is situated near the eighth peak of the Rydberg series. The expanded reaction radius is, therefore, estimated to be about 30 Aring;. Taking account of the Rydberg series can explain the known fact that excitation transfer in cyclohexane is extraordinarily fast compared with that innheptane. Kinetic analysis shows that these upper excited states at 63thinsp;000 and 78thinsp;000 cmminus;1of cyclohexane make direct excitation transfer, competing with relaxation to the lowest excited state at 49thinsp;750 cmminus;1. It is briefly discussed that the Rydberg series, which can be observed only in cyclic (or round) alkanes, may also be one of the key processes that explain molecular structural dependence observed in the excitation transfer and electron mobilities in alkanes.