To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 +/- 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state lifetimes of Ado, Guo, Cyd, and Thd were determined to be 290, 460, 720, and 540 fs, respectively (uncertainties are +/-40 fs). The decay times are shorter for the purines than for the pyrimidine bases, consistent with their lower propensity for photochemical damage. Following internal conversion, vibrationally highly excited ground state molecules were detected in experiments on Ado and Cyd by hot ground state absorption at ultraviolet wavelengths. The decays are assigned to intermolecular vibrational energy transfer to the solvent. The longest time constant observed for Ado is approximately 2 ps, and we propose that solute-solvent H-bonds are responsible for this fast rate of vibrational cooling. The results show for the first time that excited singlet state dynamics of the DNA bases can be directly studied at room temperature. Like sunscreens that function by light absorption, the bases rapidly convert dangerous electronic energy into heat, and this property is likely to have played a critical role in life's early evolution on earth.

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Indole in aqueous solution is photoionized near threshold following single photon absorption from a femtosecond laser pulse at 260 nm. Transient absorption measurements are performed using a white-light continuum probe pulse. Excited state absorption of neutral indole molecules is characterized accurately in 1-propanol where photoexcitation at 260 nm does not lead to photoionization. The presence of 0.75 M carbon tetrachloride in a solution of indole/1-propanol leads to the formation of indole radical cations on a picosecond time scale. While solvated electrons are formed in aqueous indole within our time resolution of 200 fs, measurements of the transient absorbance out to 100 ps are flat and indicate that geminate recombination is insignificant on this time scale. This result contrasts sharply with the geminate recombination dynamics observed following the photoionization of neat water. This indicates that the bimolecular reaction between indole radical cations and solvated electrons is considerably slower than the diffusion limit. We suggest that geminate recombination arising from solute photoionization in polar solvents may be slower than previously thought.

This volume contains two contributions: a chapter on the new technique of thermal-diffusion forced Rayleigh scattering (TDFRS), by W. Koehler and R. Schaefer, and a review of the area of scanning force microscopy (SFM), by S. S. Sheiko.The first chapter begins with a clear presentation of the principles of thermal and mass diffusion in polymer solutions. The TDFRS method is based on Bragg diffracton of temperature and concentration gratings optically written in a polymer solution. TDFRS can be used to determine diffusion coefficients, molar mass distributions, and molar mass averages of the solutes in subseconds, therefore avoiding the perturbation of convection. Furthermore, TDFRS is shown to be capable of measuring the linear response function for pseudostochastic noise-like excitation patterns by means of Fourier transform. By utilizing tailored pseudostochastic binary sequences for signal processing, selective excitation of certain frequency ranges of interest in TDFRS becomes possible. The appeal of the technique is the capability of determining different averages of the solution Brownian diffusion coefficient of polymers with broad molar mass distributions. This makes TDFRS, in principle, superior to photon correlation spectroscopy for polydisperse polymer samples, even though the analysis of very broad molar mass distributions is still problematic. The content of the chapter is comprehensive, and the list of the references is adequate. While the discussion on the optical wavelength used for readout with respect to Rayleigh scattering, diffraction efficiency, and signal-to-noise ratio is not deep, the chapter is a valuable reference for researchers and graduate students in the field of advanced polymer science.The chapter by S. S. Sheiko shows nicely that SFM has emerged within the last couple of years from an exclusive imaging tool to a powerful technique for the study of interfacial properties and for manipulation of nanoscale objects. It provides a clear introduction into general principles of the technique and covers recent SFM applications in the area of polymer science with special focus on nanotechnology and characterization of single macromolecules. The author also outlines the current knowledge in intermittent (tapping) and noncontact imaging, which are both very useful for the imaging of soft materials that are important in polymer and biological sciences.

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