Measurements of photoelectron-photofragment angular correlation and energy partitioning in the dissociative photodetachment of O 4 2 ͑O 4 2 1 hn ! O 2 1 O 2 1 e 2 ͒ at 532 nm are reported. Using photofragment translational energy and photoelectron spectroscopy in coincidence, vibrationally resolved product translational energy distributions and a pronounced angular correlation between the photoelectron and photofragment recoil directions are observed. These results provide insights into the molecule-fixed photoelectron angular distribution of a negative ion and structural information on O 4 2 in the gas phase. [S0031-9007 (96)01411-1] PACS numbers: 33.80.EhThe dynamics of photodetachment and photoionization processes are sensitive probes of electron-molecule interactions. Photoelectron angular distributions provide detailed information on photodetachment dynamics due to the fact that they are a function of both the magnitude and relative phases of the degenerate electron continuum channels accessible in photodetachment processes [1,2]. Studying the photoelectron angular distribution in the molecular frame provides greater insights into the photodetachment dynamics. This results from the removal of the averaging over molecular orientations inherent in a laboratory angular distribution. Theoretical studies of such moleculefixed photoelectron angular distributions (MF-PAD) have been pursued for an improved understanding of the dynamics of photoionization [1,3-6] and electron interactions with molecules oriented on surfaces [7]. Evidence of anisotropic MF-PAD's have been observed in experiments on the dissociative photoionization of H 2 [8], O 2 [9], and several halomethanes [10,11]. Direct detailed measurements of the MF-PAD in the photoionization of NO have recently been reported [5]. Measurements of the MF-PAD for the photodetachment of negative ions are of interest as they will provide insights into the nature of both the bound electron in the anion and the anisotropy of electronmolecule interaction potentials.The structure and energetics of atomic and molecular cluster anions have received considerable attention in recent years. Translational energy spectroscopy can be used to determine bond energies and photofragment internal energy distributions [12]. Photoelectron kinetic energy spectroscopy has been used to characterize vibrational and electronic states of both anionic and neutral clusters [13]. In many systems, however, photodetachment produces the neutral far from the equilibrium geometry. The repulsive energy in the cluster can then lead to rapid dissociation. To study the dissociation dynamics of such short-lived neutral clusters, it is essential to measure the photoelectron kinetic energy and the translational energy released between the atomic or molecular fragments. Such mea-surements can reveal both the internal energy distribution of the photofragments and provide a direct measure of the repulsive energy in the complex. In this Letter, we report measurements in which a complete kinematic description...

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An investigation of the photodissociation dynamics of the dimer anion O 4 Ϫ at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation ͑O 4 Ϫ ϩh→O 2 ϩO 2 Ϫ ͒ is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O 2 (a 1 ⌬ g) and ground state O 2 Ϫ (X 2 ⌸ g), indicating that O 4 Ϫ is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.

A high-collection-efficiency fast-ion-beam photoelectron spectrometer is described. In a straight time-of-flight mode, the spectrometer collects-1% of the photoelectrons and achieves an energy resolution of AEIE of-5%. For coincidence experiments requiring greater collection efficiency, a paraboloidal electrostatic mirror is used. The mirror collects-40% of the photoelectrons while maintaining AEIE=G35%. In both modes of operation, a time-and position-sensitive electron detector allows conversion of the photoelectron laboratory energy to center-of-mass energy. The fast-ion-beam photoelectron spectrometer is used to prepare mass-and energy-selected neutral molecules which are used in molecular dissociation studies. 0 I995 American Institute of Physics.

Electron energy dependence of the kinetic energy and angular distributions of O+ from dissociative ionization of O2