Femtosecond time-resolved second harmonic generation studies of the barrierless isomerization of an organic dye, malachite green (MG), have been carded out at several aqueous interfaces. A comparison of the dynamics at the air/aqueous, alkane/aqueous and silica/aqueous interfaces, indicates increased friction and increased water structure at the aqueous interfaces relative to bulk water, in support of molecular simulations, with the silica/aqueous interface being the most structured. The dynamics are slower at all of these interfaces than in bulk water, by a factor of three to five in the case of the air/aqueous and alkane/aqueous interfaces, and almost an order of magnitude in the case of the silica/aqueous interface. These investigations also indicate that the generally accepted isomerization model of twisting of the three aromatic rings about the central carbon atom requires modification in that the synchronous twisting of all three aromatic rings is not necessary for rapid internal conversion from the excited to ground electronic state. In contrast to MG, the dynamics of the activated photoisomerization of the cyanine dye, 3,3'-diethyloxadicarbocyanine iodide (DODCI), is faster at the air/aqueous interface than in bulk aqueous solution. The different dynamics of MG and DODCI suggest that the interface friction must be described in terms of the orientation and solvent structure in the vicinity of the chromophores involved in the isomerization process.
Video microscopy of nucleic acids (DNA) undergoing electrophoresis in hydroxyethyl cellulose (HEC) sieving buffers demonstrates previously unobserved shape-changing interactions between DNA and HEC molecules. We provide the first visual demonstration of entanglement between DNA and one or several discrete HEC molecules, which has been postulated to occur in ultradilute polymer solutions. Typically, nucleic acids appear to become entangled with HEC at a single region only, in both dilute and fully entangled HEC solutions. Fluctuations of the center of mass velocity of a DNA molecule and its correlation with conformation are revealed from analyses of the image data. These observations account for the success of recently reported rapid, high-resolution dc and pulsed-field capillary electrophoretic separations of nucleic acids in ultradilute hydroxyethyl cellulose solutions and hydroxyethyl cellulose/poly(ethylene oxide) solutions.
Recent experimental and theoretical studies of electron solvation in liquid water have led to a detailed kinetic picture of this process. The electron solvation dynamics can be viewed as an excited state relaxation process between different electronic energy levels of the solvated electron. Simulations have suggested the importance of direct relaxation from upper excited electronic states as well as from the lowest excited electronic state (the wet electron state) to the ground electronic state. Using this as a model to analyze the experimental data, it is found that the quantum yield of wet electron formation cannot be less than 0.5 and that the peak of the wet electron absorption spectrum shifts to the IR with a decrease of wet electron formation quantum yield, with limiting values for the peak of 1.1-1.5 eV.
The electronic relaxation and photodetachment dynamics of an electron originating from an aqueous iodide ion have been studied using femtosecond absorption spectroscopy. The results indicate the importance of an upper excited charge transfer to solvent (CTTS) state, of odd parity, in the potential well of the solvated ion.The upper excited CTTS state, which is accessed by two-photon excitation, relaxes in <50 fs to the lowest excited CTTS, which in turn relaxes to the ground state of the I-in 80 fs. It is found that most of the solvated electrons are produced by a three-photon excitation to a photoionized state, rather than from the CTTS excitation.
We have observed an intensity dependent geminate recombination of electrons in neat water. Since the recombination dynamics occurs on a time scale comparable to the salvation dynamics, the kinetics characteristic of an isosbestic wavelength are obscured. We have hypothesized that this intensity-dependent geminate recombination is due to the existence of two mechanisms for electron production with different characteristic thermalization distances. These results clarify the discrepancy between cxperiments at higher intensities and those at lower intensities
We have performed femtosecond studies of electron photodetachment from a simple halide ion in aqueous solution. After photoexcitation, the electron is trapped in the deep potential well formed by the orientational polarization of the water molecules around the anion. This trapped state of the electron is a precursor ofthe wet electron and in the case of an aqueous iodide ion has a strong absorption in the visible. We hypothesize that the electron recombines with the neutral halogen atom or crosses onto the potential surface of the wet electron by a non-adiabatic electron transfer process. We find that analogous to the case of photoionizntion of a neutral water molecule, the wet to solvatcd electron transition in the vicinity of the halogen atom can be described as two-state in character.
Using fluorescence video microscopy, DNA electrophoretic behavior under field inversion conditions has been investigated in hydroxyethyl cellulose (HEC) solutions both above and below its entanglement limit. DNA conformational fluctuation periods are found to be strongly influenced by the frequency of the applied electric field. DNA maximum extension is found to be dependent on both the frequency and the strength of the applied field. It is proposed that both above and below the HEC entanglement limit, field inversion serves to keep the average DNA conformation in a size-dependent regime intermediate between full extension and random coil. In this time-averaged geometry, efficient long-chain DNA electrophoretic separation is enabled.
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