Investigations of the Two- and Three-Body Photodissociation Processes of Free Radicals Using the Fast Radical Beam Machine

The unimolecular decomposition of small free radicals is examined using photofragment/ntranslational spectroscopy. A fast beam of neutral radicals is generated following the photodetachment/nof an anionic precursor that has been accelerated to 6-8 keV. The radicals/nintersect a high energy photon and dissociates. Two- and-three-body fragments are collected/nin coincidence via a time-and-position sensitive detector to ultimately yield mass,/ntranslational energy, and angular distributions. These distributions are used to identify the/nproducts and characterize the dynamics leading to their production./nChapter 1 provides a background on photodissociation experiments and a broad overview/nof some of the systems presented in this dissertation. Chapter 2 includes a detailed description/nof the current experimental setup and any modications and improvements made in/nrecent years, specically regarding the photoelectron spectrometer and piezo valve./nChapters 3 and 4 discuss the photodissociation of alkyl peroxy radicals, CH3OO, C2H5OO,/nand t-C4H9OO, at 248 nm. Dissociation on a repulsive electronic state leads to O (3P)/n+ alkoxy formation such that the alkoxy fragment dissociates further to yield three-body/nproducts. As the alkyl substituent increases in size, this process becomes dominant. The/ntwo smaller radicals, CH3OO and C2H5OO, exhibit repulsive O (1D) loss to yield two-body/nproducts, but this pathway is not observed in the larger t-C4H9OO. Additionally, OH loss/nfrom CH3OO and C2H5OO is observed. C2H5OO and t-C4H9OO yield some O2 and HO2/nproduction that is attributed to statistical dissociation on the ground state./nChapters 5 and 6 cover the dissociation of alkyl perthiyl radicals, CH3SS and t-C4H9SS/nat 248 nm and 193 nm. At 248 nm, both radicals predominantly lose an S atom with a large/ntranslational energy release on a repulsive excited state. At 193 nm, S2 + alkyl products/ndominate and are also attributed to form on an excited state surface. For the larger perthiyl/nradical, the alkyl product dissociates further into a variety of three-body product channels./nThe dissociation of the phenoxy radical at 533 nm, 290 nm, and 225 nm is presented in/nChapter 7. At all three wavelengths, the dominant product channel is CO + C5H5 with a/nrelatively low translational energy release. Therefore, this process is attributed to ground state dissociation. At the higher dissociation energies, the C5H5 radical can dissociate further/nto C2H2 + C3H3 or H + C5H4, again on the ground electronic state of C5H5./nUnimolecular dissociation of the indenyl (C9H7) radical at 248 nm and 193 nm is presented/nin Chapter 8. The observed products are C2H2 + C7H5, C2H2 + C3H3 + C4H2, and C2H2 +/nC2H2 + C5H2 and are all attributed to C9H7 absorbing two photons prior to dissociation. Each/nproduct channel exhibits a small translational energy release indicating that dissociation/noccurs statistically on the ground state./nChapter 9 presents recent results regarding the dissociation of the iso-propoxy radical/n(i-C3H7O) at 248 nm. The predominant photoproducts are CH3 + C2H4O and OH + C3H6./nA small amount of three-body dissociation is observed yielding CH3 + CH3 + HCO and/nCH3 + CH4 + CO and is attributed to secondary dissociation of the acetaldehyde.

Advisor: Neumark, Daniel M