FLUORESCENCE TRANSIENT AND OPTICAL FREE INDUCTION DECAY SPECTROSCOPY OF PENTACENE IN MIXED-CRYSTALS AT 2-K - DETERMINATION OF INTERSYSTEM CROSSING AND INTERNAL-CONVERSION RATES

In this paper the recently reported fluorescence transient, appearing during the onset of cw dye laser excitation of the energetically lower sites of pentacene in p-terphenyl at 2 K, is analyzed quantitatively. The role of the lowest triplet state in causing the long (compared to the fluorescence lifetime) transient, is firmly established. For the lower sites a kinetic analysis yields a T1←S1 intersystem crossing (ISC) yield of about 0.4% and an S0←T1 ISC rate of (2.2±0.1) × 104 s-1. Using these data in calculating the S1 ←S0 transition dipole moment from our earlier OFID experiments yields for the O1 site (proto) μ= 0.71±0.24 D. The S0←S1 internal conversion yield is estimated to be 22%. The fluorescence transients of the energetically higher sites of pentacene in p-terphenyl exhibit a much faster decay, which indicates a drastic increase in ISC yield from S1. Optical free induction decay (OFID) experiments performed on these sites confirm this and imply that the T 1←S1 ISC yield for these sites is ∼60%. We further discuss results of an OFID experiment on pentecene in naphthalene and derive in this case at T1←S1 ISC yield of ≲2.8% and an S0←S1 internal conversion yield of 31.5±1.5%. The off-resonance pumping effect in a 3-level system was also studied, using the parameter values for pentacene in p-terphenyl. For the lower sites, with the excitation intensities used (∼1 MW/m2) the part of the inhomogeeous absorption line that is pumped has a width of about 2.5 GHz (effective excitation bandwidth < 1 MHz). © 1979 American Institute of Physics.