A short review of the status of electric propulsion (EP) is presented to serve as an introduction to the more specialized technical papers also appearing in this Special Issue (Journal of Propulsion and Power, Vol. 14, No. 5, Sept. -Oct. 1998 ). The principles of operation and the several types of thrusters that are either operational or in advanced development are discussed rst, followed by some considerations on the necessary power sources. A few prototypical missions are then described to highlight the operational peculiarities of EP, including spacecraft interactions. We conclude with a historical summary of the accumulated ight experience using this technology.

Total mercury (Hg) concentrations were determined in seven tissues (skeletal muscle, liver, blood, gonad, brain, gill, and heart) of 59 striped bass and four tissues (muscle, liver, blood, and gonad) of 69 largemouth bass, 76 channel catfish, 12 bluegill, and 22 blue tilapia collected from Lake Mead, USA. Mercury levels generally increased according to trophic level and fish length. For striped bass, mean Hg levels (ng g(-1), wet mass) were highest in the liver (531), followed by muscle (309), heart (186), gonad (136), brain (77), gill (52), and blood (36). Similarly, Hg levels in the catfish and tilapia were liver > muscle > gonad > blood. In contrast, largemouth bass and bluegill had the highest levels in muscle, followed by liver, gonad, and blood. Generally, Hg levels were strongly correlated among the tissues, especially for blood/muscle and blood/liver. As the body burden of Hg increased, the concentration in blood and organs increased relative to the concentration in muscle. The trend was most pronounced for the liver. These relationships could form the basis of a predictive model and suggest that blood and muscle (plugs) could be useful for a non-lethal measure of Hg concentration and exposure in fish. For the striped bass, elevated Hg levels in the tissues were also correlated with degree of emaciation. Liver-to-muscle ratios were similar to literature values, except for tilapia with an average ratio of approximately 1.7, which is higher than generally reported for non-piscivores. Finally, this study demonstrates the usefulness of a solid sampling approach in trace element monitoring, especially as pertaining to in vivo analysis, analysis of a large number of samples and reduction of contamination risk.

The 584 Å photoelectron spectra of rotationally cold C2H2 and C2D2 were obtained with improved resolution, permitting the first three electronic states of the ions to be characterized in greater detail. Temperature-dependent studies led to a definitive assignment of the low intensity features in the X̃ 2∏u state, yielding ν4=837±12 cm−1 for C2H+2 and ν4=702±12 cm−1 for C2D+2. The ν5 origin of the Renner–Teller multiplet was identified. In the case of C2D+2, a Fermi resonance with this multiplet contributed intensity to the ν1 mode, facilitating its evaluation at 2572±16 cm−1. The C2h geometry of the Ã 2Ag state was determined from the two previously unobserved bending progressions, assigned to ν4 and ν5B, and evaluated at 492±12 and 605±12 cm−1 for C2H+2 and 339±12 and 516±12 cm−1 for C2D+2, respectively. A more extensive vibrational progression than previously evident, comprised of irregular spectral features indicative of nonadiabatic effects, was observed for the B 2∑+u state. Autocorrelation functions were derived from the spectra for all three electronic states, and the two electronically excited states exhibit an ultrafast decay on a 10−14 s time scale. The Ã 2Ag state decays within one period of bending vibration, while the B̃ 2∑+u state survives only 14 fs, corresponding to a single period of symmetric stretching motion.

The 584 Å photoelectron spectrum of ethylene was reexamined at higher resolution using a supersonic molecular beam source for rotational cooling. The vibrational frequencies of the C2H+4X̃ 2B3 state were measured with improved accuracy, which permitted the torsional potential energy curve to be characterized. The measured equilibrium torsional angle is 27°*2°, and the inversion barrier is 270±150 cm−1. The Ã 2B3 state is shown to be in reasonable agreement with a recent theoretical calculation which takes into account strong nonadiabatic effects. The correlation function calculated for this state is consistent with this calculation and recent calculations on the unimolecular dissociation of this state. The B̃ 2A state correlation function indicates a rapid decay mechanism is operative. The broadened vibrational structure in the fourth band results from a saddle point in the C̃ 2B2 potential energy surface from which the ion relaxes to the X̃ state by moving along a repulsive degree of freedom, which in this case is internal rotation. The C̃ state vibrational broadening as well as the decay of the correlation function yield a lifetime for this process of 7 fs.