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This contribution to ECPD2019 is dedicated to the memory of Anatoly Faenov. During a period of approximately thirteen years 1994–2006, Anatoly and his wife Tatiana Pikuz (simply “Tania” for friends), accepting the frequent invitations of the National Institute for Nuclear Physics (INFN) and of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), cooperated with many Italian research laboratories dedicated to EUV and soft X-ray generation, spread in different towns (L'Aquila, Frascati, Milano, Padova, Pisa, Roma, etc.). In spite of the fact that they could stay in Italy only about one or two months per year, their activity was so intense that more than 50 peer- reviewed publications were generated from their experimental and theoretical work (just considering only the results obtained at L'Aquila and Tor Vergata—Rome Universities and at the ENEA Research Center of Frascati), without mentioning the cultural atmosphere that they stimulated in the field of Science and Humanity. The numerous experimental spectra obtained at ENEA by means of their spherically bent mica spectrometers, together with the corresponding theoretical simulations performed in Moscow, allowed to study the changing role of different excitations mechanisms for various plasma conditions, and to characterize at best the ENEA laser-plasma source for different applications: polychromatic and monochromatic micro-radiography of dried biological samples at 1 keV, soft X-ray contact microscopy (SXCM) of living cells in the water-window spectral region, spectroscopy of hollow atoms, etc. In this memorial paper, the main results of biological samples imaging on lithium fluoride (LiF) detectors, obtained with the ENEA and Tor Vergata University laser-plasma sources, are presented. In particular, the improvement of the micro-radiography and of the SXCM techniques obtained after moving from photoresist detectors and photographic films to lithium fluoride (LiF) detectors are discussed, for both dried and wet biological samples.

Lithium fluoride crystals are irradiated at various doses by gamma rays at 77 K. The time evolution of photoluminescence signals from aggregated F 2 +, F2, F3 + and F 3 color centers, and of the absorption intensity of primary F centers are measured at various annealing temperatures. The lifetimes of anionic vacancies υa and F2 + centers, the characteristic times of concentration growth of F2, F 3 + and F3 centers, and also the activation energies of diffusion of vacancies and F2 + centers together with various processes of aggregation are determined. It is found that lifetime decreases for vacancies while increases for F2 + centers by increasing the irradiation dose. It is also shown that, after irradiation during annealing, vacancies are formed as a result of the reaction F2 ++H→υa+Fl-, where Fl- is a fluorine ion in a lattice site and H is a fluorine interstitial atom. Then these vacancies participate in color centers aggregation kinetics. The presence of F- centers in the irradiated crystal is established, and the processes which lead to the formation of F2, F3 + and F3 centers after irradiation, are unveiled. © 2013 Elsevier B.V.

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the $$ F_2^{+} $$ centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F 2 and $$ F_3^{+} $$ centers in the post-radiation period. The F 2 centers are formed when vacancies νa add to $$ F_1^{-} $$ centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction $$ F_2^{+} $$ + H → νa + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The $$ F_3^{+} $$ centers are formed both by merging $$ F_2^{+} $$ and F 1 centers and as a result of addition of vacancies to F 2 centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of $$ F_3^{+} $$ centers occurs faster than the rise in the concentration of F 2 centers.

Eight nominally pure lithium fluoride crystals, obtained from a single LiF crystal containing less than 100 ppm impurities, were irradiated by gamma-rays from a 60Co source at room temperature, with doses from 8.4 to 2.5 105 Gy, but one at -60 °C. Optical density measurements were performed to investigate the radiation-induced color centres (CCs) and to evaluate their concentrations. Thermoluminescence (TL) glow curves were collected and simulated by a first-order kinetics approach, and from the best-fit procedure ten glow peaks (GPs) spanning from 100 to 450 °C were highlighted. A comparative analysis of GP intensities and CC concentrations as a function of the irradiation dose has questioned their association as obtained in previous measurements, showing the impurities, less than 1018 cm-3, still playing a predominant role in TL spectra. New measurements on LiF crystals more pure, at least an order of magnitude, are required to establish for sure the association of GPs to CCs. © Published under licence by IOP Publishing Ltd.

