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Fast Beam Investigations of Two- and Three-Body Photodissociation by Time- and Position-Coincidence Imaging


Fast beam photofragment translational spectroscopy has been used to elucidate the photodissociation dynamics of small radicals and closed-shell anions. Imaging of photofragments in time- and position-coincidence allows the determination of mass distributions, translational energy distributions [P(ET) distributions], and in the case of three-body fragmentation channels, ternary Dalitz plots depicting the momentum disposal among the fragments. These data yield information about the potential energy surfaces responsible for dissociation as well dynamic and kinematic information about the decay mechanism itself.

Chapter 1 presents a brief overview of photodissociation dynamics and photofragment translational spectroscopy and some recurring themes of the research presented in this Thesis are explored. The use of Dalitz plots in kinematic analysis of three-body dissociation is discussed in detail.

Chapters 2 and 3 present, respectively, experimental and theoretical findings on the photodissociation of perdeuterated C3D3 isotopologs, propargyl (D2CCCD) and propynyl (CCCD3) at 248 and 193 nm. Mass distributions reveal D2 loss and three heavy mass channels: CD + C2D2, CD2 + C2D, and CD3 + C2. The D loss channel is present, but incompatible with our detection scheme and thus was not observed. P(ET) distributions indicate each dissociation proceeds by internal conversion followed by statistical dissociation on the ground state surface. In Chapter 3, results of high level ab initio calculations are presented and RRKM rate constants are determined.

Chapter 4 presents results on two- and three-body photodissociation dynamics of I2Br&hibar; from 300 to 270 nm. Two- and three-body dissociation is observed throughout this wavelength range. Four distinct two-body channels are observed: Br- + I2, I- + IBr, Br + I2-, and I + IBr-. Dalitz plots indicate three-body dissociation occurs primarily by a concerted decay mechanism. A sequential dissociation mechanism, where Br- + I2(B) is followed by electronic predissociation of the metastable I2(B) fragment into ground state I atoms is reported.

Chapter 5 presents preliminary results of two- and three-body photodissociation of ozone at 193 and 157 nm. Ozone is prepared by photodetachment of ozonide at 386 nm (about 1.1 eV above the detachment energy). Greater than expected kinetic energy release is observed for both wavelengths in the P(ET) distributions. The additional energy is attributed to two sources: extra energy available by photodetaching far above the electron affinity of ozone, and extra internal energy in the ozonide precursor. Three-body dissociation occurs at 193 nm as a 3.6% channel and at 157 nm as a 19.2% channel. Three-body dissociation proceeds at both wavelengths by a synchronous, concerted decay mechanism.

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