847left to right along the helical groove. In the X isomer this same vector is directed from right to left opposing the right-handed groove. This structural difference between the enantiomers leads to unfavorable contacts between the nonintercalating ligands on the X isomer and the sugar phosphate backbone of the DNA double helix. Because the presence of the phenyl groups increases the number of these potential contacts, this model would account for the increase in chiral discrimination observed for Ru(DIP)Z+ in comparison to that for Ru(phen)32+. Thus, this model may be useful both for designing new experiments and for developing new DNA binding agents of high site and conformation specificity.

Photoluminescence (PL) spectroscopy is a powerful technique for probing the structures of many types of III-V semiconductor materials. When a semiconductor material is excited at a particular wavelength, electron-hole pairs are generated. The most intense radiative transition is between the conduction band and valence band, and this measurement is used to determine the material band gap. Radiative and non-radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration, and thus predict device quality. At ambient temperatures, the PL signal is typically broad, as much as 100 nm in width. When cooled, structural details may be resolved, and a small spectral shift between 2 samples may represent a change in a structural parameter. Thus a system with high spectral resolution is required.In this paper, a modular Low Temperature Photoluminescence system (LTPL) for measuring optical band gap as a function of temperature is described. Results show that the optical band gap shifts towards higher energy as the sample temperature decreases.

Vibrational and total energy dependence of the reaction rate of Na with CO2 laserexcited SF6Collision induced multiple-photon absorption by SF 6 of cw CO 2 laser radiation [PI 16) 10.6pm, 8.5 W] has been carried out inside a transparent, capillary, nozzle beam source. Resonant laser excitation ofthe gas just upstream ofthe nozzle exit enhances the internal energy by 0.38 eV molecule -lover its value at Tnz = 290 K. In comparison, simple thermal heating of the nozzle to 544 K, which provides a similar velocity distribution in the supersonic beam to that found with laser irradiation, produces a beam enhanced in internal energy by only 0.03 e V molecule -I . Energies were determined using bolometer, mass filter (or flux meter), and time-of-flight measurements. Laser excitation in the nozzle induces high vibrational temperatures in the beam which cannot be achieved by simple thermal heating of the nozzle. For an assumed Boltzmann distribution of vibrational energy, the "vibrational temperature" oflaser excited SF 6 increases (from Tnz = 290 K) to T yib = 680 K. TVib for the thermally heated SF 6 is always less than the nozzle temperature. The internal energies measured are lower than those predicted using a free-jet isenthalpic expansion equation.1490

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