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Notes: Picosecond infrared pump–probe experiments are used to measure the vibrational lifetime of the asymmetric (T1u) CO stretching mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 as a function of solvent density and temperature. As the density is increased at constant temperature from low, gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the critical temperature (Tr≡T/Tc(approximate)1.01), the lifetimes are essentially constant over a wide range of densities around the critical value (ρc). When the density is increased well past ρc, the lifetimes shorten further. At higher temperature (Tr=1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W(CO)6 and Rh(CO)2acac in supercritical CO2, C2H6, and CHF3 acquired for the same isotherms show that the vibrational spectral peak shifts follow similar trends with density. The peak positions shift to lower energy as the density is increased. Near the critical point, the peak positions are density independent, and then redshift further at densities well above ρc. It is shown that critical fluctuations play a dominant role in the observed effects. Theoretical calculations ascribe the density independence of the observables to the cancellation of various rapidly changing quantities near the critical point. The theory's calculation of density independence implicitly involves averages over all local densities and does not involve any form of solute–solvent clustering. © 1997 American Institute of Physics.

Notes: Experimental measurements are reported for the temperature dependence of the vibrational lifetime, T1, of the asymmetric CO stretching mode of tungsten hexacarbonyl in supercritical ethane at constant density from just above the critical temperature to substantially higher temperatures. T1 is found initially to increase with temperature along an isochore (reaching a maximum at about 70° above the critical point of ethane), and then subsequently to decrease. Using a recent classical theory of vibrational relaxation, we attempt to rationalize the T1 data. This behavior can be semiquantitatively reproduced by the theory if quantum corrections to the classical rate expressions are assumed to be temperature independent in the limit when the transition energy is much greater than thermal energy. In this case, the theory indicates that the initial increase in T1 with temperature arises because of a competition between properties of the solvent which are changing rapidly as the temperature is raised above the critical temperature. At sufficiently high temperature, properties of the solvent vary slowly with temperature, and the explicit temperature dependence of the vibrational relaxation dominates, producing a decrease in T1 with increasing temperature. The predictions of the theory are also examined when other postulated forms of the quantum correction factors are used, and the implications of these results for theoretical approaches to vibrational relaxation are discussed. © 1997 American Institute of Physics.

Notes: Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy and leaf inclination. Leaves of Thermopsis montana had well-developed palisade and spongy mesophyll whereas the photosynthetic tissue of Smilacina stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low-light levels, leaves of S. stellata and T. montana were nearly horizontal, whereas under high-light levels, S. stellata leaves and T. montana leaves were inclined 600 and 300, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial leaf surface of T. montana plants grown under high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxial surface of low-light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem II (PSII) down regulation, measured from each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was found in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2–3 times higher than those in T. montana, while the portion of the PSII population which was found to be QB nonreducing was 4–6 times lower in high light S. stellata leaves than in T. montana. All values of Stern-Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher than the irradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxial and abaxial leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf inclination.