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Previous measurements of the birefringence of sapphire and magnesium fluoride have been extended to their respective limits of transparency, revealing anomalous dispersion effects at short wavelengths, which are qualitatively discussed. Possible practical applications to Lyot filters and quarter-wave plates for Lyman-alpha are indicated.

The authors describe the following improvements to a three-electrode vacuum spark source: reduction of the size of the triggering device, electronic control of the discharge rate, and a completely coaxial mounting of the electrodes along the optical axis. The performance thus obtained is specified, and absorption spectra are shown as examples of the use of the source in the far ultraviolet (100 A to 2000 A).

The plasma generated by focusing a 20-Hz, 330-mJ, Nd:YAG laser pulse in a rare gas at pressures from 1 to 6 bars is proposed as a source of a VUV continuum giving as many as 10(9) photons nm(-1) sec(-1) near 170 nm. Its stability makes it suited to VUV spectrophotometry. The pulse duration, ranging from 20 nsec for Ne to 40 nsec for Kr, should also permit its use in time-resolved spectroscopy.

Irradiation of an atherosclerotic plaque with different laser beams was investigated to obtain necessary parameters for the photodissociative effect. It was found that the wavelength must be short enough to generate energetical enough photons, and the peak power density must take into account the energy and temporal width of the laser pulse. It was also found that intracavity tripling of the fundamental frequency of a Nd:YAG laser provides ultraviolet irradiation with efficiency more or less comparable to that of irradiation by excimer lasers. >

A simple and safe way is presented to protect power meters when dealing with moderate waist and high peak‐power pulsed laser beams, such as Nd:YAG laser beams.

Kinetics associated with the decay of the low‐lying excited states of xenon is investigated in a density range 2.5×1018 to 2.5×1020 atoms cm−3 (pressure range between 104 and 106 Pa). Three different experiments are performed, i.e., two‐photon excitation of 6p atomic states or 0+g molecular states, VUV one‐photon excitation of the 6s[3/2]1 and 6s’[1/2]1 atomic states, and laser probing of intermediate transient species. The basic experimental data are the time‐resolved second continuum excimer fluorescence at ∼173 nm and the photoionization current. It is found that all excitation decays through 6s[3/2]1 or 6s[3/2]2 with a branching ratio 9/1. Part of the 6s[3/2]1 species (70%) decay subsequently via the 6s[3/2]2 level. The effectiveness of these atomic intermediate steps is directly demonstrated by the transient laser probing. This result, the time analysis, and the discussion allow to set a clear basis to the involved processes and to rule out part of the models proposed in the literature. In particular...

Spectral, temporal, and density behavior of dye-laser-excited dense xenon below and above breakdown threshold has been studied. Observation of three-photon resonant excitation at 147 nm is limited to very low density (\ensuremath{\rho}7\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), whereas four-photon enhanced photoionization plays a major role between 500 mbar and 15 bar of pressure (1.4\ifmmode\times\else\texttimes\fi{}${10}^{19}$ to 40\ifmmode\times\else\texttimes\fi{}${10}^{19}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). Excitation and deexcitation schemes are discussed from existing data and our spectral and temporal results obtained from either excimer (172-nm continuum) or laser spark emission.

Pressure effects on the decay rates of the Xe 6p[ 1 2 ]0 (0–800 mbar) and 6p[ 3 2 ]2 (0–200 mbar) levels have been measured up to 6 × 108 s−1 using two-photon excitation. An analysis in terms of binary and ternary total quench rates for both transitions is presented. A radiative lifetime of 28.2 ± 0.5 ns has been found for both 6p[ 1 2 ]0→6s[ 3 2 ]01 (828.0 nm) and 6p[ 3 2 ]2→6s[ 3 2 ]02 (823.2) transitions.

Reflection techniques are used to study self-broadening of the 253.65 nm mercury resonance line at high vapor density (2.7 × 10 19 −2.7 × 10 21 atoms-cm -3 ). The absorption profiles are deduced from reflectance data using a modified Kramers-Kronig analysis. Another method, based on semi-classical dispersion theory, is shown to be inadequate and is used here only in order to compare our results with previously reported values. Conclusions are stated concerning the effects of different kinds of forces, broadening rate, oscillator strength, and interatomic potential curves with the help of the available theoretical approaches for which validity ranges are discussed.

