Polarized UV-visible absorption, emission, and magnetic circular dichroism of a series of porphycenes are reported and interpreted in terms of the classical perimeter model as well as semiempirical CNDO/S, INDO/S, and PPP calculations. Differences relative to porphyrins follow readily from their different topology. Two results are particularly striking: (i) unlike the soft MCD chromophores, porphyrins, the porphycenes are negative-hard chromophores and provide a clear example of the competition between the µ+ contributions and the µ" contributions to the B terms of the Soret bands and (ii) fluorescence polarization results for free-base porphycene suggest the existence of a fast intramolecular proton-transfer process in the first singlet excited state, which calls for a closer examination by time-resolved methods.

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Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. For some examples, it is shown numerically that this mechanism may account for the largest contribution to the two-photon cross section in polar molecules, if the dipole moment strongly alters with excitation. Possible consequences for the two-photon detection of "forbidden" states in polar polyene systems are discussed.

The low-lying A states of trans-stilbene are investigated by means of two-photon excitation spectroscopy. The experimental findings are compared with theoretical results. From the combined information of oneand two-photon spectroscopy we can assign at least seven excited singlet states in the energy range below 50 000 cm-'.The newly obtained information is discussed with regard to the mechanism proposed for the photoisomerization of stilbene.

Valence and low-lying Rydberg states of acetylene (C 2 H 2 ) are reexamined in the singlet as well as in the triplet manifold. The major goal of this work is a better understanding of the valence states that contribute to the low-energy electron-energy-loss spectrum recorded under conditions where transitions to triplet states are enhanced. An appropriate theoretical treatment of these states has to include the low-lying Rydberg states because of their energetic proximity to some of the valence states. The CASSCF/CASPT2 method provides a suitable framework for such a task. For some important states the geometry was optimized at the CASPT2 level to allow a comparison with the results of other highly accurate methods that have been applied to acetylene in the past.

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The one- and two-photon spectra of trans-stilbene are reinvestigated and compared to the spectra of two stilbene derivatives in which rotation around the vinyl-phenyl bonds is prevented by methylene or ethano bridges. The spectra are compared to recent theoretical results that did not agree with some of the earlier assignments. The new experimental data lead to a consistent assignment of the low energy part of the singlet spectrum of trans-stilbene up to an excitation energy of about 50 000 cm-1, which is in excellent agreement with the theoretical findings. Special consideration is given to the weaker of the first two allowed singlet transitions whose location has been under discussion for quite some time.