Inactivation of bacteria in plasma.

The photo-inactivation rate of bacteria by methylene blue, MB, was found to be significantly lower in plasma than in water, saline, and PBS solutions. The spectroscopic data and ultrafast time resolved transient spectra and kinetics presented show that methylene blue photo-bleaches faster and to a larger degree in plasma and the MB excited singlet and triplet state populations in plasma are much lower in plasma than in water and PBS solutions. The optical density, OD, of MB in plasma was found to decrease by ~50% after a minute of illumination with 661 nm light, while under identical conditions the OD in PBS solution decreased by only 1%. Based on these data and the effect of the plasma proteins on MB photochemistry, a mechanism is proposed that accounts for the low inactivation rate of bacteria in plasma.

Two-photon three-dimensional optical storage memory.

We describe the design and construction of ultrahigh capacity three-dimensional, 3D, optical storage devices that operate by two-photon absorption. The molecular systems and their properties that are used as two photon media for writing and one photon for accessing the stored information within the volume of the device are presented in some detail and the nonlinear two-photon absorption mechanism is briefly visited. The optical system and its components, which facilitated writing and reading, are also described and the bit density, bit error rate, store and access speeds, cycle times, and stability of the materials under various experimental conditions are also topics addressed in this review. The first ever storage of terabyte data in a removable storage disk is described in detail.

Photolysis of CBr4 and its transient solvent stabilized (CBr3+//Br-)solv ion pair.

In this paper we describe the mechanism of the photodissociation of CBr4 in various solvents. We have monitored and identified the intermediate species and determined the growth and decay rates of the excited states and metastable species formed during the photodissociation process by means of ultrafast time-resolved spectroscopy. On the basis of the experimental data presented in this paper, the observed transient spectra and species have been assigned to the stabilized solvated ion pair (CBr3+//Br-)solv.

Nonvolatile read-out molecular memory.

A versatile molecule is described that performs as a nondestructible read-out optical-storage molecular memory. This molecular memory is composed of two distinct molecules that are chemically bonded to each other to form a single molecule with unique properties. One component is a photochromic fulgimide, and the other is a strongly fluorescing oxazine dye. This composite molecule was specifically designed and synthesized to display, under specific conditions, both the photochromic functions of the first component and the fluorescence properties of the dye. To store information, the polar, closed form of the photochromic component is illuminated with 530-nm light, which converts it to the open, nonpolar form. The information is accessed by excitation at the 650-nm band of the oxazine dye component, causing the dye to fluoresce. However, the dye emits intense fluorescence under a nonpolar environment, which is attained only when the fulgimide component is in its open, nonpolar structure. The ultrafast kinetics, spectroscopy, and mechanism of the photoreaction of this molecule and its photoinduced intermediates have been measured, and fluorescence quantum yields and cross sections were determined.

Structures of bromoalkanes' photodissociation in solution by means of ultrafast extended x-ray absorption fine-structure spectroscopy.

The structures of initial and final products of bromoalkanes' photodisociation reaction in cyclohexane solution have been measured with a bond length accuracy of 0.02 A by means of ultrafast time-resolved extended x-ray absorption fine structure spectroscopy. The photoredaction mechanism is also discussed.

Dynamical principles in biological processes: a model of charge migration in proteins and DNA.

The generalized master equations (GMEs) that contain multiple time scales have been derived quantum mechanically. The GME method has then been applied to a model of charge migration in proteins that invokes the hole hopping between local amino acid sites driven by the torsional motions of the floppy backbones. This model is then applied to analyze the experimental results for sequence-dependent long-range hole transport in DNA reported by Meggers et al. [Meggers, E., Michel-Beyerle, M. E., & Giese, B. (1998) J. Am. Chem. Soc. 120, 12950-12955]. The model has also been applied to analyze the experimental results of femtosecond dynamics of DNA-mediated electron transfer reported by Zewail and co-workers [Wan, C., Fiebig, T., Kelley, S. O., Treadway, C. R., Barton, J. K. & Zewail, A. H. (1999) Proc. Natl. Acad. Sci. USA 96, 6014-6019]. The initial events in the dynamics of protein folding have begun to attract attention. The GME obtained in this paper will be applicable to this problem.

