Scanning tunneling microscopy was used to characterize the structure of partial monolayers of C60 and C70 on the Au(111) surface. Both 2√3 × 2√3 R30° and 7 × 7 lattice symmetries were observed for C60 monolayers, in accordance with previous results. For C70 monolayers, structures are observed with rotation angles of 0°, 30°, and 14° with respect to the underlying substrate; we propose a previously unreported √13 × √13 R13.9° lattice structure to explain this last observation. Time sequences of STM images show that while fullerene monolayers are largely stable, molecular motion can be observed on the timescale of minutes or hours. For C60, thermal diffusion is the predominant cause of this motion, and STM perturbation of the sample is negligible. In contrast, C70 is observed to diffuse far more slowly; under normal scanning conditions, tip-induced motion is the major effect.
The nonlinear optical technique of degenerate four-wave mixing (DFWM) is shown to be an effective tool for the interrogation of nascent product molecules generated during a photodissociation event. By combining an absorption-based, beam-like response with the sensitivity afforded by full resonant enhancement, this scheme provides an attractive alterative to the ubiquitous laser-induced fluorescence (LIF) methodology. DFWM spectroscopy has been used to probe the unrelaxed hydroxyl (OH) radicals formed upon 266 nm photolysis of hydrogen peroxide (HOOH), with a sub-Doppler experimental configuration enabling extraction of both scalar and vector properties. In particular, the rovibrational population distribution of the ground electronic state fragments, as well as the spatial alignment of their rotational angular momenta, have been measured. These results are compared with those obtained in previous LIF studies, thereby demonstrating the utility of four-wave mixing techniques for the quantum state-specific characterization of nascent reaction products.
A new folded variant of optical-optical double resonance has been developed for the detailed characterization of polyatomic species that contain chemically significant quantities of vibrational excitation. Based upon a novel implementation of phase-conjugate degenerate four- wave mixing (DFWM) spectroscopy, this technique provides a quantum state-specific probe of molecular topography and dynamics that offers substantial advantages over more conventional methods. Despite the third-order nonlinearity inherent to the DFWM process, the tremendous resonant enhancement that accompanies this Doppler-free interaction permits facile detection of double resonance signals even under the rarified conditions present in a molecular beam environment.
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