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Studies of ionic alkali halide excimer molecules excited by laser-produced plasmas

By Shoichi Kubodera


New ionic states of alkali halide molecules, which are isoelectronic to the neutral rare gas halide molecules, have been generated by direct photoionization using the soft x-ray flux from a laser-produced plasma. The fluorescence emission of three ionic excimer transitions has been observed at 185 nm for CsF, 208 nm for CsCl, and 130 nm for RbF. The emission wavelengths are in good agreement with those calculated using Rittner-type potentials. The vibrational structure in the emission spectrum of CsF has been calculated and excellent agreement with the experiment is found. A model for photoionization of these molecules has revealed that the vibrational population of the bound photoionized state is Boltzmann distributed if the neutral ground state has a thermal vibrational distribution. Molecular halogen transitions of Cl$\sb2$* at 258 nm and F$\sb2$* at 158 nm have been observed in CsCl and RbF, respectively, as a result of predissociation of the ionic states of these molecules. Rate constants for the molecular halogen formation and lifetimes for the precursor states have been experimentally evaluated. The predissociation probability of the ionic state by adiabatic curve crossing has been estimated to describe the observed spectral intensity. A new kinetic model has been developed to describe the photoionization of CsF vapor. The calculated number density of the Cs$\sp{2+}$F$\sp-$ ionic state of 6 $\times$ 10$\sp{13}$ cm$\sp{-3}$ is in good agreement with the measured one. Based on the kinetic model, it is concluded that a CsF ionic excimer laser is feasible when a short pulse excitation with a line focusing geometry is employed. The absorption spectra of CsF, CsCl, and RbF in the UV and VUV have been determined using a VUV plasma continuum probe light source. The observed spectral peaks have been identified as the transitions from the ground states to the neutral covalent states. The spectra have been analyzed using the reflection method, providing potential curves of the neutral covalent states. The absorption oscillator strengths of these transitions have been evaluated from the absorption spectra

Topics: Molecular physics, Physical chemistry, Electronics, Electrical engineering
Year: 1991
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