Raman spectra of low-pressure methane gas both at room temperature and near condensation have been studied in the frequency range from 0 to 50 cm '. The spectra include two contributions: One is due to collision-induced scattering (CIS) and the other to an unresolved dimer rotational band. The latter component is most prominent at low temperatures. Both the dimer band and CIS spectrum have been computed for a Lennard-Jones interaction and point-dipole polarizability. At large frequency shifts ( & 20 cm ) it is suAicient to use classical collision dynamics to compute the CIS spectrum, but at lower shifts it is necessary to use quantum theory and add the dimer contribution to obtain good agreement with the experimental spectrum.From analysis of the spectra we determine that the fractional depolarized scattering from dimers is 10% at 300'K and 43% at 115'K. The shape of the far wing (20-50 cm ') provides a sensitive means of testing the form of the induced polarizability anisotropy. For methane, excellent agreement with experiment is obtained for the point-dipole model.
Sodium and thallium surfaces were heated in vacuum by a short intense pulse from a CO2 TEA laser. The time-dependent emission of neutral atoms following the laser pulse was measured in the density range 1011–1012 atoms/cm3 by monitoring the absorption from a beam of resonance light. The temperature of the metal surface and the depth of heating after the laser pulse were obtained using a theoretical model. The results show that at the end of the laser pulse the temperature of the surface is close to the boiling point for the metal independent of laser energy in the range 106–109 W/cm2. The surface temperatures were 593 and 934 °K for sodium and thallium, respectively. The depth of heating is greater than can be explained by heat conduction alone. It is suggested that during the laser pulse the metal is heated by a shock wave. The results also point out the limitations of pulsed laser vaporization in the production of neutral atoms from refractory materials.
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