The first experimental evidence is presented for a strong coupling between electronic states in isolated, supercooled 4-(N,N-dimethylamino)benzonitrile, DMABN. In the fluorescence excitation spectrum, measured under isolated supercooled conditions, two types of transitions were found (B-type and C-type). Rotational contour analysis of the B-type bands (ππ* character) shows that there is hardly any difference between the geometries of the ground and excited states; the molecule retains its planar conformation. In the out-of-plane transition (C-type band) the equilibrium position of the dimethylamino group twists 90°with respect to the benzonitrile part. The broadening of the rotational lines indicates that the fluorescence is preceded by a fast radiationless intramolecular process (internal conversion) on a picosecond time scale. For the B-type bands it was found that the broadening increases with the vibrational energy.

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The phosphorescence of phenanthrene, pyrene, and naphthalene in fluid solutions has been studied. The radiative lifetimes are calculated from the quantum yields and actual lifetimes and vary only slightly or not at all with temperature within the experimental error. For phenanthrene, pyrene, naphthalene, and anthracene the radiative lifetimes at 77°K in ethanol are 25, 30, 75, and 180 sec, respectively. The temperature dependence and the absolute value of the triplet–triplet annihilation efficiency vary from molecule to molecule. The results are discussed in terms of a re-encounter association of the annihilation partners to form an excimer.

Results of quantum calculations on rotationally and vibrationally inelastic scattering of 1Au(S1) trans-glyoxal by H2,He, and Ar(E=80 meV) are presented. For scattering of 00 glyoxal and 72 glyoxal by H2, a quantitative comparison with a recent crossed beam experiment is made. The calculated cross sections for rotationally inelastic scattering are in good agreement with the experimental values, and the trend that σ(72, k=0→72, k′) falls off faster with k′ than σ(00, k=0→00, k′) is reproduced. Our calculations do not reproduce the experimental finding that, for the initial 72 level, the cross sections for the Δv7=−1 and Δv7=+1 transitions are nearly identical. The calculations on 72 glyoxal + He and Ar show important differences with 72 glyoxal + H2 in the competition of rotational excitation with vibrational excitation. Model calculations also presented here yield some interesting trends in the k′ dependence of the cross sections σ(00, k=0→x1, k′) on the vibrational mode x excited.

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