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Rearrangements and Tunneling Splittings in Small Water Clusters

Recent far-infrared vibration-rotation tunneling (FIR-VRT) experiments pose new challenges to theory because the interpretation and prediction of such spectra requires a detailed understanding of the potential energy surface (PES) away from minima. In particular we need a global description of the PES in terms of a complete reaction graph. Hence all the transition states and associated mechanisms which might give rise to observable tunneling splittings must be characterized. It may be possible to guess the detailed permutations of atoms from the transition state alone, but experience suggests this is unwise. In this contribution a brief overview of the issues involved in treating the large amplitude motions of such systems will be given, with references to more detailed discussions and some specific examples. In particular we will consider the effective molecular symmetry group, the classification of rearrangement mechanisms, the location of minima and transition states and the calculation of reaction pathways. The application of these theories to small water clusters ranging from water dimer to water hexamer will then be considered. More details can be found in recent reviews.Comment: 15 pages, 5 figures. This paper was prepared in August 1997 for the proceedings volume of the NATO-ASI meeting on "Recent Theoretical and Experimental Advances in Hydrogen Bonded Clusters" edited by Sotiris Xantheas, which has so far not appeare

International Cometary Explorer (ICE)

The primary mission objectives of the International Cometary Explorer (ICE) Comet Mission are to determine the composition and physical state of the Giacobini-Zinner Comet's nucleus; to determine the processes that governs the composition and distribution of neutral and ionized species in the cometary atmosphere; and to investigate the interaction between the solar wind and the cometary atmosphere. The spacecraft was in a halo orbit around the Sun-Earth libration point until it was moved 10 Jun. 1982 to the Earth's Geomagnetic Tail (GT). The spacecraft reached the GT in Jan. 1983 and remained there until Dec. 1983, at which time a lunar swing-by placed the spacecraft in a trajectory heliocentric orbit which encountered the comet Giacobini-Zinner in Sep. 1985. The spacecraft provided observations of solar wind upstream of Halley's Comet in 1986. Information is presented in tabular form and includes the following areas: Deep Space Network support, frequency assignments, telemetry, command, and tracking support responsibilities

Theoretical study of finite temperature spectroscopy in van der Waals clusters. II Time-dependent absorption spectra

Using approximate partition functions and a master equation approach, we investigate the statistical relaxation toward equilibrium in selected CaAr$_n$ clusters. The Gaussian theory of absorption (previous article) is employed to calculate the average photoabsorption intensity associated with the 4s^2-> 4s^14p^1 transition of calcium as a function of time during relaxation. In CaAr_6 and CaAr_10 simple relaxation is observed with a single time scale. CaAr_13 exhibits much slower dynamics and the relaxation occurs over two distinct time scales. CaAr_37 shows much slower relaxation with multiple transients, reminiscent of glassy behavior due to competition between different low-energy structures. We interpret these results in terms of the underlying potential energy surfaces for these clusters.Comment: 10 pages, 9 figure