We have studied the performance of a fast photodetector for use in the vacuum ultraviolet (λ Nous avons etudie les performances d'un recepteur rapide destine a l'ultraviolet a vide (λ

Medium resolution spectra of molecules subjected to a pressure of argon indicate a broadening and a shift of Rydberg transitions, dependent on the shape of the outer orbital. Valence transitions are comparatively little affected even at 200 bar. In the case of the very complicated spectrum of NO, the washing out of the Rydberg bands allows the valence bands to show up even at a moderate pressure, so that the spectrum tends toward the spectrum of NO isolated in a solid rare gas matrix, from which spectrum discrete Rydberg bands are absent. Vibronic terms in homogeneous interaction behave, under pressure, in a way which reflects closely their degree of interaction in the free molecule.

Self-broadening of the 146.96 nm xenon line and 123.58 nm krypton line is studied in the density range 1-100 amagat (2.7 x 1019 — 2.7 × 1021 at. cm-3). The absorption profile is deduced from reflection experiments by a modified Kramers-Kronig analysis. A detailed discussion of the criteria for validity of available theories is given, allowing the determination of the extent to which the cases studied are related to many-body interactions and/or pure resonance forces. The quasistatic theory, in the nearest neighbour approximation, is shown to be valid for interpreting the far wings. Thus it is possible at increasing density, to underline the specific effect of the various kinds of forces : resonance, Van der Waals and repulsive, and to specify the 1u excited state potential which is found weakly attractive for Xe2 and Kr2.

Previous low resolution studies indicated that, in the absorption spectrum, transitions to Rydberg states are far more sensitive to pressure than transitions to valence states in accordance with the complete vanishing of Rydberg bands and the deperturbation of the valence bands in the spectrum of NO trapped in a solid rare gas matrix. The present study of NO is the first extensive investigation of medium and high resolution molecular electronic spectra subjected to pressure of foreign gases. It is observed that, at the highest pressure attained (∼1000 bar), almost all of the Rydberg bands have vanished whereas the remaining valence bands stay relatively sharp. While high pressure medium resolution spectra give information about the deperturbation of intensities and of vibrational spacings of the valence bands, inspection of low pressure high resolution spectra reveals that the rotational lines react to pressure in a way reflecting closely the degree of configuration mixing between Rydberg and valence states.

Absorption spectra of high-density pure krypton and xenon gases, in the wavelength range 118-150 nm and the density range 1-80 amagat, exhibit a red shift of all the atomic lines. The shifts are interpreted by taking into account the high-density interactions with the help of the local field correction of Lorentz introduced in classical dispersion theory.

On calcule les coefficients de Fresnel d'un cristal uniaxe tres absorbant dans le cas general d'une orientation arbitraire de l'axe optique. On en deduit des expressions simples des pouvoirs reflecteurs dans un certain nombre de cas et l'on examine les applications possibles a la determination des constantes optiques.

Description d'un dispositif de mesure du dichroïsme linéaire dans l'ultraviolet à vide par modulation de la polarisation au moyen d'un prisme de Rochon tournant. La sensibilité absolue est contrôlée par la mesure de la transmission pseudo-dichroïque d'une lame de quartz fondu inclinée. Le signal détectable minimal est 10 -3. Le spectre dichroïque d'un biopolymère orienté est présenté comme exemple.

On montre qu'il est possible, dans le cas d'un materiau reflechissant homogene et isotrope, de calculer d'une facon relativement simple, a partir des constantes optiques supposees connues, les angles d'incidence qui offrent des proprietes particulieres (maximum de P = (R s m R p)/(R s + R p), minimum de R p, angle d'incidence principale). Le cas de l'angle d'incidence principale est traite de facon complete. La consideration des courbes P et R s en fonction de l'angle d'incidence permet de definir les proprietes d'un polariseur par reflexion comme le montrent les differents exemples pris a la longueur d'onde de 584 +.

The four optical constants of sodium nitrate have been determined from reflectance measurements with polarized light. A wide band observed between 1900 and 2400 A arises from a transition to a non- planar excited state of the NO - 3 . Similarities with isoelectronic and isomorphous carbonates are reported.

Pulsed superfluorescence has been observed in the visible spectrum of 130 Te 2 and identified by comparison with Fourier transform fluorescence spectra.

It is shown that the triggering of a vacuum spark source is easily obtained by focusing onto the cathode a pulsed laser beam of short duration and rather low energy per pulse. The main advantages of this kind of triggering are discussed in comparing with the previous designs based on an auxiliary sliding spark.

