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Picosecond, time-resolved, fluorescence depolarization spectroscopy is used to measure the rotational reorientation times of rhodamine 6G (R6G) and p-terphenyl (PTP) as a function of solvent viscosity. The viscosity is varied either by changing the solvent or by changing the pressure in a single solvent. The differences between the two molecules, PTP and R6G, provide a means of evaluating the role of solute structure and solute–solvent interactions on the dynamics of rotational reorientation. The rotational behavior of PTP is well described by simple hydrodynamic models as embodied by the Stokes–Einstein–Debye equation. In contrast, the rotational reorientation dynamics of the charged molecule R6G are not well described by these models. It is demonstrated that dielectric friction plays an important role in governing the rotational motion of charged molecules in polar solvents. When the solvent dielectric properties are varied, the dielectric friction model accurately predicts the observed experimental trends under a wide variety of experimental conditions. This model is also shown to explain anomalous effects previously attributed to the presence of solute–solvent hydrogen bonded complexes.

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Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10−1–10−2) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm−1. Quantitative, state-resolved measurements of concentration vs time were made in pure H2CO and in mixtures with O2, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H2CO was (4.3±1) ×10−12 cm3 molecule−1 s−1. Reaction rates for HCO+NO→HNO+CO and HCO+O2→HO2+CO were measured as (1.4±0.2) ×10−11 and (4.0±0.8) ×10−12 cm3 molecule−1 s−1, respectively. Approximate rates were determined for the radical–radical reactions H+HCO→H2+CO and 2HCO→H2CO+CO as 10−9.26±0.3 and 10−10.2±0.5, respectively.

A broadly tunable picosecond laser source and an ultrafast streak camera were used to measure temporally and spectrally resolved emission from intact phycobilisomes and from individual phycobiliproteins as a function of excitation wavelength. Both wild-type and mutant phycobilisomes of the unicellular cyanobacterium Synechocystis 6701 were examined, as well as two biliproteins, R-phycoerythrin (240 kilodaltons, 34 bilins) and allophycocyanin (100 kilodaltons, 6 bilins). Measurements of intact phycobilisomes with known structural differences showed that the addition of an average of 1.6 phycoerythrin disks in the phycobilisome rod increased the overall energy transfer time by 30 +/- 5 picoseconds. In the isolated phycobiliproteins the onset of emission was as prompt as that of a solution of rhodamine B laser dye and was independent of excitation wavelength. This imposes an upper limit of 8 picoseconds (instrument-limited) on the transfer time from "sensitizing" to "fluorescing" chromophores in these biliproteins. These results indicate that disk-to-disk transfer is the slowest energy transfer process in phycobilisomes and, in combination with previous structural analyses, show that with respect to energy transfer the lattice of approximately 625 light-harvesting chromophores in the Synechocystis 6701 wild-type phycobilisome functions as a linear five-point array.

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Articles you may be interested inUltraviolet photochemistry of buta-1,3-and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurements Absolute quantum yields have been measured for the gas phase photolysis of H,CO and D,CO as a function of excitation energy. Over the spectral region examined, 290 nm ::;,,::; 360 nm, formaldehyde can dissociate into either molecular or free radical products: H,CO + h v-lH, + CO or H,CO+hv-lH+HCO. For H,CO the total photochemical quantum yield was <1>'0101= 1.0±0.1 for ,,::;326 nm and 0.68 at ,,= 353 nm. For D,CO, 'otal = 1.0 ±0.1 for,,::; 331 nm and drops to zero near the band origin. The mechanism of formyl radical recombination in pure formaldehyde was shown to be 2HCO-lH,CO+CO. Addition of NO to the photolysis mixture allowed determination of the radical/molecule branching ratio by introducing the scavenging reactions: HCO + NO-lHNO + CO, and H+NO+M-lHNO+M. The threshold for production of radicals was found to be between 339 nm and 330 nm for H,CO, and between 326 nm and 320 nm for D,CO. Radical quantum yields increase steeply at the threshold and remain constant near 0.4 at higher energies.1264