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We report measurements of the absolute yield of 123.6-nm vacuum ultraviolet ͑VUV͒ radiation produced in a resonant four-wave-mixing scheme in krypton enhanced by electromagnetically induced transparency. One of the mixing fields, in the ultraviolet ͑UV͒ at 212.55 nm, was in two-photon resonance with the 4p 6 1 S 0 -4p 5 5p͓0, 1 2 ͔ transition of krypton, while a second field ͑the coupling field͒ at 759 nm was resonant with the 4p 5 5p͓0, 1 2 ͔-4p 5 5s͓1, 1 2 ͔ transition. Electromagnetically induced transparency has been predicted to be induced by this coupling field and to enhance the efficiency of the generation of the field at 123.6 nm, on the 4p 5 5s͓1, 1 2 ͔ to 4p 6 1 S 0 transition. This was confirmed by measurements of the absolute VUV radiation yield using a calibrated photodiode in the limit of a large density-length product. Energies of ϳ30 nJ were produced, which gave a conversion efficiency, from the coupling field energy of ϳ1%. Higher yields are thought to be achievable by increasing the coupling laser intensity, the path length of the medium, and by the use of a transform-limited UV pulse of the same duration as the coupling field.PACS number͑s͒: 42.50.Ϫp Schemes employed to generate coherent radiation at vacuum ultraviolet ͑VUV͒ wavelengths have commonly made use of four-wave mixing in atomic gases conducted off resonance in order to avoid reabsorption of the generated light. Typical conversion efficiencies in the region 10 Ϫ5 -10 Ϫ6 have been achieved for input powers in the range 1-5 MW ͓1,2͔. These schemes have been widely used in linear spectroscopy but the relatively low pulse energies generated have largely precluded other uses. Applications such as nonlinear spectroscopy, photolithography, and photochemistry, for which coherent radiation at longer wavelengths has been used widely, have been difficult, if not impossible, in the VUV. More powerful sources of narrowband and coherent radiation have therefore been sought.The third-order nonlinearity of the medium ( (3) ) controls the mixing of the three applied fields, and though this can be enhanced by resonance ͑for example two-photon resonance ͓3,4͔͒ the generated field must be far from resonance to avoid reabsorption. The dispersion of the medium at resonance also varies steeply, causing disruption to the phase matching. Earlier work using two resonant lasers and a third near-resonant laser mixing scheme in mercury vapor reported high conversion efficiency ͑up to 5%͒ into the VUV at 130.2 nm ͓5͔. This high conversion efficiency was attributed by the authors to the use of collimated unfocused beams over a 1-m path length and two-photon resonant and near single-photon resonant enhancement of (3) for all fields.In 1990 Harris, Field, and Imamoglu proposed that the phenomenon of electromagnetically induced transparency could be used to suppress the absorption of generated light which is in resonance with an atomic transition while also improving the dispersive properties of the medium ͓6͔. The advantages of this type of scheme are that the nonli...

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The dynamic Jahn-Teller effect has been used in the analysis of the temperature-dependent paramagnetic susceptibilities of Ti 3ϩ :Al 2 O 3 . Two important changes have been made from the original analysis: ͑1͒ Parameters as determined from the dynamic Jahn-Teller effect are incorporated, and ͑2͒ All Van Vleck susceptibilities have been forced to sum to zero as required by quantum theory. The results of the analysis show that ʈ contains a constant term of the order 1.22ϫ10 Ϫ8 emu/g that does not arise from Ti 3ϩ singles. The data over the temperature range 18 to 100 K indicate that g ʈ 1 ϭ2.54, a value 25% larger than that determined from the infrared Zeeman effect. It is possible to determine the perpendicular Van Vleck susceptibility for the ground state which is governed by the magnetic interaction of the lowest two energy levels with a substantial Jahn-Teller contribution. Finally, the suppression of the susceptibilities below 4.12 K can be modeled by a factor for the removal of singles from the susceptibilities by interactions with nearest neighbors, next-nearest neighbors, etc.