ULTRAFAST FORMATION OF A 3-ELECTRON-BONDED RADICAL-ANION (CH3S-THEREFORE-SCH3-) IN A LIQUID ORGANIC SULFUR COMPOUND

The elementary steps of an electron photodetachment triggered by the U V excitation of pure liquid dimethyl sulfide, (CH3)(2)S, have been inv estigated by femtosecond absorption UV-IR spectroscopy at 294 K. The b uildup of a long-lived UV band centered around 420 nm (3.26 eV) is obs erved at the sub-picosecond time scale. This spectral band is assigned to a radical anion (CH3S therefore SCH3-) characterized by a sulfur-s ulfur bond with an antibonded third electron (2c, 3e). A very short-li ved electronic state, whose rise time equals 180 +/- 10 fs, exhibits a spectral overlap with this UV radical. The frequency and time depende nces of induced absorption signals are analyzed in the framework of a kinetic model for which an early electron transfer yields an ultrashor t-lived anion radical ({RSR-}(RSR) or {RSR [GRAPHICS] RSR}, R = CH3). The decay rate of this UV state (1/tau = 3.7 x 10(12) s(-1)) is ration alized by postulating an ultrafast ion-molecule reaction and the picos econd formation of a disulfide radical anion (CH3S therefore SCH3-) ch aracterized by a 2 sigma/1 sigma bond. A second electron-transfer cha nnel leading to a delayed formation of a disulfide anion radical (RS t herefore SR-) has been identified by time-resolved IR spectroscopy. Th ese femtosecond investigations argue for an ultrafast formation of a s ulfur-sulfur bond with C-S bonds breaking. It is suggested that the de nsity-state fluctuations of organic sulfur molecules influence the ene rgy of early electron-thioether couplings (electron attachment or loca lization) and would govern competitive branchings between ultrafast el ectron photodetachment channels.