David N. Nikogosyan

Two-photon ionization and dissociation of liquid water by powerful laser UV radiation

The effect of two-photon absorption of powerful laser UV radiation by liquid water has been investigated. The two-photon absorption coefficients were measured at a number of wavelengths from 266 to 316 nm. Using a method of picosecond spectroscopy it is shown that the absorption of two UV quanta with λ = 266 nm by water results in its ionization with a quantum yield of 15% or in its dissociation with a quantum yield of 13%. The dependence of the quantum yield of ionization on the UV radiation wavelength has been obtained. The measured ionization limit of liquid water is 6.5 eV. The products of two-photon laser photolysis of water were studied. The reaction of water with the soluted oxygen resulting in superoxide ion (O 2 − ) formation is shown to be the main transformation of the hydrated electron e aq − . Other, less effective, channels of e aq − transformation are reactions resulting in the formation of the hydroxyl ion (OH − ). The measured lifetime of e aq − is0.3 μs. The relative contribution of e aq − , OH 2 − , O 2 − and H 3 O + to the photoconductivity of water under powerful UV laser radiatio been studied.

Two-quantum UV Photochemistry of Nucleic Acids: Comparison with Conventional Low-intensity UV Photochemistry and Radiation Chemistry

The action of high-intensity laser u.v. radiation on nucleic acid molecules and their constituents in vitro and in vivo is compared with the results of low-intensity u.v. photolysis and gamma-radiolysis.

Beta barium borate (BBO)

The paper contains a review of crystallographic, optical and nonlinear optical properties of beta barium borate (β-BaB2O4 or BBO) crystal and presents a description of its typical applications in nonlinear optics and quantum electronics.

Femtosecond measurements of two-photon absorption coefficients at λ = 264 nm in glasses, crystals, and liquids

Using ultraviolet femtosecond pulses with high irradiance stability, we measured the two-photon absorption (TPA) coefficients in a number of substances with a total accuracy of approximately 10%. Six commercial fused-silica samples (KU-1, Coming 7940, SQ, Suprasil, Herasil, and Infrasil) possess TPA coefficients (beta values) of approximately 2 x 10(-11) cm/W. For crystalline quartz and sapphire, the following beta values were obtained: (1.2 +/- 0.2) x 10(-11) and (9.4 +/- 1.2) x 10(-11) cm/W, respectively. In beta-barium borate crystal the TPA coefficient depends on crystal cut, beam polarization, or both and varies from (47 +/- 5) x 10(-11) to (68 +/- 6) x 10(-11) cm/W. For eight liquids that were studied (water, heavy water, ethanol, methanol, hexane, cyclohexane, 1,2-dichloroethane, and chloroform) the beta value lies from (34 +/- 3) x 10(-11) to (95 +/- 11) x 10(-11) cm/W.

Fiber Bragg grating inscription by high-intensity femtosecond UV laser light: comparison with other existing methods of fabrication

By use of high-intensity (approximately 200 GW/cm2) femtosecond 264-nm laser light and a phase mask technique, Bragg grating inscription in a range of different photosensitive and standard telecom fibers (both H2-free and H2-loaded) was studied. The dependences of the induced refractive index modulation versus the incident fluence as well as the thermal decay curves were compared with similar dependences for gratings fabricated by other existing methods. It was shown that with high-intensity UV laser irradiation, two-quantum photoreactions occur in the irradiated fiber core, that result in a significant photosensitivity enhancement of the investigated fibers in comparison with conventional low-intensity 248-nm exposure (by 6-128 times, depending on fiber type and irradiation intensity).

Inscription of fiber Bragg gratings by ultraviolet femtosecond radiation

We report on what is to our knowledge the first fabrication of fiber Bragg gratings by UV femtosecond radiation. The Bragg gratings, with photoinduced refractive-index modulation up to 1.92 x 10(-3) in H2-loaded SMF-28 and up to 1.05 x 10(-3) in Nufern GF1 fibers, were written by high-intensity (31-77-GW/cm2) femtosecond pulses at 264 nm. The dependence of the refractive-index modulation on intensity at equal fluences points to a two-photon absorption mechanism for grating inscription.