Dynamical principles in biological processes.

The purpose of this paper is to propose certain dynamical principles in biological systems, which can be used to explain the effectiveness of charge transfer or excitation transfer in biological systems. Some of these systems are accessible experimentally.

Experimental characterization of a two-photon memory.

We demonstrate the recording of 100 planes of digital images in a page-oriented two-photon memory and characterize the images in terms of signal-to-noise ratio and bit error rate. Possible error sources in the recording are discussed, and methods for compensating for some of these effects are presented. Looking at the distributions of the normalized bit intensities, we are able to estimate the minimum achievable bit error rate for this system.

Two-photon-induced photochromic reactions in spirobenzopyran-doped poly(methyl methacrylate) thin-film waveguides.

We present experimental results of photochromic reactions induced by single- and two-photon excitations in a poly(methyl methacrylate) thin film doped with spirobenzopyran. We also demonstrate the operation of a spirobenzopyran-doped poly(methyl methacrylate) planar waveguide, prepared by cast spinning the media onto fused-quartz slides. The following phenomena were shown to take place in this waveguide: single-twophoton coloring, f luorescing, and bleaching. The results indicate that such devices may be suitable for many applications, including optical waveguides, volume optical storage, and optical sensors.

Picosecond X-Ray Pulses Generated in a Diode Driven by 193-nm Picosecond Laser Pulses.

A new technique for the generation of picosecond x-ray pulses is presented. Picosecond laser pulses are used to generate ultrashort electron pulses in a conventional x-ray diode in which the cathode is replaced by a photocathode. In the experiments presented, 9-ps UV pulses are employed to produce characteristic x-ray pulses with duration in range of 20-90 ps at a repetition rate of 300 Hz. This technique is inherently capable of subpicosecond x-ray pulses and synchronization of the x-ray pulses with the laser pulses or other ultrafast devices with picosecond accuracy.

Third-harmonic generation in barium borate.

The cubic nonlinear susceptibility of barium metaborate was employed for the generation of phase-matched third-harmonic generation (THG) of 1.053-microm pump radiation. The various processes that are responsible for THG in this crystal are discussed and experimental results for types I and II phase matching are presented. THG power of 50 microW is recorded with an average pump power of 800 mW of 1-kHz repetition rate picosecond pulses.

Potentials of two-photon based 3-D optical memories for high performance computing.

The advent of optoelectronic computers and highly parallel electronic processors has brought about a need for storage systems with enormous memory capacity and memory bandwidth. These demands cannot be met with current memory technologies (i.e., semiconductor, magnetic, or optical disk) without having the memory system completely dominate the processors in terms of the overall cost, power consumption, volume, and weight. As a solution, we propose an optical volume memory based on the two-photon effect which allows for high density and parallel access. In addition, the two-photon 3-D memory system has the advantages of having high capacity and throughput which may overcome the disadvantages of current memories.

Three-dimensional optical storage memory.

A novel three-dimensional (3-D) optical memory device is presented that allows fast random access of the information and extremely high bit densities. This device is based on two-photon writing, reading, and erasing of the information in a photochromic material embedded in a polymer matrix. Absorption and emission data show that two-photon writing and reading of information is feasible. The advantages and properties of such a 3-D optical memory are discussed.

Measurement of x((3)) and phase shift of nonlinear media by means of a phase-conjugate interferometer.

We have constructed a new interferometer that uses two phase-conjugate mirrors. This device allows for the measurement of the relative phase shift and the ratio of the nonlinear susceptibilities of materials. We have been able to measure weak reflectivity signals not detectable by normal degenerate four-wave mixing methods. It has been determined that the x((3)) value of colloid semiconductor glass OG530 at 532 nm is 7 x 10(-12) esu.