The advent of scanning near-field optical microscopy (SNOM) has augmented at a microscopic level the usefulness of optical spectroscopy in the region between 300 nm and 10 mum. Two-dimensional imaging of chemical constituents makes this a very attractive and powerful new approach. In this paper we show SNOM results obtained in several geometrical configurations on boron clusters in silicon, Li clusters embedded in a LiF sample and BN growth on silicon. We also show some results on the wavelength dependence of the reflectivity (R) in the near infrared (IR) of biological cells in liquid environment with the observation of the local fluorescence. The SNOM images revealed features that were not present in the corresponding shear-force (SF) images and which were due to localized changes in the bulk properties of the sample. The size of the smallest detected features clearly demonstrated that near-field conditions were reached both in the visible and infrared region. (C) 2004 Elsevier B.V. All rights reserved.

Results of investigations of radiative F 3 + -color centers in lithium fluoride obtained by the methods of one- and two-photon absorption, polarization spectroscopy, and luminescence in singlet, triplet and triplet-singlet channels are reported. The scheme of energy levels of the F 3 + -center is revised. The probabilities of singlet-triplet conversion and the rates of depletion of the lower triplet state in the range of 80–350 K are presented. Recommendations are given on the technology of radiative LiF coloration that provide an increase in the F 3 + -center concentration and a decrease in the concentration of some other centers that prevent lasing on the LiF:F 3 + active medium from being obtained. The phototransformations of color centers in LiF are analyzed. The parameters of lasers with LiF:F 3 + active media that provide generation of radiation tunable in the green region of the spectrum are considered. Based on spectroscopic data the maximum attainable time characteristics of such lasers are discussed. The prospects for further investigations of LiF:F 3 + lasers are outlined in brief.

Self-diffraction of light has been investigated in KCl with F/sub A/ centers in order to elucidate the role of F/sub A/-center kinetics in holographic recording under resonant optical excitation. It is shown that diffraction efficiency for holographic grating recorded in the spectral range of the F/sub A1/ transition can be substantially increased by populating the orientations which immediately interact with the polarized light by means of lateral illumination of the crystal with an auxiliary beam. The optimum range of auxiliary beam intensities leading to improvement of grating quality is found. Such a range is directly defined by the parameter /spl chi/, characterizing the selection rules violation for F/sub A/ center in KCl. It has been found that the experimentally determined value /spl chi/=0.04 is consistent with the self-diffraction data obtained in this paper.

The optical properties of Ni2+ -ion impurities in a new crystal, the inverted fluoroperovskite BaLiF3, have been studied. In particular we have investigated the fine structure of the transition by means of absorption and emission spectra. By analogy to previous results we have assigned the structured absorption band around 1200 nm to the 3A2 ↠ 3Ti transition. Four zero-phonon lines are clearly visible in the absorption spectrum. The emission in the region 1250–1600 nm, due to the same 3A23T1 transition, consists of two sharp lines with their vibrational sidebands. © 1995, Taylor & Francis Group, LLC. All rights reserved.

The Shroud of Turin displays a weak front and back body image which inter alia is very superficial and possesses three-dimensional properties. Up to now it has been impossible to reproduce all its characteristics at the same time albeit several attempts were made in many laboratories. But lately, coloration similar to that of the Shroud has been imprinted on dry linen cloths with a powerful VUV radiation of an excimer laser, which however does not exclude the fact that a different laser source could do the same. In fact, a new hypothesis is advanced here which is based on the irradiation of wet linen cloths with a short and powerful pulse of a CO2laser. Besides the well known features of pulsed IR laser, the hypothesis is based solely on the peculiar optical and thermal properties of water and vapor, and therein an experimental investigation is proposed to prove its validity. Key words: Body image, Shroud of Turin, light, lasers, heat, vapor.

A detailed investigation on F-aggregate centers in KF crystals has been performed by means of optical absorption and luminescence measurements. The spectroscopic features of the absorption bands due to F 3 and F 4 defects were confirmed and studied as a function of temperature. Two new emission bands at 958 and 1180 nm were detected, and their dependence on the excitation wavelength and temperature was determined. Assignment of the 958 nm luminescence as due to F 3 centers and of the 1180 nm luminescence to F 4 defects is proposed according to the results of excitation spectra. A remarkable influence of the crystal preparation on the optical properties of color centers in KF has been evidenced.

Here we show a new, simple method to observe soft x-ray microradiographs of biological material. A thin film of lithium fluoride (LiF) works as image detector, storing the microradiograph obtained exposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the stored image, collecting the optically stimulated visible luminescence emitted by the LiF active color centers locally produced by the x rays, a scanning near-field optical microscope is used with an optical aperture of 50 nm, i.e., L/12, where L is the wavelength of the collected photoluminescence.

