IL NUOVO CIMENTO VoL 63 B, N. 1 11 5[aggio 1981 KrF-Laser-Induced Fluorescence of Benzene and its Fluorinated Derivatives in the Gas Phase (*). W. ItACI~: and W. LANGEL Max-Planck-Institut ]iir StrSmungs/orschun 9 - Bdttingerstr. 4-8, D-3400 GStti~gen (ricevuto il 24 Novcmbre 1980) Summary. -- The fluorescence of C6H6, C6HsF, 1, 4-CGH4F 2 and C6F~ excited by a KrF laser has been observed in the gas phase. The collision- less lifetime of C6H6 (zilBz~) with 2000 to 3000era -x of vibratrional energy is 28 ns, which is a factor of two less than earlier results. Self- quenching of benzene leads to redistribution of vibrational energy in the S 1 state. C6F~ yields CFo when irradiated by the laser. 1. - Introduction. Benzene has been an object of pioneering research in photophysics for many years. Its well-resolved and easily detected absorption and fluorescence spectra were first assigned in 1939 (1). Since then it has proven to be more advantageous to extend the knowledge on the <~ prototype ~ benzene molecule and then transfer to other molecules, rather than starting with these molecules ab initio. One important feature of benzene photophysics is the increase of decay rate and decrease of fluorescence quantum yield with the vibrational energy in the first excited singlet state (2). A so-called (( third channel ~), the nature (*) Paper preseuted at the (~ European Conference on the Dynamics of Excited States ~, held ill Pisa, April 14-16, 1980. (1) H. SPONER, G. NORDHEI~r A. L. SKLAR and E. TELLER: J. Chem. Phys., 7, 207 (1939). (2) K. G. SPEARS and S. A. RICE: J. Chem. Phys., 55, 5561 (1971). 207