6 research outputs found
Optical constants of solid methane
Methane is the most abundant simple organic molecule in the outer solar system bodies. In addition to being a gaseous constituent of the atmospheres of the Jovian planets and Titan, it is present in the solid form as a constituent of icy surfaces such as those of Triton and Pluto, and as cloud condensate in the atmospheres of Titan, Uranus, and Neptune. It is expected in the liquid form as a constituent of the ocean of Titan. Cometary ices also contain solid methane. The optical constants for both solid and liquid phases of CH4 for a wide temperature range are needed for radiative transfer calculations, for studies of reflection from surfaces, and for modeling of emission in the far infrared and microwave regions. The astronomically important visual to near infrared measurements of solid methane optical constants are conspicuously absent from the literature. Preliminary results are presented on the optical constants of solid methane for the 0.4 to 2.6 micrometer region. Deposition onto a substrate at 10 K produces glassy (semi-amorphous) material. Annealing this material at approximately 33 K for approximately 1 hour results in a crystalline material as seen by sharper, more structured bands and negligible background extinction due to scattering. The constant k is reported for both the amorphous and the crystalline (annealed) states. Typical values (at absorption maxima) are in the .001 to .0001 range. Below lambda = 1.1 micrometers the bands are too weak to be detected by transmission through the films less than or equal to 215 micrometers in thickness, employed in the studies to date. Using previously measured values of the real part of the refractive index, n, of liquid methane at 110 K, n is computed for solid methane using the Lorentz-Lorenz relationship. Work is in progress to extend the measurements of optical constants n and k for liquid and solid to both shorter and longer wavelengths, eventually providing a complete optical constants database for condensed CH4
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Experimental studies of self-suppression of vacuum ultraviolet generation in Xe
Vacuum ultraviolet light in the range 116 nm to 117 nm was produced by using a two-photon resonant four-wave mixing scheme in Xe. The buildup of coherent cancellation of the two-photon resonant transition employed in the generation of the vacuum ultraviolet, with resulting limitations imposed on the achievable vacuum ultraviolet intensity was investigated. Under certain predicted conditions, increases in the intensity of one of the pumping beams, approx.1500 nm infrared, or tuning this beam towards resonance with the 5p/sup 5/7s(3/2)/sub 1/ level of Xe led, not to increases, but decreases in the vacuum ultraviolet generated. 3 refs., 3 figs
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New coherent cancellation effect involving four-photon excitation and the related ionization
We describe here an effect which occurs when a first laser is tuned near a dipole allowed three-photon resonance and a second laser is used to complete a dipole allowed four-photon resonance between the ground state /vert bar/0 > and an excited state /vert bar/2 >. In this process three photons are absorbed from the first laser and one photon from the second; so that if the /vert bar/0 >--/vert bar/2 > transition is two-photon allowed the transition is also pumped resonantly by the third harmonic field due to the first laser and the second laser field. When the second laser is strong enough to cause strong absorption of the third harmonic light, and the phase mismatch, /DELTA/kappa is large and dominated by the nearby resonance, a destructive interference occurs between the pumping of the /vert bar/0 >--/vert bar/2 > transition by two- and four-photon process. 7 refs
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Effect of the coherent cancellation of the two-photon resonance on the generation of vacuum ultraviolet light by two-photon reasonantly enhanced four-wave mixing
Many of the most impressive demonstrations of the efficient generation of vacuum ultraviolet (VUV) light have made use of two- photon resonantly enhanced four-wave mixing to generate light at ..omega../sub VUV/ = 2..omega../sub L1/ +- ..omega../sub L2/. The two-photon resonance state is coupled to the ground state both by two photons from the first laser, or by a photon from the second laser and one from the generated VUV beam. We show here that these two coherent pathways destructively interfere once the second laser is made sufficiently intense, thereby leading to an important limiting effect on the achievable conversion efficiency. 4 refs