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences. (C) 2004 Elsevier B.V. All rights reserved.

New easy spectrally tunable backlighting schemes based on a spherically bent crystal are considered. Contrary to traditional backlighting scheme, in which the investigated objects should be placed between the backlighter and the crystal, for the considered schemes an object is placed downstream of the crystal, before the tangential or after the sagittal focus and an image of the object is recorded at the distance from the object corresponding to the needed magnification. The magnification is defined by the ratio of the distances from the sagittal focus to the detector and from the object to the sagittal focus. A ray-tracing modeling and experimental images of test meshes, obtained at incidence angles of the backlighter radiation of 10° and 22°, are presented. It is demonstrated that a simple linear transformation of the obtained astigmatic images allows reconstructing them as a stigmatic with an accuracy of 5–15%. For the spectral range around 9 Å a spatial resolution about 10 μm in a field of view of some square millimeters is achieved experimentally and confirmed by ray-tracing simulations.

A new imaging detector for EUV or soft-X-ray radiation based on optically stimulated luminescence (OSL) of lithium fluoride (LiF) films or crystals is presented. The first micro-radiography images of biological samples and of meshes obtained on LiF using a laser-plasma source or an X-ray laser are shown, and (up to now) a resolution better than one micron is demonstrated. The dependence of the coloration density vs the deposited X-ray dose is considered and the advantages of this new diagnostic technique for both coherent and non-coherent EUV sources, compared with CCDs detectors, photographic films and photoresists are discussed. This new detector is extremely suitable for laser plasmas and for X-ray lasers sources.

We report the results of the morphological, electrical and optical characterisation of double-layer Alq3-based organic emitting diodes with a lithium fluoride (LiF)/Al cathode. A detailed electron microscopy investigation of their cross-section shows the presence of LiF isolated grains underneath the Al film. Due to the introduction of the LiF layer, luminance was larger than 30,000 cd/m2 at a bias voltage VB=25 V with a maximum external luminous efficiency as large as 46 lm/W at VB=20 V. Performing on/off VB cycles at a very low frequency, each time a recovery of the initial electric and luminous performance was observed. The non-exponential decay of both electric current and luminance during each on-cycle has been interpreted as the signature of dispersive transport controlled by multiple trapping on localised states in an amorphous system.

The efficient coloration of LiF material, in the form of bulk and films, by EUV and soft X-rays emitted by a laser-plasma source is demonstrated. The short penetration depth of soft-X-rays is exploited to obtain high spatial resolution luminescent patterns while the high dynamic range of proportionality between X-ray dose and coloration is exploited for using LiF as image detector in micro-radiography and soft X-ray microscopy applications.

We present a survey on five years of experiments of excimer laser irradiation of linen fabrics, seeking a coloration mechanism able to reproduce the microscopic complexity of the body image embedded onto the Shroud of Turin. We achieved a superficial, Shroud-like coloration in a narrow range of irradiation parameters. We also obtained latent coloration that appears after artificial or natural aging of linen following laser irradiations that, at first, did not generate any visible effect. Most importantly, we have recognized photochemical processes that account for both coloration and latent coloration.

Optical absorption and photoluminescence spectra were measured on gamma colored lithium fluoride crystals at five different irradiation doses. By accurate Gaussian best-fit procedures of the absorption measurements the peak intensity of F and M bands and the concentrations of F, F2 and F3+ color centers in gamma irradiated LiF crystals at different doses were obtained. A quadratic relationship between F and F2 defect concentrations as a function of the irradiation dose was found. Photoluminescence measurements, excited by an Argon laser at 457.9 nm, were used to study the effect of the laser power on the F2 and F3+ emission spectra of gamma irradiated LiF crystals. The influence of the triplet state on the optical cicle of F3+ color center was highlighted for several excitation laser power and the ratio of the transitions probability W1 and W2 to and from the triplet state was derived.

The controversial far-infrared optical absorption in Li-doped alkali halides is now attributed to transitions between the low-lying energy levels of an anharmonic oscillator composed of three halogen-ion pairs centred at the cation site which vibrate in theT 1u ungerade mode. This oscillator is electronically coupled to the impurity dragging it into rotation over 〈111〉 sites. Several experimental features derive simply from our model such as the absence of any clear-cut lithium-isotope effect and the low-frequency range. The capital effect of an F centre at a 〈001〉 nearest-neighbour (nn) site is to reduce the dimensionality of vibration forcing the impurity to rotate in the perpendicular (001)-plane. This upgrades the optical frequency in proportion to (3/2)1/2 and shrinks the off-centre displacement. We finally show that recent computer-simulation data on Li-doped alkali halides provide a solid ground for regarding the Li off-centredness as a pseudo-Jahn-Teller phenomenon.

