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

  • Author(s): Sullivan, Erin
  • Advisor(s): Neumark, Daniel M
  • et al.

The unimolecular decomposition of small free radicals is examined using photofragment

translational spectroscopy. A fast beam of neutral radicals is generated following the photodetachment

of an anionic precursor that has been accelerated to 6-8 keV. The radicals

intersect a high energy photon and dissociates. Two- and-three-body fragments are collected

in coincidence via a time-and-position sensitive detector to ultimately yield mass,

translational energy, and angular distributions. These distributions are used to identify the

products and characterize the dynamics leading to their production.

Chapter 1 provides a background on photodissociation experiments and a broad overview

of some of the systems presented in this dissertation. Chapter 2 includes a detailed description

of the current experimental setup and any modications and improvements made in

recent years, specically regarding the photoelectron spectrometer and piezo valve.

Chapters 3 and 4 discuss the photodissociation of alkyl peroxy radicals, CH3OO, C2H5OO,

and t-C4H9OO, at 248 nm. Dissociation on a repulsive electronic state leads to O (3P)

+ alkoxy formation such that the alkoxy fragment dissociates further to yield three-body

products. As the alkyl substituent increases in size, this process becomes dominant. The

two smaller radicals, CH3OO and C2H5OO, exhibit repulsive O (1D) loss to yield two-body

products, but this pathway is not observed in the larger t-C4H9OO. Additionally, OH loss

from CH3OO and C2H5OO is observed. C2H5OO and t-C4H9OO yield some O2 and HO2

production that is attributed to statistical dissociation on the ground state.

Chapters 5 and 6 cover the dissociation of alkyl perthiyl radicals, CH3SS and t-C4H9SS

at 248 nm and 193 nm. At 248 nm, both radicals predominantly lose an S atom with a large

translational energy release on a repulsive excited state. At 193 nm, S2 + alkyl products

dominate and are also attributed to form on an excited state surface. For the larger perthiyl

radical, the alkyl product dissociates further into a variety of three-body product channels.

The dissociation of the phenoxy radical at 533 nm, 290 nm, and 225 nm is presented in

Chapter 7. At all three wavelengths, the dominant product channel is CO + C5H5 with a

relatively low translational energy release. Therefore, this process is attributed to ground state dissociation. At the higher dissociation energies, the C5H5 radical can dissociate further

to C2H2 + C3H3 or H + C5H4, again on the ground electronic state of C5H5.

Unimolecular dissociation of the indenyl (C9H7) radical at 248 nm and 193 nm is presented

in Chapter 8. The observed products are C2H2 + C7H5, C2H2 + C3H3 + C4H2, and C2H2 +

C2H2 + C5H2 and are all attributed to C9H7 absorbing two photons prior to dissociation. Each

product channel exhibits a small translational energy release indicating that dissociation

occurs statistically on the ground state.

Chapter 9 presents recent results regarding the dissociation of the iso-propoxy radical

(i-C3H7O) at 248 nm. The predominant photoproducts are CH3 + C2H4O and OH + C3H6.

A small amount of three-body dissociation is observed yielding CH3 + CH3 + HCO and

CH3 + CH4 + CO and is attributed to secondary dissociation of the acetaldehyde.

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