Photochemical conversion and optical absorption spectra of V centers in coloured KI crystals

Abstract

The initial V₂ and V₃ bands observed after quenching of coloured KI crystals are found to be the superposition of absorption bands due to two kinds of V centres with ?110? and ?111? symmetries. The ratio of concentrations of the former to the latter V centers is estimated to be about 3:1. When a quenched crystal is irradiated at 19 K with V₃-light in the 270-nm region, V centers are bleached forming I_2-like centers. On the basis of the I₃ molecule-ion model bound to a cation vacancy, a review is given of recent work on the photochemical conversion of V centers at low temperatures.     

Passive compression of KrCl excimer laser pulses in naphthalene solutions

The width of KrCl laser pulses has been compressed from 5.2 ns to less than 800 ps using naphthalene as the saturable absorber dye. It was found that the width of the compressed laser pulse decreased with both the input laser intensity and the concentration of naphthalene in the solution. The pulse shortening mechanism is attributed to excited state S₂-S_n transitions in naphthalene.     

Degradation of gallium arsenide under irradiation with an excimer laser

The results of a study of degradation of the surface of gallium arsenide resulting from irradiation with a power excimer laser at power densities ranging from the threshold power to the power level causing local melting of the surface are presented. Two degradation mechanisms have been identified, one of which causes the formation of a thin near-surface layer of modified nonstoichimetric gallium arsenide at a power level higher than 1×10⁷ W/cm² and the other of which causes the formation of a separate gallium phase. The formation of the separate gallium phase can be produced either by a single pulse of laser radiation with a power density exceeding 2.7×10¹¹ W/cm² or by a few less powerful pulses. An empirical relationship has been established between the power density and the number of pulses causing the formation of the separate gallium phase. It has also been established that as a result of laser irradiation at the boundary of “cold” and “hot” gallium arsenide, periodically ordered defects in the form of blocks aligned along the [100] directions emerge.     

Defect-density switching wave in crystals under pulsed laser irradiation

A model is proposed for the ignition of a defect-density switching wave in a crystal by powerful laser pulses. It is shown that the switching wave arises as a result of the nonlinear dependence of the activation energy of the defect-formation process on the strain field due to defects. The conditions under which a switching wave arises and the profile, velocity, and propagation direction of the wave are discussed. Zh. Tekh. Fiz. 68, 73–77 (August 1998)     

Photoabsorption of BCl3 Gas under pulsed ArF excimer laser irradiation

The gas immersion laser doping (GILD) technique requires the measurement of the fraction of incident light absorbed in the gas phase during the irradiation with a pulsed laser. Here we report the absorption of boron trichloride (BCl₃) gas at the wavelength of a pulsed ArF excimer laser (=193 nm). We have determined the one-photon (₁) and two-photon () absorption cross sections of this dopant gas for 193 nm. The values of ₁ and are 3.6×10^–20 cm² and 9×10^–45 cm⁴·s, respectively. However, the distinction between simultaneous and sequential absorption has not been possible. Based on these results, we have established a relationship which allows the calculation of the fraction of incident light absorbed as a function of incident intensity and gas pressure.     

Determination of the unsaturated losses and of the saturation intensity in the KrCl excimer laser

The unsaturated losses , and the saturation intensityI _s, were measured in an uv-preionized KrCl laser for optimized He and Ne based laser gas mixtures. The measurements were made as a function of the specific power loading and of the total pressure of the laser mixtures. Higher values for andI _s were found for the Ne-based laser mixture than for the He-based mixture. At 45 kV charging voltage and at 355 kPa of total pressure we measured =0.053 cm^–1 andI _s =9.4MW/cm² for the Ne based mixture, and =0.035 cm^–1 andI _s =5.1MW/cm² for the He based mixture.     

Surface ablation by excimer laser irradiation of Ti and Ti6Al4V alloy

This paper presents the surface microstructure of Ti and Ti6Al4V alloy irradiated with a high output energy XeCl ( = 308 nm) excimer laser. The treatments are carried out on both materials at two beam fluences and the effects of single- and multiple-pulse irradiation are compared. The results of the scanning electron microscopy and of the X-ray diffraction techniques suggest the possible influence of both time-behaviour and energy fluence of the laser pulse on the relative weight of the ablation rate and of the reaction product deposition rate at the sample surface.     

Formation of coniform microdischarges in KrCl and XeCl excimer lamps

The dynamics of formation of the steady-state regime in KrCl and XeCl double-barrier excimer lamps excited by a pulsed-periodic discharge is studied. Diffusive microdischarges in the form of two cones with joint vertices are shown to appear for about 1 s. Over this time interval, the initially volume exciting discharge (within several early pulses) transforms into a spark (immediately before the formation of the coniform microdischarges). It is demonstrated that the spark-diffusive discharge transition may be associated with fast electron generation.     

Amorphous film thickness dependence for epitaxy of perovskite oxide films under excimer laser irradiation

We have studied the epitaxial growth of perovskite manganite LaMnO₃ (LMO) on SrTiO₃(1 0 0) in the excimer laser assisted metal organic deposition process. The LMO was preferentially grown from the substrate surface by the KrF laser irradiation. The study of amorphous LMO film thickness dependence on epitaxial growth under the excimer laser irradiation revealed that the photo-thermal heating effect strongly depended on the amorphous film thickness due to a low thermal conductivity of amorphous LMO: the ion-migration for chemical bond-forming at the reaction interface would be strongly enhanced in the amorphous LMO film with the large film thickness about 210 nm. On the other hand, the photo-chemical effect occurred efficiently for the amorphous film thickness in the range of 35-210 nm. These results indicate that the epitaxial growing rate was dominated by the photo-thermal heating after the photo-chemical activation at the growth interface.     