Photolysis of aromatic amino acids in aqueous solution by nanosecond 248 and 193 nm laser light

Abstract An investigation of the photochemical properties of three aromatic amino acids, tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe) was carried out using 248 and 193 nm laser irradiation and high performance liquid chromatography analysis. The quantum yields of formation of hydrated electrons (Φe−) and decomposition of substrate (Φd) were determined at room temperature in aqueous solutions at pH 6–7. From the dependences of Φe−vs. the laser intensity I, we found that the mechanism of photoionization of the aromatic amino acids is monophotonic (single-step) for λexc = 193 nm throughout and mainly biphotonic (two-step) for λexc = 248 nm. In the latter case a minor single-step process contributes; the values for I→O are Φe− = 0.05 (Trp), 0.03 (Tyr) and 0.01 (Phe), compared with those of 0.24 (Trp), 0.36 (Tyr) and 0.17 (Phe) at I = 1.2 × 1011 W m−2. The Φd, values under argon and in the presence of electron scavengers (O2 and N2O) are interpreted on the basis of photoionization as the dominant process for decomposition and a proposed reaction scheme. The efficiencies for decomposition per ionization event range from 0.3 for Trp under argon to about 2 for Phe under N2O for λexc = 193 nm. A comparison between our new and previous experimental results for Φd in the presence of oxygen using 254, 266 and 213 nm irradiation was made. In addition, the threshold energy for photochemical induced splitting of water (dissociation) was found to be in the region 6.4–6.7 eV.

DETERMINATION OF PARAMETERS OF EXCITED STATES OF DNA and RNA BASES BY LASER UV PHOTOLYSIS

Abstract— The mechanism of photodecomposition of nucleic acid bases in a neutral aqueous solution upon two‐step excitation of high‐lying electronic states by a powerful laser UV radiation is discussed. Experimental dependences of photodecomposition efficiency versus UV radiation intensity are measured both under picosecond and nanosecond laser UV irradiations. By comparison of experimental dependences with a theoretical model, we obtain some characteristics of excited states, such as lifetime t1 of the first electronic excited state S1 intersystem crossing yield φ, photosensitivity from an intermediate excited state and others for all five nucleic acid bases.

Lithium triborate (LBO)

The paper contains a review of crystallographic, optical and nonlinear optical properties of the lithium triborate (LiB3O5 or LBO) crystal and presents a description of its typical applications in nonlinear optics and quantum electronics.

Nonlinear laser photophysics, photochemistry and photobiology of nucleic acids

ConclusionThe present paper is concerned with photophysics, photochemistry and photobiology of nucleic-acid molecules and their components under intense laser UV irradiation. Attempts have been made to follow the effect of nonlinear two-quantum excitation on the course of subsequent physical-chemical and biological processes. It has been shown that high-intensity laser UV irradiation of nucleic-acid molecules and their components with nanosecond and picosecond pulses enables us to realize two-step excitation of chromophorebases above the ionization limit. At the same time in some cases two-photon excitation of the solvent, water, forming reactive products occurs. Nonlinear two-quantum excitation of biomolecules and water provides realization of chemical reactions with the dissolved molecules participating, and the products of such reactions differ qualitatively from those of classical linear photochemistry. Besides, as the irradiation intensity increases, the quantitative relation between one-step and two-step nucleic-acid lesions changes, since the quantum yield of one-step lesions drops with increasing intensity and the quantum yield of two-step lesions increases. The products formed in nonlinear photochemical reactions may be important in biology. As a result, the biological processes after nonlinear excitation also begin to depend on intensity, which is illustrated in some instances in the last section of the paper. Thus the nonlinearity of the process of excitation of molecules under intense laser UV radiation (nonlinear photophysics) is responsible for the nonlinear character of subsequent chemical and biological processes (nonlinear photochemistry and photobiology).