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Measurements have been made of the concentration dependence of the rise and decay time characteristics of the monomer and excimer fluorescence of deoxygenated solutions of pyrene in cyclohexane at temperatures from 293 to 340 °K. Two independent methods were employed, one using a pulsed light source and a pulse-sampling oscilloscope, and the other a modulated light source and a phase and modulation fluorometer. In conjunction with observations of the monomer and excimer fluorescence quantum efficiencies, the results have been analyzed to determine the six rate parameters which describe the behaviour of the system. Values of 6.8 x 10 -7 and 0.9 x 10 -7 s are obtained for the radiative lifetimes of the monomer and excimer, respectively. Excimer formation is shown to be a diffusion-controlled collision process, in which every collision between excited and unexcited molecules is effective. From the difference in the activation energies for excimer dissociation and formation, the excimer binding energy is found to be 0.34eV.

Measurements have been made of the fluorescence spectra, lifetimes and quantum efficiencies and of the absorption spectra of several organic molecules in dilute solution. These have been used to test the validity of the theoretical relation between the radiative lifetime and the absorption intensity, and of the mirror-image relation between the fluorescence and absorption spectra. The former is found to be valid provided there is no major change in the nuclear configuration between the ground and excited states, but the latter is usually more sensitive to such changes. The effect of a change in nuclear configuration, which is particularly marked in the higher diphenylpolyenes, is to increase the radiative lifetime and to distort the spectral intensity distribution.

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When heat treated, amorphous alumino‐silicate fibres will devitrify into crystalline phases which on cooling become stable at room temperature. X‐ray diffraction has been used to identify and quantify the phases present in such products; mullite and two distinct forms of cristobalite have been observed. One form of cristobalite has been identified as α‐cristobalite, the second form has been designated as α′‐cristobalite. An internal standard approach was used to enable calibration curves to be obtained for the three crystallite phases, including the α′‐ cristobalite for which no commercially available standard exists. The accuracy of the methods was estimated as ± 5 wt% for all three crystalline phases present. This was achieved by quantifying the proportion of α′‐cristobalite in a standard alumino‐silicate fibre after heat treatment for 48 h at 1773 K and then using this material as the `standard' for α′‐cristobalite. This `standard' contained 32.9 wt% α′‐cristobalite, 1.6 wt% α‐cristobalite and 65.5 wt% mullite. The exact nature of the α′‐ cristobalite phase has also been examined using high‐temperature X‐ray diffraction and differential scanning calorimetry. It has been shown to be more similar in nature to α‐cristobalite than β‐cristobalite since it undergoes an α′→β phase transition on heating. Compared with α‐cristobalite it has a different lattice size, and both the temperature and enthalpy of the α′→β phase transition are lower than for the α→β phase transition. It has been proposed that the α′‐cristobalite observed is a defect form of α‐cristobalite but with a constant amount of defects/substituted cations which gives it a well defined and consistent structure.

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