Gain measurements in the KrCl discharge laser

The small-signal net gain of the KrCl laser, excited in an UV-preionized transverse discharge, was measured using the passive absorption cell method. A maximum net gain of 0.13 cm^-1 was measured in a 0.09% HCl/10.1% Kr/1.5% He/ 88.3% Ne mixture at a total pressure of 355 kPa and at a specific power loading of about 23 MW/cm³. Smaller gains were measured when He was used instead of Ne in the lasing mixture.     

Study of color centers produced in thulium doped YLF crystals irradiated by electron beam and femtosecond laser pulses

In this work, we report the influence of the presence of photochromic and color centers in the photobleaching of thulium ions blue emission in YLF (YLiF₄) crystals doped with 1 mol% Tm (3+). The samples were irradiated at room temperature both with electron beam and high intensity ultrashort pulses from a Ti:Sapphire CPA laser system. In both irradiations the production of photochromic and color centers was observed via the absorption bands in the UV and visible ranges. Pure LiF and pure and oxygen doped YLF crystals were used to identify the color centers produced and their optical properties. From a phenomenological model it was possible to study the interaction between color centers and thulium ions, and their effect in photobleaching and photodarkening behaviors. Finally, the blue up laser level population was computed using a rate equation analysis.     

V color centers in electrolytically colored hydroxyl-doped sodium chloride crystals

Hydroxyl-doped sodium chloride crystals were successfully colored electrolytically by using pointed anode and flat cathode at various temperatures and under various electric field strengths. V₂ and V₃ color centers were produced in the colored crystals. Current-time curves for the electrolytic colorations were given, and activation energy for the V₂ and V₃ color center migration was determined. Production of the V₂ and V₃ color centers and formation of current zones for the electrolytic colorations of the hydroxyl-doped sodium chloride crystals are explained.     

Absorption modification by laser irradiation in DKDP crystals

Laser-induced modification at 355 nm of deuterated potassium dihydrogen phosphate(DKDP) crystals following exposure to nanosecond(ns) and sub-ns laser irradiation is investigated in order to probe the absorption mechanism in damage initiation. Laser damage resistance is greatly improved by sub-ns laser conditioning,whereas only a little improvement occurred after ns laser conditioning at the same laser fluence. Moreover, scattering and transmittance variations after the two types of laser conditioning indicate similar reduction of linear absorption. However, by contrast, large differences on nonlinear absorption modification are discovered using Z-scan measurement. This characteristic absorption modification by laser irradiation provides evidence that a nonlinear absorption mechanism plays a key role in damage initiation at 355 nm.     

Comparison of luminescence spectra of natural spodumene under KrCl laser and e-beam excitation

Spectral characteristics of pulsed photoluminescence (PL) and pulsed cathodoluminescence (PCL) of a natural spodumene were investigated. PL was excited by laser radiation at 222 nm with pulse duration of 10 ns at FWHM. PCL was excited by electron beams with pulse duration from 0.1 up to 4 ns and with current densities of 40-200 A/cm². There was a dominant broad band at 600 nm due to the manganese impurity in PCL spectra. But in PL spectra, the orange band had the intensity comparable with intensities of intrinsic defect bands. At sample cooling by liquid nitrogen, the intensity of orange band in the PCL spectrum increased by two times and the short-wave shoulder of the band reduced.     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Diamond-like carbon layer formation on graphite by excimer laser irradiation

2 fluence results in the formation of a thin hydrogen-free diamond-like carbon (DLC) film on the surface. The density of the laser-formed DLC layer is approximately 2.8 g/cm³, as calculated from area-selective Raman spectroscopy and atomic force microscopic (AFM) measurements. The proportion of the sp³ hybrid states of carbon is estimated to be approximately 50%. Annealing in air at 650 °C for 30 min recovers the graphite structure. Received: 26 March 1998/Accepted: 27 March 1998     

Damage to optically transparent crystals with a macroscopic crack under pulsed laser irradiation

An investigation was made of the influence of a macroscopic crack on the mechanical strength of NaCl, LiF, and CaCO₃ single crystals exposed to pulsed laser irradiation. The probability of growth of the initial macrocrack being activated with optical breakdown of the sample is estimated. A mechanism is proposed for the growth of an initial macroscopic crack in which cavities, microcracks, and macrocracks formed as a result of the heating and burnout of absorbing inclusions combine with the initial crack. Zh. Tekh. Fiz. 68, 34–37 (December 1998)     

Bleaching of color centers in NaCl crystals with laser pulses

Abstract

Bleaching of F centers in gamma irradiated NaCl crystals at room temperature and elevated temperatures using laser pulses from pulsed laser systems is studied in this paper. The rate of decay of F centers is monitored and the decay constants are evaluated. A comparison on the decay kinetics is made for CW and pulsed bleaching with a He-Cd laser. F center to C center conversions using a thermo-optic scheme are carried out with nanosecond dye laser pulses in gamma irradiated NaCl crystals.     

Spectroscopy of defects in KBr(NO 2 − ) and KI(NO2 −) crystals under low-temperature UV irradiation

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 3, pp. 410–415, September, 1990.