Lifetimes, radiative Subject

The first electronic transition in butadiene has been the subject of many experimental and theoretical studies.The absorption, which has a maximum at 2100 A., is strong and represents a tt tt transition from a ground singlet to an upper singlet state. Analysis of the spectrum, which shows very little structure, has not been carried out. Since no fluorescent radiation has ever been detected on excitation of any of the simple dienes even at low temperature, a definite assignment of the 0 — 0 band has not been made. The 0 — 0 band had been placed at 2300 A. (124 kcal./mole), at which point the absorption is only /so as intense as at its maximum. The oscillator strength is 0.53, which leads to a radiative lifetime of 10 sec. Since emission of radiation has not... 

Another factor which determines the efficiency of LEDs is the photoluminescence (PL) efficiency, i.e. the fraction of photoexcited states which recombine radiatively. Since the radiative lifetime of most conjugated polymers is less than 1 ns and there are relatively few non-radiative channels for relaxation, the PL efficiency can be quite high. Many conjugated polymers have photoluminescence efficiencies higher than 60%. A subject of substantial debate is whether or not the electroluminescence (EL) efficiency can be as high as the photoluminescence (PL) efficiency. As summarized in Section IVB, EL efficiency as high as 50% of the PL efficiency has been demonstrated [158]. 

The relationships between measurable quantities related to absolute transition probability (e.g. absorption cross section, molar absorption or extinction coefficient, radiative lifetime) and the fundamental quantities used to describe and inter-relate the observable quantities axe fraught with difficulties of unit conversions and internally consistent treatments of initial- and final-state degeneracies. Several excellent papers on this subject exist (Hilborn, 1982 and 2002, Larsson, 1983, Tatum, 1967, Schadee, 1978, and Whiting, et al., 1980). Much of Section 6.1.1 is based on or checked against Hilborn (1982 and 2002), although slightly different notation and definitions are used. 

The powerful technique of two-photon absorption, which permits limitations of Doppler broadening to be overcome (see Chapter 1), has been used in a study on sodium atoms in which the Stark effect in the 5s 2Si and 4d D and D levels was observed.188 The radiative lifetimes of the S and D Rydberg levels of Na,189 the use of laser-induced resonance fluorescence for the measurement of small concentrations of Na vapour,170 the quenching of Na(32P) and K(42P) by N2, 02, H2, and HaO,171 the chemi-ionization reactions of photoexcited atoms,172 and excitation of the K(42P ) level in collisions with rare-gas atoms173 have been the subjects of recent reports. 

From the beginning of this century experiments using resonance radiation and resonance fluorescence have been largely responsible for our increasing understanding of excited atoms and their interaction with radiation. In the 1920 s the polarization of resonance fluorescence from atoms subjected to external magnetic fields was studied in detail by Hanle and used to measure radiative atomic lifetimes. 

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