A recent comparison of NO profiles at 100-200 km with modeling calculations has been presented by Cleary.21 The measurements were obtained from observations of solar scattering in the NO y, , and t band systems. The model was based on a total of 36 processes-11 photoabsorptive and 25 reactive. Good agreement was obtained between the observations and the model, in spite of the fact that the N(2D) + 0(3P) rate coefficient used was 5 X 10~13 cm3 molecule™1 s™1.As discussed above, the only reactions leading to NO formation are N(2D) and N(4S) interactions with 02. With a drastic reduction in NO produced by N(2D) + 02, the only apparent compensating effect would be an increase in the N(4S) + 02 rate coefficient. This is a well-studied reaction, and there is no reason to believe that the measured activation energy34 of 6.3 kcal/mol is too large.A further aspect of the dominance of N(2D) quenching by 0(3P) is that the electronic state of the oxygen product becomes of interest. Is it O('D) or 0(3P)? If O^D) it will contribute to 02(b12g+) generation by 0('D)/02 transfer and to the N2 vibrational temperature by 0('D)/N2 transfer.Calculations by Abreu et al.26 indicate that quenching of 0('D) by 0(3P) is relatively rapid, with a rate coefficient of 1 X 10™11 cm3 molecule™1 s™1. Thermally, the quenching of 0(*D) by 0(3P) is the equivalent of 0(3P) being the N(2D)/0(3P) product; in each (34) Baulch, D.

Picosecond transient absorption studies of two Mn(III) porphyrins in coordinating and noncoordinating solvents have revealed two resolvable transient states in each case. The lifetimes of the transients are <30 ps and 80 or 140 ps, the faster component being instrument-limited and the slower one depending on the complex. We attribute the transients to the two lowest energy tripmultiplet states that arise from coupling of the ring (ir,ir*) triplet and the unpaired metal electrons. The rapid deactivation of both tripmultiplets probably proceeds via lower energy CT or (d,d) excited states.

We have investigated, using time-resolved and steady-state optical techniques, some of the photophysical properties of a lanthanide-porphyrin dimer. The complex contains a Ce(IV) ion sandwiched between two octaethylporphyrin macrocycles. The dimer is nonfluorescent (0f < 2 X 10"5). The absorption changes produced by pulse excitation decay largely (>90%) by return of the molecules to the ground state in <10 ps. There is evidence for low energy quenching states in the ground state absorption spectrum. In order to explain these observations, we have considered, the importance of neutral (ir,tr*) exciton states of the two rings, ring-to-ring charge-transfer states and ring-to-metal charge-transfer states. We believe that excited states having substantial charge-transfer character may provide the most effective routes for rapid radiationless decay.

No abstract

C-phycocyanin and allophycocyanin are light-harvesting proteins that occur in the phycobilisome in cyanobacteria.1 They represent relatively simple systems in which the structural factors that govern the direction and rate of excitation transfer can be addressed through the methods of picosec-ond spectroscopy. Several aspects of the structure of C-phycocyanin, as recently determined by X-ray crystallographic methods,2 present challenges to current theories for excitation transfer in light-harvesting proteins. We have begun a detailed characterization of the picosecond transient absorption and dichroism kinetics of C-phycocyanin in order to compare measured patterns of excitation transfer in C-phycocyanin with those indicated by direct calculation, using the known coordinates and orientations of the pigment molecules bound by the protein. We are also examining excitation transfer processes in allophycocyanin, a similar phycobiliprotein that directs excitation energy received from C-phycocyanin towards the photosystem II reaction center.