We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. %'e have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation. PACS numbers: 78.47.+p, 72.20.3v, 78.55.Kz, 78.66.gn The photophysics and resonant nonlinear optical properties of conducting polymers are dominated by the locations and natures of the excited-state energy levels.These excited states include singlet excitons with odd (8") and even (As) parity, the continuum band (CB), two-electron (biexciton) states, and the triplet manifold [1,2]. Recent theoretical advances in the area of subgap third-order optical nonlinearity [3,4] provide information about a subset of the excited states, which include the lowest B"exciton (18"),a dominant As exciton (hereafter the mAs), and the CB threshold. The relative locations of the 18"and the lowest As (2As) excitons are determined by a sensitive interplay between electronelectron interaction and alternation (b) in the tr electron transfer integral along the polymer chain [5]. For realistic Coulomb interaction and small 8 [5], the optical gap Eg to the 1 B"exciton is relatively small, the 2Ag lies below the 18",and, due to the dipole forbidden character of the lowest singlet, photoluminescence (PL) is weak. Large b results in larger Es, state ordering E(2As) )E(18"), and consequently high PL efficiency with promising applications in displays [such as light emitting diodes (LED) [6]]. The benzene ring in the backbone structure of poly(p-phenylenevinylene) (PPV) yields an effective 8 for the extended n states that is large [2], and therefore PPV belongs to the latter category. Nevertheless, Coulomb interaction among the tr electrons in PPV leads to behavior qualitatively different from the predictions of single-particle Hiickel or SSH models. Recent subpicosecond PL [7] and site-selection PL [8] have demonstrated that the primary excitation in PPV is to an exciton, and that the associated lattice relaxation energy is small. This already suggests a subsidiary role of the electron-phonon interaction. The location of the mug exciton has been determined by two-photon luminescence [9], whereas long-lived triplet excitons have been found in thin films [10,11] and LEDs [12). In the present work, we present a more complete picture of the various photoexcitations and excited states in PPV, based on a variety of optical probes including picosecond transient and cw photomodulation (PM) and PL and their excita-tion dependence, and the electroabsorption (EA) technique. We have tentatively mapped the most relevant singlet and triplet electronic manifolds, including the CB threshold and the lowest biexciton, that are not seen in direct optical absorpt...

EDITOR'S NOTE[This paper] reports on the research that was recognized by two awards, the Gordon Bell Award and the Karp Prize, at IEEE's COMPCON 1988 meeting in San Francisco on March 2.The Gordon Bell Award recognizes the best contributions to parallel processing, either speedup or throughput, for practical, full-scale problems. Two awards were proposed by Dr. Bell: one for the best speedup on a general-purpose computer and a second for the best speedup on a special-purpose architecture. This year the two awards were restructured into first through fourth place awards because of the nature of the eleven December 1987 submissions. Bell presented the first place award of $1,000 to the authors of [this paper].Following the Second Conference in Parallel Processing in November 1985, Dr. Alan Karp challenged the scientific community to demonstrate a speedup of at least 200 for a real scientific application on a generalpurpose, MIMD computer. At COMPCON, Karp presented the authors with a plaque and his $100 check (to a charity of their choice) in recognition of their achievement.The editors of SISSC are very pleased to publish this paper for many reasons. First, of course, is the natural interest in work that achieves such a high degree of parallelism for important problems. Second, the editors believe that this paper will provide the reader unfamiliar with parallel computing with an excellent overview of the issues one confronts when considering the use of a parallel architecture. Third, this paper is well written and makes its content easily accessible to the reader. For this reason, the editors have decided to publish this paper in its entirety and as rapidly as possible, though it is broader in scope and longer than those that typically appear in SISSC. It was received on March 10, revised and resubmitted on March 25. Key words. fluid dynamics, hypercubes, MIMD machines, multiprocessor performance, parallel computing, structural analysis, supercomputing, wave mechanics AMS(MOS) subject classifications. 65W05, 68M20, 68Q05, 68Q10 1. Introduction. We are currently engaged in research [5] to develop new mathematical methods, algorithms, and application programs for execution on massively parallel systems. In this paper, massive parallelism refers to general-purpose Multiple-Instruction, Multiple-Data (MIMD) systems with 1000 or more autonomous floating-point processors, rather than Single-Instruction, Multiple-Data (SIMD) systems of one-bit processors such as the Goodyear MPP or Connection Machine.The suitability of parallel architectures, such as hypercubes [20], of up to 64 processors has been demonstrated on a wide range of applications [5,9,10,13,14,16]. The focus here is on the 1024-processor environment, which is very unforgiving of old-fashioned serial programming habits. The large number of processors forces one to reexamine every sequential aspect of a program. It also leads one to reexamine the traditional paradigm for measuring parallel processor performance.In this paper, we examine the relationship betwe...

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