Excitation Energy and Deuteration Dependence of Fluorescence in Naphthalene Vapor

Abstract

Combining fluorescence quantum yields with the lifetime data of Laor and Ludwig [J. Chem. Phys. 54, 1054 (1971)] radiative and nonradiative decay rates of dilute naphthalene vapor were determined as a function of excess vibrational energy in the lowest excited singlet state (S1). It is shown that the nonradiative decay probability increases, first approximately linearly then nearly exponentially, as the excess energy is varied over a range of 15 000 cm−1. The deuterium isotope effect on fluorescence yield is found to be large only in the region of high excess energy in which nonradiative decay rate exhibits an exponential energy dependence. These results are interpreted in terms of two different pathways of intersystem crossing which compete in the nonradiative deactivation of molecules in S1. In the low excess energy region, an intersystem crossing to a close-lying triplet state apparently dominates, so that the nonradiative decay rate is not sensitive to the deuteration of the molecule and it exhibits a linear variation with excess vibrational energy. At high excess energy, on the other hand, the dominant radiationless transition appears to be the S1→T1 (the lowest triplet) intersystem crossing, and a relatively large S1—T1 energy gap leads to the near exponential energy dependence of its decay rate and a rather large deuteration effect on fluorescence quantum yield. The near linear variation of the radiative decay rate with excess vibrational energy is accounted for on the basis of Herzberg-Teller intensity borrowing mechanism.