Thomas W. Ebbesen

Picosecond flash photolysis apparatus with high absorbance resolution

A new picosecond flash photolysis apparatus is described which makes extensive use of fiber optics in the probing beam pathway. The apparatus is simple and easy to use. The absorbance resolution and accuracy are greatly enhanced and allow for quantitative determination of properties of transient species. The capability of the system is illustrated by determining the excited singlet state absorbance properties of erythrosine and rose bengal.

Laser flash photolysis of chlorobenzene in polar solvents

Abstract The laser flash photolysis of chlorobenzene at 266 nm was studied in methanol (MeOH) and other polar solvents. The triplet state properties in MeOH were determined: φT = 0.7 ± 0.2; λmax = 305 nm; ϵ305 = 6500 ± 800 M−1 cm−1; τ = 715 ± 20 ns. In the presence of LiCl a long-lived transient absorption with λmax = 350 nm was observed and it was assigned to Cl-rad2. Part of the absorption was formed immediately after the laser pulse and a slow growth was also observed in the following microsecond. This was interpreted as being partly due to a singlet state very fast CCl bond homolysis and partly due to a triplet-mediated generation of Cl·. Also the characteristic absorption of solvated electrons was found in MeOH. A photo-ionization quantum yield of 0.03 ± 0.01 was estimated. The triplet lifetime was determined in various solvents and it was found to decrease with increasing dielectric constant. The photolysis mechanism of chlorobenzene is discussed in terms of a singlet state photosubstitution reaction with the radical cation as an intermediate, and a singlet and triplet homolytic CCl bond breakage.

DISTORTIONS IN LASER FLASH PHOTOLYSIS ABSORPTION MEASUREMENTS. THE OVERLAP PROBLEM

Abstract Lack of overlap between the laser beam and the analyzed volume in laser flash photolysis experiments may lead to significant error in the analysis of transient absorbances. A simple theoretical treatment is given to calculate the error for a given overlap. The consequences of bad overlap is discussed for a number of standard experiments and finally a method to determine the quality of the overlap in an experimental setup is given.

Photophysics of chlorobenzene in cyclohexane

Abstract The photophysical parameters of chlorobenzene in cyclohexane were determined. The fluorescence quantum yield was found to be 0.007 ± 0.0005 and the fluorescence lifetime was 0.74 ± 0.09 ns independent of the chlorobenzene concentration. The triplet state properties were studied by laser flash photolysis at an excitation wavelength of 265 or 248.8 nm. The triplet—triplet absorption spectrum with λmax = 300 nm (ϵ300 = (6250 ± 300) M−1 cm−1) was obtained. The triplet decay was found to be a function of the laser intensity and a rate constant of (7.8 ± 1.5) × 109 M−1 s−1 was obtained for a triplet—triplet annihilation process. The triplet lifetime extrapolated at zero triplet concentration was 1.6 ± 0.1 μs independent of the chlorobenzene concentration within the experimental error. At low laser intensities and high chlorobenzene concentrations a second maximum appears at 365 nm in the transient absorption spectrum. The species responsible for this band is present immediately after the laser pulse and is very much longer lived than the triplet state. The implications of these results for the continuous photolysis mechanism of chlorobenzene are discussed.

Spectral evidence for long-lived radical-electron pairs: photoionization of phenol in alcohols

Abstract The transient solvated electron generated by photoionization of phenol shows a 100 nm blue spectral shift in both ethanol and 1-propanol with respect to the known absorption spectra. No spectral change is observed in methanol. However, in all three solvents, the photoionized electron disappears by non-homogeneous kinetics. The spectral shift decreases during the electron decay in ethanol. These results and others are discussed in terms of the formation of long-lived radical-solvated electron pairs.

Cis-trans radical photoisomerization—A pulse radiolysis-flash photolysis study

Abstract The combination of pulse radiolysis and laser flash photolysis is a very useful tool to study radical photochemistry. The criteria for designing such double pulse experiments are discussed and a typical system is described. The technique is illustrated with recent results from the study of olefin radical photoisomerization.