Dr. Valentini is supported by a grant fom The Physical Chemistry Program to study the structure and dynamics of transition states in gas phase chemical reactions. Valentini has developed a spectroscopic technique which permits the indirect observation of the short-lived intermediates (transition states) which have long been theoretically predicted but never observed in chemical reactions. Dr. Valentini's research involves the studies of so-called Feshbach resonances in rotational excitation cross sections which are observed for chemical reactions in the gas phase. The direct observations involve state-to-state dynamics in which the vibrational and rotational quantum states (v,j) of the reactants are prepared by free-jet expansion cooling and probed by stimulated Raman excitation. The collision energy of the reactants is selected, varied, and precisely specified through laser photolysis of a diatomic precursor to generate the reactant atom. Vibrational and rotational (v',j') analysis of the reaction products under single collision conditions are effected using resonance-enhanced multiphoton ionizaiton. Dr. Valentini's research program is motivated by his recent discovery of Feschbach resonances in the ortho-, para-hydrogen exchange reaction. This observation permitted him to infer the vibrational energy states of the linear triatomic transition state which occurs in this reaction. Further studies in this area will involve the hydrogen deuterium exchange reaction, the reaction of chlorine with hydrogen chloride, and the reaction of iodine and bromine with their corresponding hydrogen halides. Coherent Raman spectra of these species will be obtained to elucidate the nature of the theoretically predicted vibrational bonding.
