Femtosecond Dynamics and Photochemistry in Solution

Abstract: The reaction dynamics of several polyatomic systems have been investigated in solution by femtosecond spectroscopy. Tetranitromethane-naphtalene complexes excited in the charge-transfer band at 400 nm undergo dissociative electron transfer, resulting in three primary products: [naphthalene].+, NO2 and C(NO2)3-. It was shown that this triad is formed within 200 fs. The subsequent competing reactions of naphthalene were followed. After ~1 ns, the main product in dichloromethane is the trinitromethylnaphtalenyl radical. In acetonitrile, the [naphthalene].+, preferentially reacts with ground state naphthalene to a dimer cation. The kinetics were reproduced by a model in which the ion pair separation rate was the primary solvent dependent parameter. Excitation of ICH2CH2I, ICF2CF2I and ICF2CF2Br at 266 nm produces haloethyl radicals and iodine atoms within 200 fs. In both fs-ps and ns-ƒÝs experiments, evidence for decomposition of these radicals were found. I2 formation within a few ps was observed from ICF2CF2I in different solvents, most likely by secondary dissociation of the radicals followed by geminate combination of iodine atoms. The predissociation rates of halobenzenes were determined in the gas phase and that of bromobenzene also in solution. Ab initio calculations on bromobenzene showed that the state reached upon excitation at 266 nm is (ƒà,ƒà'), which interacts with a repulsive triplet state. The predissociation of bromobenzene takes 28 ps in the gas phase and 9 ps in acetonitrile. The thesis also includes a study of isomerization of CH2ICl in acetonitrile and discussion of the use of two-photon absorption in neat solvents for determining pump-probe cross-correlation functions.