Optical free induction and fluorescence decay of pentacene in p-terphenyl following narrow band laser excitation
✍ Scribed by Harmen De Vries; Philippus De Bree; Douwe A. Wiersma
- Publisher
- Elsevier Science
- Year
- 1977
- Tongue
- English
- Weight
- 627 KB
- Volume
- 52
- Category
- Article
- ISSN
- 0009-2614
No coin nor oath required. For personal study only.
✦ Synopsis
Optical free induction and fluorescence decay experiments on pentacene following narrow band laser excitation are rcported. From the results we conclude that pentacene for its lowest * Bzu + ' Alg transition is a perfect two-level system and from the power saturation term in the OFID we calculate a transition dipole moment of 1.5 + 0.2 debye. A long-lived transient observed in the fluorescence during the onset of cw dye laser excitation is ascribed to a bottle-neck in the optical pumping cycle.
We have recently reported in a series of papers [l-3] on the photonecho decay of pentacene (PTC) in a p-terphenyl mixed crystal using a pulsed dye laser as excitation source. In dilute (10H8M) mixed crystals, at temperatures below 2.5 K, the coherence lifetime (T2) was found to be determined by the fluorescence lifetime. PTC, for its lowest t BLu * 1 A,, transition, therefore seemed to be a perfect example of a twolevel system, which is of great interest in the study of self-induced transparency effects [I ,4] .
Recent work by Zewail et al. [5,6] however, seemed to indicate that, under narrow band conditions, PTC might behave as a multilevel system_ In fact, after cw dye laser excitation, a "fluorescence" decay time almost three orders of magnitude longer than after pulsed dye laser excitation was reported.
In this paper we show that the optical free induction decay (OFID) and fluorescence decay of PTC after cw dye laser excitation yield optical decay constants (T1 and T2) identical to those obtained using a pulsed dye laser [3] _ The two-level nature of the lowest spin-allowed electronic transition in PTC is thus hereby firmly established. More precisely all intramolecular decay channels to the triplet as well as to the ground state correspond to the statistical limit.