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Streak cameras have enabled kinetic data in the picosecond time regime to be obtained from single laser shots. Digitization of the streaked image with an optical multichannel analyser results in accurate fluorescent decay curves from which accurate fluorescence lifetimes may be calculated, and in which non-exponential decays are readily detected. This paper describes the characteristics of the picosecond laser system at the University of Melbourne. For excitation, a Nd3+ : glass mode-locked laser, Pockels cell pulse selector and various harmonic generators are used; a streak camera/OMA system linked directly to a computer is used to detect fluorescence.

The time-resolved fluorescence of tetracene crystals excited by a single 7-ps pulse of 530-nm light has been measured with a streak camera/OMA system. The singlet exciton lifetime was found to be 30030 ps at 293 K. Singlet-singlet exciton annihilation was observed, and the rate constant found to be 5 x 10-9 cm3 s-1. In order to fit the observed decay curves, an upper limit of c. 1 x 10-9 cm3 s-1 must be set on the singlet-triplet exciton annihilation rate. The measured singlet-singlet exciton annihilation rate is consistent with a hopping model for singlet exciton motion at room temperature.

Spectra of stable dimers of azulene in an isopentane-methylcyclohexane glass at 77 K are reported. In the 700 nm region the dimer spectrum shows a marked hyperchromism and blue shift of 100 cm-1 compared to the monomer. Assumption of a stacked 'head-to-tail' configuration for the dimer and use of the dipole-dipole approximation enables the spectral shift to be accounted for. In the U.V. region, a large loss of intensity is observed for the S2 ← So and S3 + So transitions and a number of new dimer bands are observed. The intensity redistribution is assigned to configuration mixing of the monomer states in the dimer. No luminescence could be observed from the dimer.

The crystal structure of acenaphthylene has been calculated by assuming the intermolecular forces may be represented as a sum of pairwise interactions between atoms. The calculated structure is shown to be remarkably insensitive to the values of the parameters used in an ?exp- 6? type atom-atom potential; variation of the parameters associated with the attractive part of the exp-6 potential has less effect than variation of the repulsive parameters. It is deduced that the actual values of the parameters in the atom-atom potential have little significance and that the correct crystal structures could be predicted by any approximately correct repulsive potential.

Stable dimers are formed by controlled softening of glasses at low temperature. The spectra of dimers of anthracene and 9,10- dichloroanthracene have been studied in detail. Polarized excitation spectra of the polarized fluorescence of the anthracene dimer show the presence of two oppositely polarized components in the absorption spectrum. An estimate of about 700 cm-1 for the excitation exchange matrix element follows from these results. Hyperchromism is explained by configuration interaction with a second 1B2u state near 44000 cm-1. This configuration mixing leads to hypochromism for the sandwich dimer. The 1B3u, state of anthracene near 40000 cm-1 appears as a broad band in the dimer with a maximum at 49600 cm-1. Neither delayed fluorescence nor phosphorescence can be observed for the stable dimers of anthracene and 9,10-dichloroanthracene.