electronic lifetime T~~ at a temperature slightly higher than 21 1 K. At even lower temperatures, the greatest part of the molecules reorient very slowly or they cannot reorient, thus, 7, becomes so high that all the emission is observed from the X state and is well described by a single exponential function. These results are in good agreement with those deduced from the study of the spectral shifts at the same dilution.1° The variation of T~ with temperatures agrees well with the considerations made and the results obtained by other authors6*" in different compounds.In conclusion, a solutesolvent reorientation during fluorescence gives a coherent interpretation of the decay time curves and an estimation of the reorientation times at different temperatures. Acknowledgment. The authors are very grateful to Drs. A.Molecular complexes of H20, H202, CO, and C02 in solid O2 have been prepared by a conventional matrix deposition method using an appropriate gas mixture. FTIR spectra and vibrational assignments of matrix-isolated H20 dimer and trimer, H20C02, H 2 0 C 0 , H20~H202, and H202.nC0 in solid O2 are presented. Hydrogen bonds of the type. 0-H-0 involving H,O, species and C.-H-0 involving H,O,-nCO species are found except for H20.C02. In the H20"202 complex, H202 is an electron acceptor (acid) and H 2 0 is an electron donor (base). The frequency shift of the 0-H stretch in the electron acceptor molecule of the complex increases in order of H 2 0 C 0 , H202.C0, (H20),, and H20.H202.

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Nonradiative transitions in the first excited singlet state of perfluorocyclobutanone: Fluorescence decay times and fluorescence excitation spectroscopy Radiative and nonradiative transitions from the first excited singlet state of methylsubstituted naphthalenes Radiative and nonradiative rates of asymmetric linear aldehydes (C 2 -C 6 ) have been measured in the gas phase. Observe radiative lifetimes are compared to calculated radiative lifetimes obtained from the Strickler-Berg (SB) expression. The calculated values are about the same as the observed values in large aldehydes, but the former is about six times greater than the latter in acetaldehyde. The radiative rates are about four orders of magnitude smaller than the nonradiative rates. The two most important nonradiative processes are S\--! T\ intersystem crossing and type II processes, the latter playing an increasingly important role at high energy in the aldehydes with y·hydrogens. The contrasting radiative behavior of the asymmetric aldehydes from that of the symmetrical ketones is intriguing and suggests important differences in their excited state eqUilibrium geometries and transition moments.