Photoelectron-Photofragment Angular Correlation and Energy Partitioning in Dissociative Photodetachment

Abstract

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...

Femtosecond Time-Resolved Photoelectron Spectroscopy

“…As shown by Continetti and co-workers for the case of nanosecond laser photodetachment, correlated photofragment and photoelectron velocities can provide a complete probe of the dissociation process. 113,114 The photofragment recoil measurement defines the energetics of the dissociation process and the alignment of the recoil axis in the laboratory frame, the photoelectron energy provides spectroscopic identification of the products, and the photoelectron angular distribution can be transformed to the recoil frame in order to extract vector correlations such as the photofragment angular momentum polarization. The integration of photoion-photoelectron timing imaging (energy and angular correlation) measurements with femtosecond time-resolved spectroscopy was demonstrated, using timing imaging detectors, in 1999 by C. C. Hayden and co-workers 115,116 at Sandia National Laboratories.…”

Time‐Resolved Photoelectron Spectroscopy of Nonadiabatic Dynamics in Polyatomic Molecules

“…For the case of nanosecond laser photodetachment, correlated photofragment and photoelectron velocities can provide a complete probe of the dissociation process [130,136]. The photofragment recoil measurement defines the energetics of the dissociation process and the alignment of the recoil axis in the LF, the photoelectron energy provides spectroscopic identification of the products and the photoelectron angular distribution can be transformed to the recoil frame in order to make measurements approaching the MF PAD.…”

“…Thus, these detectors allow for full 3D velocity vector measurements, with no restrictions on the symmetry of the distribution or any requirement for image re-construction techniques. Very successful methods for full time-and-position sensitive detection are based upon interpolation (rather than pixellation) using either charge-division (such as "wedge-andstrip") [129] or crossed delay-line anode timing MCP detectors [130]. In the former case, the avalanche charge cloud is divided amongst three conductorsa wedge, a strip and a zigzag.…”

Molecular frame photoelectron angular distribution for oxygen 1s photoemission from CO2molecules

“…Our coincidence momentum imaging apparatus is based on a measurement of electron and ion time of flight (TOF) with two multi-hit two-dimensional position sensitive detectors [23][24][25]. Similar apparatus are used by other groups as well [9,26,27]. A supersonic jet of CO 2 in the vertical direction crosses the photon beam in the horizontal direction.…”