Spectroscopic characterization of nitrated purple membranes.

Light-adapted purple membranes were modified with tetranitromethane by a new light-dependent procedure at pH 5.5 which results in a blue-shifted chromophore absorbing at 530nm. This modification affects two aromatic residues. The modified bacteriorhodopsin's ground state chromophore structure is probed by circular dichroism and resonance raman spectroscopy while its photocycle is studied by laser-flash photolysis in the picosecond, microsecond and millisecond time scale. After nitration, the main findings are 1) Interactions between neighboring chromophores are lost, 2) Modified bacteriorhodopsin contains a conformationally changed chromophore but retains a protonated Schiff's base as evidenced by a resonance raman band at 1652 cm-1, 3) A red-shifted intermediate is formed in less than 10 ps after laser excitation, 4) The decay of the M-intermediate is not significantly affected whereas the rise time of the intermediate is enhanced about two fold. These observations are relevant to the role of aromatic acid residues of the apoprotein in the determination of the chromophoric characteristics in bacteriorhodopsin.

Bathorhodopsin intermediates from 11-cis-rhodopsin and 9-cis-rhodopsin.

Bathorhodopsin-rhodopsin difference spectra of native 11-cis-rhodopsin and regenerated 9-cis-rhodopsin were measured at room temperature with a double-beam laser spectrophotometer after excitation at 532 nm. A detailed analysis of data obtained at 85 psec after excitation suggests that the bathorhodopsins generated from 11-cis- and 9-cis-rhodopsin differ in their extinction coefficients and that their absorption maxima are shifted in wavelength by about 10 nm from one another. The ratio of quantum yields for photochemical production of the 11-cis-bathorhodopsin and the 9-cis-bathorhodopsin approximates 1. Implications that the early photochemical processes in vision are more complex than previously considered are explored.

Biological applications of picosecond spectroscopy.

Technological advances in picosecond spectroscopy have permitted the mechanisms of various chemical, physical and biological processes to be elucidated and understood to a greater degree than ever before. By means of picosecond emission, absorption and Raman spectroscopy, one can probe and measure directly the transient intermediates and kinetics of primary events in complex biological processes. A description of two current types of laser systems--solid-state and synchronously pumped dye lasers--and their application to determining the primary events in the biological processes of dissociation of oxy- and carboxymyoglobin, excited-state relaxation of porphyrins and visual transduction, illustrate the power of picosecond spectroscopy.

Advances in picosecond spectroscopy.

Progress in the technology of picosecond spectroscopy in the past few years has made possible the generation of well-characterized pulses emitted by synchronously pumped tunable dye lasers. In addition, the development of sensitive emission and absorption detection methods and the advent of picosecond Raman and coherent anti-Stokes Raman spectroscopy make possible the direct observaton of picosecond transient spectra and lifetimes. The information obtained allows the complete determination of mechanisms through the identification of transient states, radicals, and ions that evolve during the course of a chemical or biological reaction.

Picosecond spectroscopy of Cu(II) cytochrome c.

We have observed a strong pH dependence in the relaxation rate of Cu(II) cytochrome c following excitation at 532 nm. At pH 8.0 the excited state relaxes with a lifetime of 10 +/- 5 ps while at pH extremes of 2.5 and 13.0 we find that the lifetime becomes longer than 1 ns. This change of more than two orders of magnitude in the lifetime may be due to the Cu coordination number, which is six at neutral pH but five at pH extremes.

Kinetics and temperature dependence of carboxymyoglobin ligand photodissociation.

We have observed the rate of oxymyoglobin (MbO2) photodissociation at room temperature and carboxymyoglobin (MbCO) photodissociation as a function of temperature (260-10 K) by means of picosecond spectroscopy. The Mb + O2 and Mb + CO photodissociated states have also been characterized. Based on the picosecond experimental data, we postulate that the photodissociation of ligated myoglobin is a nonactivitated process, and the mechanism involves either a small enthalpy barrier or none at all.