The experimental values of the off-axis angle of FA(II) dipoles in KCl:Li+, RbCl:Li+ and KF:Li+ are in good agreement with those obtained from theoretical estimates of the off-centre displacements of the Li+ ion calculated with the shell model potentials. On this basis, one can quantitatively predict the off-axis behaviour of FA(II) centres from the calculated off-centre positions of small impurity ions in alkali halides.

Complex F-aggregate centers were studied in pure and Li 1 -doped KF, by means of optical excitation. Three new emission bands in the near infrared region were revealed. Among them, the luminescence at 1040 nm was identified as the emission of F 3 defects because of its excitation, its peak position and its enhancement under irradiation at room temperature with F light. The luminescences at 950 and at 1250 nm were too weak and could not be unambiguously identified. However, they are probably due to complex F-aggregate centers, like F 4 defects.

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The off-axis configuration of the F A centers in KF : Li + has been evaluated from photostimulated reorientation experiments. We performed transient and kinetic measurements at 77 K of the crystal absorption coefficient and stationary emission measurements at 2 K. The experiments give an average off-axis angle θ = 14° ± 2°. Moreover, the spectral features of the F A (Li) emission band in KF have been determined, and the temperature dependence of the band width has been found to follow the expected behavior.

The lineshapes of the absorption bands ofFA centres in alkali halides have been studied for the first time. The new method used for this investigation is based on the determination of the overlap between theFA1 andFA2 bands from luminescence measurements. The experimental results have been compared with calculated values deduced from the theoreticalFA bands of different shapes. For bothFA (I) centres in KCl: Na+ andFA (II) centres in KCl: Li+ and RbCl: Li+ the absorption lineshape at low temperature is much closer to a sum of two Lorentzian curves than that of two Gaussian or Poissonian bands. This result shows an unexpected difference with theF centres, whose absorption lineshape is known to be Poissonian at the same temperatures.

Type-I or type-II behaviour of the FA centres in alkali halides has always been thought of as a feature dependent on the crystal lattice parameter and on the impurity size. We have found experimental evidence suggesting for the first time that both behaviours are present in RbCl:Li+, where the FA centre at low temperature (T < 15 K) exhibits a luminescence of type-I switching to type-II at higher temperature. This effect is probably related to the possibility for the Li+ ion of being, at high temperature, a mobile and off-centre ion. Both these conditions seem to be necessary for having a type-II behaviour.

The off-axis configuration of FA centers in RbCl: Li+ has been investigated by means of luminescence measurements. A first quantitative evaluation of the off-axis angle has been obtained.

The photostimulated reorientation of theFA centres in KCl:Li+ is described by a simple model that takes into account the off-axis angle of the centres and the overlapping of the tails of the two absorption bands. With this model we have interpreted the dependence of the luminescence intensity on the polarization of the exciting beam. The fitting of the experimental data gives an off-axis angle of approximately 5° and an overlapping of the bands larger than that predicted for Gaussian line shapes.

Complex colour centres in RbCl and RbCl:Li+ are extensively studied. (F2)A centres, never observed so far in this system, are formed and their optical absorption and emission determined. The kinetics of the equilibria among aggregate and associate defects are investigated in order to obtain the highest concentration of (F2)A centres. On a etudie en detail les centres de couleur complexes dans le RbCl et le RbCl:Li+. Les centres (F2)A ont ete produits pour la premiere fois dans ce systeme; on a mesure leur absorption optique et leur emission. Les equilibres entre les defaults agreges et ceux associes ont ete etudies du point de vue cinetique pour atteindre la concentration des centres (F2)A la plus haute possible.

Lithium fluoride (LiF) films irradiated by low energy electrons were employed as active spacers in all-solid, dielectric optical microcavities emitting in the visible spectral range. We present the results of optical characterization of the spontaneous emission from F 2 color centers embedded in a LiF layer confined inside a planar microcavity. These structures seem promising for the realization of novel kinds of solid-state miniaturized emitting devices.