Assuming a lorentzian shape for the imaginary part ϵ2 of the dielectric constant, in the frame of the so-called rigourous classical dispersion theory, we show that without further approximations the frequency shift of the absorption coefficient relative to ϵ2 can be calculated as a function of the damping coefficient.

The four optical constants of dolomite are determined from reflectance measurements with polarized light. A wide band observed between 1400 and 1800 A is associated, as for calcite, with a transition to a non-planar excited state of the CO 2- 3 .

Resume Absorption indices of calcite in the 1910-1690 A region have been determined from transmittance at non normal incidence of polished thin calcite plates cut obliquely to the optic axis.

The four optical constants of magnetise have been determined from reflectance measurements with polarized light. As in the case of calcite and dolomite, a wide band between 1300 and 1700 A is observed in the curve of ko and it arises from a transition of the planar CO2-3 to a non-planar excited state.

On décrit un dispositif à deux faisceaux pour études d'absorption optique jusqu'à 500 Å de gaz condensés. La source est une étincelle initiée sous vide et l'échantillon est déposé sur un substrat fluorescent. Le fonctionnement de l'électronique de détection est décrit en détail. Les performances de l'instrument sont illustrées par deux exemples.

Les auteurs décrivent un réflectomètre destiné à des mesures en lumière polarisée dans l'ultraviolet à vide, sur des matériaux massifs ou évaporés. Cet appareil offre la possibilité de faire varier de façon continue l'angle d'incidence de 4° à 86° et de changer l'orientation du plan d'incidence. Il est associé à un miroir torique qui joue un rôle de polariseur et d'élément de focalisation et l'on montre comment on peut déterminer le degré de polarisation de la lumière incidente sur l'échantillon.

A small size mirror mount is described, whose high sensitivity is obtained by the use of coaxial differential screws.

A clean demountable dye cell is described which is more advantageous than the usual commercial cuvettes for obtaining good quality light beams from pulsed dye lasers.

It is demonstrated that, in case of transverse excitation, amplified spontaneous emission is the best source of pure continuum background for high-resolution pulsed laser spectroscopy.

The four optical constants of calcite are determined from reflectance measurements with polarized light. In the curve of k o a wide band observed between 1400 and 1700 A is associated with a planar excited state of the CO 2- 3 .

The absorption spectrum of solid thin films of neon has been recorded at 4.2 K in the photon energy range from 15 to 25 eV. The first three terms of the hydrogenic series, corresponding to the resonance doublet of the free atom, have been identified and the energy gap at the $\ensuremath{\Gamma}$ point is calculated to be 21.42 eV.

An experimental setup including the synchronized coupling of a VUV spark source with a laser is described. Experimental results on the depletion of Ar(2), Kr(2), and Xe(2) excimer fluorescence illustrate the capability of the system.

The authors describe improvements to a Ballofet-Romand-Vodar continuum source, viz., accurate synchronization of the main spark, better cooling of the anode. High resolution absorption spectra with a 10-m concave grating spectrograph are shown as examples of the performance thus obtained.

A complete vacuum UV spectrophotometric device, including the source and a computerized data acquisition system, for both reflectance and transmittance measurements in the 105-200-nm wavelength range is described. The apparatus is suitable for various experimental situations such as measurements in the -200 to +100 degrees C temperature range for gases, liquids, and solids (for gases and liquids within the 0-150 bar pressure range). Drift of 0.07% per day for ambient temperature measurements and absolute reflectance as low as 0.05% are reported, illustrating the high performance reached. The setup is particularly well suited for long cycles of low temperature, high pressure experiments.

Measurements of the birefringence of sapphire, magnesium fluoride, and synthetic quartz from 3000 A to 1500 A are reported and are followed by a discussion of the methods of achromatizing quarter-wave plates and of linearizing the path retardation of a multiwave plate against wavelength (equichromatic wave plates).

Measurements of the ordinary and extraordinary refractive indices of synthetic quartz in the vacuum uv down to 1440 A are reported as deduced from the determination of the orders of interference fringes in a thin parallel plate.

For an absorbing plane parallel plate, a simple reciprocal transformation exists between the expressions for the reflectance R and transmittance T, enabling easy passage from one to the other. Then it is shown that the expression A=1−R−T is always positive and, further, may be identified with the energy computed for the dissipation by the Joule effect in the plate, provided that the resultant electric field is calculated at each point inside the plate.