Nonradiative transitions in the first excited singlet state of perfluorocyclobutanone: Fluorescence decay times and fluorescence excitation spectroscopy
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Titel:
Nonradiative transitions in the first excited singlet state of perfluorocyclobutanone: Fluorescence decay times and fluorescence excitation spectroscopy
Beschreibung:
<jats:p>Fluorescence decay times (τF) of an ``isolated'' molecule of perfluorocyclobutanone (PFCB) in its first excited singlet state (S1) with little vibrational energy content and that of a vibrationally relaxed species are 110±4 nsec; the nonradiative lifetime is 112 nsec and the radiative lifetime is 5.4 μsec. These low energy PFCB (S1) species decay chiefly via the S1↝ T1 intersystem crossing route. The value of τF (λex=3577 Å) at ``low pressure'' is 49 nsec, indicating the probable existence of a new electronic relaxation channel below λex=3800 Å. Fluorescence excitation spectroscopy at ``low pressure'' provides the intrinsic fluorescence quantum yield (φF) as a function of exciting wavelength, and it was found that φF begins to decrease below 3800 Å and it is reduced by a factor of 100 below 3480 Å. Collisional lengthening of the τF values at λex=3577 Å indicates that ≤ 1 kcal/mol is lost per collision with a PFCB (S0). From the overlap of the absorption and emission spectra, the 0–0 band of the first singlet n-π* transition is estimated at 4100 Å.</jats:p>