LASER OPTOGALVANIC EFFECTS CAUSED BY FORMATION OF NEGATIVE IONS

Dans ce rapport deux types d'effets optogalvaniques sont présentés et étudiés. En premier lieu, il a été démontré la possibilité d'obtenir un effet optogalvanique dans les gaz moléculaires électronégatifs (SF6, CCl2F2, I2). L'effet du au piégeage volumique d'électrons est observé à bas voltage dans une diode fonctionnant en régime de charge d'espace et dans un magnétron à triode. Les molécules SF6, CCl2 F2 sont excitées vibrationnellement par absorption d'une transition d'un laser à CO2 ou par un processus Raman résonnant (I2) sous l'action combinée des rayonnements du deuxième harmonique du laser à YAG et du laser à colorants organiques. La possibilité d'utiliser ces effets liés au piégeage volumique d'électrons et leurs propriétés sont étudiées en spectroscopie moléculaire optogalvanique. En second lieu, la physique de la chimionisation laser-induite par collisions thermiques entre des atomes de métaux alcalins et quelques molécules électronégatives, est présentée. Les expériences ont été faites sur le sodium dont les atomes sont excités par un laser à colorants organiques. Nous avons mesuré la vitesse absolue de l'ionisation collisionnelle pour l'état 4d de Na avec des gaz-cibles tels que O2, SF6 CH3Br, CH3I. Les données expérimentales sont comparées à la théorie pour des transitions nonadiabatiques dans la théorie de Landau-Zener modifiée par Zembekov. Il est montré que le modèle des croisements multiples est valable et que le maximum Landau-Zener de la section efficace est atteint.In this paper two kinds of optogalvanic effects are discussed and investigated. First, it was demonstrated the possibility of obtaining optogalvanic effects in electronegative molecular gases (SF6, CCl2F2, J2) due to volume electron capture. Effects have been observed at low voltage in space charge limited diode and triode magnetron. The molecules were vibrationally excited at absorption of CO2-laser radiation (SF6, CCl2F2) or at the resonance Raman-like process (J2) under second harmonic YAG-laser and dye laser radiation. The properties and the probability using of these effects for molecular optogalvanic spectroscopy are discussed. At second, the physics of laser-assisted chemo-ionization at thermal collisions between alkali metal atoms and some electronegative molecules are considered. Experiments were made with sodium atoms whose were excited by dye laser radiation. We measured the absolute rate of the collisional ionization for 4d Na state with gas-target as O2, SF6, CH3Br, CH3J. The experimental data are compared with Landau-Zener theory of nonadiabatic transitions modified by Zembekov. It is discovered that the model of multiple-crossing are valid and the Landau-Zener maximum of the cross-section is realized

Explosive evaporation of Rb or K fractal clusters by low power CW radiation in the presence of excited atoms

In this paper we describe a new, spectacular, unpredictable effect of the explosive evaporation of metallic Rb or K fractal clusters, only in the presence of excited atoms stimulated by resonant CW laser radiation in a heat-pipe glass cell. Evaporation occurs at low laser-power density, in the presence of a buffer gas. The effect consists of the generation of optically thick, sharply localized alkaline metals vapour clouds propagating in the cell against the laser beam. These clouds are charged and exhibit a strong luminescence of Rb or K spectral lines. We believe that the explosive evaporation of metallic fractal clusters observed is explained by the laser excitation of alkali atoms. The excited atom collides into the surface of the clusters and transfers its internal energy to the surface locally. This energy greatly raises the temperature of this local part of the clusters surface, melts it and decreases the fractal surface area. Because, in general, any fractal cluster systems have a high surface energy, some of processes which leads to decreasing their surface area can liberate the surface energy. This energy increases the total temperature of the clusters and eventually leads to the thermal explosion of the cluster

Infra-red stimulated Raman generation effects of gain focusing on threshold and tuning behaviour

It is shown that for generation of infrared radiation by stimulated Raman scattering, the diffraction spread of the Stokes wave can have a significant effect on the threshold. Compared with an analysis in which gain focussing is neglected, the threshold powers may be much higher with a corresponding reduction in tuning range. The design of a Raman oscillator is considerably influenced by these diffraction effects, and also it is found that the Stokes wave is subject to frequency-pulling which is dependent on the pump power